Model 7220 DSP Lock-in Amplifier Instruction Manual 190171-A-MNL-C Copyright © 1996 EG&G INSTRUMENTS CORPORATION
FCC Notice This equipment generates, uses, and can radiate radio-frequency energy and, if not installed and used in accordance with this manual, may cause interference to radio communications. As temporarily permitted by regulation, operation of this equipment in a residential area is likely to cause interference, in which case the user at his own facility will be required to take whatever measures may be required to correct the interference.
Table of Contents Table of Contents Chapter One, Introduction 1.1 How to Use This Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1 1.2 What is a Lock-in Amplifier? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2 1.3 Key Specifications and Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TABLE OF CONTENTS 3.3.02 Relative Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9 3.4 Full-Scale Sensitivity and AC Gain Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9 3.5 Dynamic Reserve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9 3.
TABLE OF CONTENTS 4.2.07 PREAMP POWER Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7 4.2.08 REF MON Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7 4.2.09 REF TTL Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7 4.2.10 SIG MON Connector . . . . . . . . . . . . . . . . . .
TABLE OF CONTENTS 6.3 RS232 and GPIB Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 6.3.01 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 6.3.02 RS232 Interface - General Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 6.3.03 Choice of Baud Rate . . . . . . . .
TABLE OF CONTENTS Appendix B, Pinouts B.1 RS232 Connector Pinout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1 B.2 Preamplifier Power Connector Pinout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1 B.3 Digital Output Port Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TABLE OF CONTENTS vi
Introduction Chapter 1 1.1 How to Use This Manual This manual gives detailed instructions for setting up and operating the EG&G Instruments Model 7220 Digital Signal Processing (DSP) dual phase lock-in amplifier. It is split into the following chapters:Chapter 1 - Introduction Provides an introduction to the manual, briefly describes what a lock-in amplifier is and the types of measurements it may be used for, and lists the major specifications of the model 7220.
Chapter 1, INTRODUCTION Appendix D Shows the connection diagrams for suitable RS232 null-modem cables to couple the unit to an IBM-PC or 100 % compatible computer. Appendix E Gives an alphabetical listing of the computer commands for easy reference. Appendix F Provides a listing of the instrument settings produced by using the default setting function. If you are a new user, it is suggested that you unpack the instrument and carry out the procedure in chapter 2 to check that it is working satisfactorily.
Chapter 1, INTRODUCTION 1.3 Key Specifications and Benefits The EG&G Instruments Model 7220 represents the latest in DSP Lock-in Amplifier technology at an affordable price, and offers:n Frequency range: 0.001 Hz to 120 kHz n Voltage sensitivity: 20 nV to 1 V full-scale n Current input mode sensitivities: 20 fA to 1 µA full-scale 20 fA to 10 nA full-scale n Line frequency rejection filter θ outputs n Dual phase demodulator with X-Y and R-θ n Very low phase noise of < 0.
Chapter 1, INTRODUCTION 1-4
Installation & Initial Checks Chapter 2 2.1 Installation 2.1.01 Introduction Installation of the model 7220 in the laboratory or on the production line is very simple. Because of its low power consumption, the model 7220 does not incorporate forced-air ventilation. It can be operated on almost any laboratory bench or be rack mounted, using the optional accessory kit, at the user’s convenience.
Chapter 2, INSTALLATION AND INITIAL CHECKS Instruments are normally shipped from the factory with the line voltage selector set to 110-130 V AC, unless they are destined for an area known to use a line voltage in the 220-260 V range, in which case, they are shipped configured for operation from the higher range. The line voltage setting can be seen through a small rectangular window in the line input assembly on the rear panel of the instrument (figure 2-1).
Chapter 2, INSTALLATION AND INITIAL CHECKS installed, close the plastic door firmly. The correct selected voltage setting should now be showing through the rectangular window. Ensure that only fuses with the required current rating and of the specified type are used for replacement. The use of makeshift fuses and the short-circuiting of fuse holders is prohibited and potentially dangerous. 2.2 Initial Checks 2.2.01 Introduction The following procedure checks the performance of the model 7220.
Chapter 2, INSTALLATION AND INITIAL CHECKS the CONTROL SETUP menu is displayed, which will look similar to the following:- Figure 2-3, Control Setup Menu 7) Press one of the keys on the right-hand side of the left-hand display once. This will set all the instrument’s controls and displays to a known state.
Technical Description Chapter 3 3.1 Introduction The model 7220 lock-in amplifier is capable of outstandingly good signal recovery performance, provided that it is operated correctly. This chapter describes the design of the instrument, enabling the best use to be made of its facilities. Of particular importance is the correct adjustment of the AC Gain parameter, described in section 3.2.04. 3.2 Principles of Operation 3.2.
Chapter 3, TECHNICAL DESCRIPTION 3.2.02 Signal-Channel Inputs The signal input amplifier may be configured for either single-ended or differential voltage mode operation, or single-ended current mode operation. In voltage mode a choice of AC or DC coupling is available and the input may be switched between FET and bipolar devices. In current mode two conversion gains are selectable to allow for optimum matching to the signal input.
Chapter 3, TECHNICAL DESCRIPTION The primary purpose of the DC coupling facility is to enable the use of the instrument at reference frequencies below 0.5 Hz. It may also be used to reduce the effect of phase and magnitude errors introduced by the AC coupling circuitry below a few hertz. However, the use of DC coupling introduces serious problems where the source has a DC offset or is of such high impedance that bias currents cause significant offsets.
Chapter 3, TECHNICAL DESCRIPTION 3.2.04 AC Gain The signal channel contains a number of analog filters and amplifiers, the gain of which are defined by the “AC Gain” parameter, which is specified in terms of decibels (dB). For each value of AC Gain there is a corresponding value of the INPUT LIMIT parameter, which is the maximum instantaneous (peak) voltage or current that can be applied to the input without input overload, as shown in table 3-1 below.
Chapter 3, TECHNICAL DESCRIPTION This alias signal is indistinguishable from the output generated when a genuine signal at frequency falias is sampled. Hence if the frequency of the unwanted signal were such that the alias signal frequency produced from it was close to, or equal to, that of the wanted signal then it is clear that a spurious output would result. For example, if the sampling frequency were 160 kHz then half the sampling frequency would be 80 kHz.
Chapter 3, TECHNICAL DESCRIPTION There is one situation where this automatic correction might not be sufficient to give good performance. Consider the case where the signal being measured is at 73 kHz, which is 10 kHz away from half the sampling frequency. If there were also a strong interfering signal at 93 kHz (i.e. 166 kHz/2 + 10 kHz), then an alias of this would give rise to a spurious output.
Chapter 3, TECHNICAL DESCRIPTION 3.2.08 Internal Oscillator The model 7220, in common with many other lock-in amplifiers, incorporates an internal oscillator which may be used to drive an experiment. However, unlike most other instruments, the oscillator in the model 7220 is digitally synthesized with the result that the output frequency is extremely accurate and stable. The oscillator operates over the same frequency range as the lock-in amplifier, 1 mHz to 120 kHz.
Chapter 3, TECHNICAL DESCRIPTION and Y channel output filters. The outputs of these in turn drive two 16-bit digital to analog converters (DACs) which generate the instrument’s FAST X and FAST Y analog outputs. In addition, the signals are fed to further low-pass filters before subsequent processing by the instrument’s host microprocessor. 3.2.10 Output Processor Although shown on the block diagram as a separate entity, the output processor is in fact part of the instrument’s main microprocessor.
Chapter 3, TECHNICAL DESCRIPTION 3.3.02 Relative Accuracy The majority of lock-in amplifier measurements are concerned with the variation of the input signal with time, temperature, etc. or with the comparison of two different specimens. In these cases the absolute accuracy is of less importance than the accuracy with which readings can be transferred from range to range. A new feature of the model 7220 is the introduction of a separate control function (“AC Gain”) for the gain of the signal channel.
Chapter 3, TECHNICAL DESCRIPTION reserve is often expressed in decibels, for which DR( in dB) = 20 log(DR( as a ratio)) Applying this formula to the model 7220 we may put in the maximum value of INPUT LIMIT (3 V) and the smallest available value of FULL-SCALE SENSITIVITY (20 nV) to reach a value of about 1E8 or 160 dB for the maximum available dynamic reserve.
Chapter 3, TECHNICAL DESCRIPTION The circuits connected to the REF IN socket actually detect a positive-going crossing of the mean value of the applied reference voltage. Therefore when the reference input is not sinusoidal, its effective phase is the phase of a sinusoid with positive-going zero crossing at the same point in time, and accordingly the reference phase is defined with respect to this waveform.
Chapter 3, TECHNICAL DESCRIPTION The 6 dB/octave filters are not satisfactory for most purposes because they do not give good rejection of periodic components in the demodulator output, including the inevitable component at double the reference frequency. However, the 6 dB/octave filter finds use where the lock-in amplifier is incorporated in a feedback control loop, and in some situations where the form of the time-domain response is critical.
Chapter 3, TECHNICAL DESCRIPTION with respect to the reference input) and is computed by the output processor in the lock-in amplifier and made available as the “magnitude” output. The phase angle between Vs(t) and the X demodulation function is called the “signal phase”: this is equal to the angle of the complex quantity (Vx + jV y) (where j is the square root of -1) and is also computed by the processor by means of a fast arctan algorithm.
Chapter 3, TECHNICAL DESCRIPTION 3.10 Noise Measurements The noise measurement facility is available only in the baseband mode (i.e. at reference frequencies less than 60 kHz) and uses the output processor to perform a noise computation on the Y output where it is assumed that the waveform is Gaussian with zero mean.
Chapter 3, TECHNICAL DESCRIPTION 3.12.02 Auto-Sensitivity This function only operates when the reference frequency is above 1 Hz. A single Auto-Sensitivity operation consists of increasing the full-scale sensitivity range if the magnitude output is greater than 90 % of full-scale, or reducing the range if the magnitude output is less than 30 % of full-scale. After the Auto-Sensitivity function is called, Auto-Sensitivity operations continue to be made until the required criterion is met.
Chapter 3, TECHNICAL DESCRIPTION 3.12.05 Auto-Measure This function only operates when the reference frequency is greater than 1 Hz. It performs the following operations: The line filter is disabled, AC coupling is established, the voltage measurement mode is entered, with the single-ended A input mode, the FET input device is selected and the FLOAT mode is set.
Front and Rear Panels Chapter 4 4.1 Front Panel Figure 4-1, Model 7220 Front Panel Layout As shown in figure 4-1 there are four BNC connectors with associated LED indicators, two LCD display panels, an edge-indicating analog meter, eight double and three single keys mounted on the model 7220’s front panel. The following sections describe the function and location of these items. 4.1.
Chapter 4, FRONT AND REAR PANELS 4.1.03 REF IN Connector This is the input connector for a general purpose external reference signal. When external reference mode is selected the LED adjacent to the connector will be lit (see figure 4-3). Under unlock conditions the LED will flash. Figure 4-3, OSC OUT and REF IN Connectors 4.1.
Chapter 4, FRONT AND REAR PANELS precision. In these cases a significant number of keypresses are required to make adjustments. Adjustment of the latter type of control is made easier by the use of one or other of the two methods described below.
Chapter 4, FRONT AND REAR PANELS Figure 4-6, Active Cursor Operation The double keypress action can also be performed with one finger by firmly pressing the center of the up and down ADJUST key rocker which will deform to press both keys. The active cursor can be used to set any particular digit. For example, if you only want to adjust the reference phase in 1 degree steps leave the cursor over the first digit to the left of the decimal point of the reference phase value. 4.1.
Chapter 4, FRONT AND REAR PANELS Figure 4-8, Setup Menu Mode - Left and Right-hand LCD Displays In the setup menu mode, the left-hand SELECT keys adjacent to the left-hand display cycle through a series of twelve setup menus. In general each menu allows three controls to be adjusted, one via the right-hand side of the left-hand display and the other two via the right-hand display. The setup menu description is shown on the left-hand side of the left-hand display.
Chapter 4, FRONT AND REAR PANELS keypress” feature. To perform such a switch, simply press both sides of the SELECT keys simultaneously. This feature avoids the need to cycle through a number of outputs, thereby reducing the number of keypresses needed. The edge-indicating, analog panel meter is linked to the display on the left-hand side of the right-hand display, with full-scale corresponding to a digital reading of 100 %.
Chapter 4, FRONT AND REAR PANELS 4.2.03 RS232 Connector This 9-pin D type RS232 interface connector implements pins 1, 2, 3 and 7 (Earth Ground, Transmit Data, Receive Data, Logic Ground) of a standard DTE interface. To make a connection to a PC-compatible computer, it is normally sufficient to use a three-wire cable connecting Transmit Data to Receive Data, Receive Data to Transmit Data, and Logic Ground to Logic Ground.
Chapter 4, FRONT AND REAR PANELS 4.2.11 CH1, CH2 Connectors The signal at these connectors is an analog voltage corresponding to a selected output, such as X, Y, R, θ, etc., as specified in the Output Setup menu. The minimum time constant that can be used is 5 ms. The full-scale output voltage range is ±10.0 V although the outputs remain valid to ±12.0 V to provide some overload capability. 4.2.
Front Panel Operation Chapter 5 5.1 Introduction This chapter describes how to operate the model 7220 using the front panel controls, and describes its capabilities when used in this way. Chapter 6 provides similar information in the situation where the unit is operated remotely using one of the computer interfaces. Readers should refer to chapter 4 for a detailed description of the use of the SELECT and ADJUST keys, and the functions of the left and right-hand display panels.
Chapter 5, FRONT PANEL OPERATION the right-hand display. The setup menu description is shown on the left-hand side of the left-hand display. Figure 5-1 makes this clear. One further press of the MENU key causes the instrument to leave the setup menu mode and return to the main display mode. On leaving the setup menu mode the last menu displayed is held in memory and will be displayed on re-entry. The following sections describe each menu in sequence. 5.2.
Chapter 5, FRONT PANEL OPERATION the front panel BNC connector marked “B/I”, and uses a low-noise (LN) current to voltage converter. Input This control has four settings:- Flt/DC The shells of the “A” and “B/I” connectors are connected to chassis ground via a 1 kΩ resistor and the signal channel input is DC coupled. Flt/AC The shells of the “A” and “B/I” connectors are connected to chassis ground via a 1 kΩ resistor and the signal channel input is AC coupled.
Chapter 5, FRONT PANEL OPERATION 5.2.02 Reference Setup Menu Figure 5-3, Reference Setup Menu In this menu, shown in figure 5-3, there are three controls affecting the reference channel of the instrument. They are:- Ref Source This control allows selection of the source of reference signal used to drive the reference circuitry, and has three settings:- INTERNAL The lock-in amplifier’s reference is taken from the instrument’s internal oscillator.
Chapter 5, FRONT PANEL OPERATION ON When the Demodulator Monitor is switched ON and the instrument is operating in External Reference mode, the signal at the OSC OUT connector changes from that of the internal oscillator to an analog representation of the drive from the reference channel to the X output demodulator.
Chapter 5, FRONT PANEL OPERATION OFF Output expansion is turned off. X ONLY A ×10 output expansion is applied to the X output only. Y ONLY A ×10 output expansion is applied to the Y output only. X & Y A ×10 output expansion is applied to both the X and Y outputs. CH1 OUTPUTS CH2 This control, shown on the right-hand LCD, allows the two rear panel analog outputs CH1 and CH2 to be connected to the required instrument outputs.
Chapter 5, FRONT PANEL OPERATION MAG %fs CH1/2 Voltage +120 12.0 V +100 10.0 V 0 0.0 V -100 -10.0 V -120 -12.0 V PHASE1 When set to PHASE1 the corresponding rear panel CH1/CH2 connector will output a voltage related to the PHA deg front panel display as follows:PHA deg +180 +90 0 -90 -180 CH1/2 Voltage 9.0 V 4.5 V 0.0 V -4.5 V -9.
Chapter 5, FRONT PANEL OPERATION 5.2.04 Control Options Menu Figure 5-5, Control Options Setup Menu This menu, shown in figure 5-5, has three controls affecting the line frequency rejection filter, AC Gain control and output time constants, as follows:- Linefilt This control sets the mode of operation of the internal line frequency rejection filter. Early instruments have two possible settings for this control, ON or OFF.
Chapter 5, FRONT PANEL OPERATION MANUAL In this setting the AC Gain may be manually adjusted from the main display. AUTOMATIC In this setting the AC Gain value is automatically selected by the instrument, depending on the full-scale sensitivity. TCs This control affects the output time constants, and has two settings:- SYNC When set to SYNC, the actual time constant used is chosen to be some multiple of the reference frequency period.
Chapter 5, FRONT PANEL OPERATION 5.2.05 Miscellaneous Options Menu Figure 5-6, Miscellaneous Options Setup Menu This menu, shown in figure 5-6, has three controls affecting the auxiliary ADC trigger rate and the front panel display as follows:- Trigger This control selects the trigger which is used to initiate the conversion of voltages applied to the rear panel ADC1 and ADC2 connectors, as follows:- 200Hz In this setting, ADC conversions occur at a 200 Hz rate.
Chapter 5, FRONT PANEL OPERATION 5.2.06 RS232 Setup 1 Menu Figure 5-7, RS232 Setup 1 Menu This menu, shown in figure 5-7, has three controls affecting the RS232 computer interface, as follows:- BaudRate Thirteen values of Baud Rate are available in the range 75 to 19200 bits per second.
Chapter 5, FRONT PANEL OPERATION 5.2.07 RS232 Setup 2 Menu Figure 5-8, RS232 Setup 2 Menu This menu, shown in figure 5-8, has three controls affecting the RS232 computer interface, as follows:- Prompt This function can be switched ON or OFF:- ON The prompt character is sent out by the lock-in amplifier after each command response to indicate that the response is finished and the instrument is ready for a new command. It can also be used to signal overload conditions (see section 6.3.
Chapter 5, FRONT PANEL OPERATION MP command (report Magnitude and Phase). Hence it is necessary for the controlling program to be able to determine when all of the first value has been sent. The delimiter is a separator character sent between each response which may be used for this purpose. The control allows any ASCII character with decimal value between 32 and 125, or 13, to be used. 5.2.
Chapter 5, FRONT PANEL OPERATION 5.2.09 GPIB Setup 1 Menu Figure 5-10, GPIB Setup 1 Menu This menu, shown in figure 5-10, has two controls affecting the GPIB computer interface, as follows:- Address Each instrument used on the GPIB interface must have a unique address, in the range 0 to 31, and this control is used to set this address. The default setting is 12.
Chapter 5, FRONT PANEL OPERATION 5.2.10 GPIB Setup 2 Menu Figure 5-11, GPIB Setup 2 Menu This menu, shown in figure 5-11, has two controls affecting the GPIB computer interface, as follows:- SRQ Mask The instrument includes the ability to generate a Service Request on the GPIB interface, to signal to the controlling computer that urgent attention is required.
Chapter 5, FRONT PANEL OPERATION 5.2.11 Digital Outputs Setup Menu Figure 5-12, Digital Outputs Setup Menu This menu, shown in figure 5-12, is used to control the 8 TTL lines of the rear panel digital output port, used for controlling external equipment. Decimal The bit pattern appearing at the digital output port is set by an unsigned eight-bit binary number, the decimal equivalent of which can range from 0 to 255.
Chapter 5, FRONT PANEL OPERATION 5.2.12 Control Setup Menu Figure 5-13, Control Setup Menu The final setup menu, shown in figure 5-13, is used to set the instrument to a known state and to adjust the sampling rate of the main analog to digital converter. Default Setting Pressing a key adjacent to this label will set all of the instrument’s controls to a known state. This can be useful when performing the initial checks procedure, or after taking the instrument over from another user.
Chapter 5, FRONT PANEL OPERATION 5.3 Auto Functions Menu When in the Main Display mode, one press of the MENU key accesses the AUTO MENU showing four auto functions that are built into the instrument. The left-hand LCD changes to that shown in figure 5-14. Figure 5-14, Auto Functions Menu - Left-hand Display To activate one of the auto functions press one of the keys adjacent to it, as shown in figure 5-14.
Chapter 5, FRONT PANEL OPERATION are steady, implying the signal phase is stable, when the procedure is called. If a zero error is present on the outputs, such as may be caused by unwanted coupling between the reference and signal channel inputs, then the following procedure should be adopted:1) Remove the source of input signal, without disturbing any of the connections to the signal input which might be picking up interfering signals from the reference signal.
Chapter 5, FRONT PANEL OPERATION The line filter is disabled; AC coupling is established; the voltage measurement mode is entered, using the single-ended, A, input; the FET input devices are enabled; the FLOAT mode is set.
Chapter 5, FRONT PANEL OPERATION Figure 5-15, Sensitivity Control - Voltage Input Mode When set to current input mode, the instrument’s full-scale current sensitivity may be set to any value between 20 fA and 1 µA (wide bandwidth mode) or 20 fA and 10 nA (low-noise mode), in a 1-2-5 sequence.
Chapter 5, FRONT PANEL OPERATION AC Gain If the AC Gain control is set to Manual (using the Input Setup menu), then this control allows it to be adjusted from 0 dB to 90 dB in 10 dB steps, although not all settings are available at all full-scale sensitivity settings. Figure 5-18, AC Gain Control Time Constant The time constant of the output filters is set using this control.
Chapter 5, FRONT PANEL OPERATION Slope The roll-off of the output filters is set, using this control, to any value from 6 dB to 24 dB/octave, in 6 dB steps. Note this control does not affect the roll-off of outputs at the FAST X and FAST Y connectors which are fixed at 6 dB/octave. Figure 5-20, Output Filter Slope Control Oscillator Frequency The frequency of the instrument’s internal oscillator may be set, using this control, to any value between 1 mHz and 120 kHz with a 1 mHz resolution.
Chapter 5, FRONT PANEL OPERATION Oscillator Amplitude The amplitude of the instrument’s internal oscillator may be set, using this control, to any value between 1 mV and 5 V rms with a 1 mV resolution. Adjustment is faster if use is made of the Active Cursor control - see section 4.1.04. Figure 5-22, Internal Oscillator Amplitude Control DAC 1 This control sets the voltage appearing at the rear panel DAC1 output connector to any value between +10 V and -10 V with a resolution of 1 mV.
Chapter 5, FRONT PANEL OPERATION DAC 2 This control sets the voltage appearing at the rear panel DAC2 output connector to any value between +10 V and -10 V with a resolution of 1 mV. Adjustment is faster if use is made of the Active Cursor control - see section 4.1.04. Figure 5-24, DAC 2 Output Control Offset This control allows an output offset to be added to the X output, Y output, neither or both outputs.
Chapter 5, FRONT PANEL OPERATION Reference Phase This control allows the reference phase to be adjusted over the range -360° to + 360° in 10m° steps, although readers will appreciate that a setting of -180° is equivalent to +180°, and that ±360° is equivalent to 0°. The Auto-Phase function also affects the value displayed here. Figure 5-26, Reference Phase Control 5.
Chapter 5, FRONT PANEL OPERATION X %fs Figure 5-27, X Output as % Full-Scale Shows the X output as a percentage of the selected full-scale sensitivity setting. Hence if the sensitivity setting were 100 mV and a 50 mV signal were applied, with the instrument’s reference phase adjusted for maximum X output, the display would read 50.00 % Y %fs Figure 5-28, Y Output as % Full-Scale Shows the Y output as a percentage of the selected full-scale sensitivity setting.
Chapter 5, FRONT PANEL OPERATION MAG %fs Figure 5-29, Magnitude Output as % Full-Scale Shows the signal magnitude, where magnitude = √((X output)2 + Y output) 2), as a percentage of the selected full-scale sensitivity setting. Hence if the sensitivity setting were 100 mV and a 50 mV signal were applied, regardless of the setting of the instrument’s reference phase, the display would read 50.
Chapter 5, FRONT PANEL OPERATION Phase in Degrees Figure 5-31, Phase Output in Degrees Shows the relative phase, where phase = tan-1 (Y output/X output), in degrees. Reference Frequency Figure 5-32, Reference Frequency Display Shows the reference frequency at which the lock-in amplifier is operating. Note that the display shows values in kHz only when the frequency is greater than 3 kHz. At all other values the units are Hz.
Chapter 5, FRONT PANEL OPERATION X Volts (or Amps) Figure 5-33, X Output in Volts or Amps Shows the X output directly in terms of volts or amps (depending on whether voltage or current input mode is selected). Hence if the sensitivity setting were 100 mV and a 50 mV signal were applied, with the instrument’s reference phase adjusted for maximum X output, the display would read 50.
Chapter 5, FRONT PANEL OPERATION MAG Volts (or Amps) Figure 5-35, Magnitude Output in Volts or Amps Shows the signal magnitude, where magnitude = √((X output)2 + (Y output)2), directly in terms of volts or amps (depending on whether voltage or current input mode is selected). Hence if the sensitivity setting were 100 mV and a 50 mV signal were applied, regardless of the setting of the instrument’s reference phase, the display would read 50.
Chapter 5, FRONT PANEL OPERATION Ratio Figure 5-37, Ratio Output Shows the ratio, where ratio = (X output / ADC1 Input), usually used to compensate for source intensity fluctuations in optical experiments. Log Ratio Figure 5-38, Log Ratio Output Shows the logarithm to base 10 of the ratio, where ratio = (X output / ADC1 Input), usually used to compensate for source intensity fluctuations in optical experiments.
Chapter 5, FRONT PANEL OPERATION ADC1 Volts Figure 5-39, ADC 1 Input Shows the voltage applied to the rear panel ADC1 auxiliary input. ADC2 Volts Figure 5-40, ADC 2 Input Shows the voltage applied to the rear panel ADC2 auxiliary input.
Chapter 5, FRONT PANEL OPERATION X Offset Figure 5-41, X Output Offset Control This control allows the X output offset to be adjusted, using the lower ADJUST keys adjacent to the displayed value. Note that although this control adjusts the level of the offset, it is only applied if the OFFSET control on the left-hand LCD is set to X or BOTH.
Chapter 5, FRONT PANEL OPERATION warning message OFFSET! The instrument provides a quick way to switch between the following pairs of outputs, by simply pressing simultaneously both ends of the SELECT keys adjacent to their description:X %fs Y %fs Mag %fs Noise %fs ↔ ↔ ↔ ↔ X volts or amps Y volts or amps Mag volts or amps Noise volts/√Hz or amps/√Hz 5.
Chapter 5, FRONT PANEL OPERATION 5-36
Computer Operation Chapter 6 6.1 Introduction The model 7220 includes both RS232 and IEEE-488 (also known as GPIB for General Purpose Interface Bus) interface ports, designed to allow the lock-in amplifier to be completely controlled from a remote computer. All the instrument’s controls may be operated, and all the outputs read, via these interfaces. In addition, there are some functions, such as curve storage and oscillator frequency sweeps which may only be accessed remotely.
Chapter 6, COMPUTER OPERATION 6.2.04 Internal Oscillator Frequency Sweep Generator The instrument’s internal oscillator may be swept in frequency both linearly and logarithmically over a specified range. This facility allows the instrument to function as a simple swept-frequency oscillator, or, in conjunction with the curve storage capability, allows frequency response curves to be recorded. 6.3 RS232 and GPIB Operation 6.3.
Chapter 6, COMPUTER OPERATION The main advantages of the RS232 interface are: 1. It communicates via a serial port which is present as standard equipment on nearly all computers, using leads and connectors which are available from suppliers of computer accessories or can be constructed at minimal cost in the user’s workshop. 2. It requires no more software support than is normally supplied with the computer, for example Microsoft’s GWBASIC, QBASIC or Windows Terminal mode.
Chapter 6, COMPUTER OPERATION Where the RS232 parameters of the terminal or computer are capable of being set to any desired value, an arbitrary choice must be made. In the model 7220 the combination set at the factory is even parity check, 7 data bits, and one stop bit (fixed) because these are the MS-DOS default. 6.3.06 Auxiliary RS232 Interface The auxiliary RS232 interface allows up to sixteen model 7220s or a mixture of compatible instruments to be connected to one serial port on the computer.
Chapter 6, COMPUTER OPERATION Because of the parallel nature of the GPIB and its very effective use of the control lines including the implementation of a three-wire handshake (see below), comparatively high data rates are possible, up to a few hundred thousand bytes per second. In typical setups the data rate of the GPIB itself is not the factor that limits the rate of operation of the control program. 6.3.
Chapter 6, COMPUTER OPERATION parameter byte. The default (power-up) state of this bit is for it to be asserted. The program RSCOM2.BAS in section C.2 illustrates the use of the echo handshake. 6.3.09 Terminators In order for communications to be successfully established between the lock-in amplifier and the computer, it is essential that each transmission, i.e.
Chapter 6, COMPUTER OPERATION sent without the optional parameters, the response consists of a transmission of the present values of the parameter(s). Any response transmission consists of one or more numbers followed by a response terminator. Where the response consists of two or more numbers in succession, they are separated by a delimiter (section 6.3.11). Some commands have an optional floating point mode which is invoked by appending a .
Chapter 6, COMPUTER OPERATION respectively. In GPIB communications, the use of these bits can lead to a useful simplification of the control program by providing a response subroutine which is the same for all commands, whether or not they send a response over the bus. The principle is that after any command is sent, serial poll operations are repeatedly executed.
Chapter 6, COMPUTER OPERATION 6.3.14 Service Requests The interface defined by the IEEE-488 standard includes a line (pin 10 on the connector) called the SRQ (service request) line which is used by the instrument to signal to the controller that urgent attention is required. At the same time that the instrument asserts the SRQ line, it also asserts bit 6 in the status byte.
Chapter 6, COMPUTER OPERATION 6.4 Command Descriptions This section lists the commands in logical groups, so that, for example, all commands associated with setting controls affecting the signal channel are shown together. Appendix E gives the same list of commands but in alphabetical order. 6.4.
Chapter 6, COMPUTER OPERATION CP [n] Input coupling control The value of n sets the input coupling mode according to the following table: n 0 1 SEN[.
Chapter 6, COMPUTER OPERATION ASM Perform an Auto-Measure operation The instrument adjusts its full-scale sensitivity so that the magnitude output lies between 30 % and 90 % of full-scale, and then performs an auto-phase operation to maximize the X output and minimize the Y output. ACGAIN [n] AC Gain control Sets the gain of the signal channel amplifier. Values of n from 0 to 9 can be entered, corresponding to the range 0 dB to 90 dB in 10 dB steps.
Chapter 6, COMPUTER OPERATION n 0 1 Notch filter mode 60 Hz (and/or 120 Hz) 50 Hz (and/or 100 Hz) Units made prior to June 1996 generate an Invalid Command (bit 1 of the serial poll status byte is asserted) to the LINE50 command. SAMPLE [n] Main analog to digital converter sample rate control The sampling rate of the main analog to digital converter, which is nominally 166 kHz, may be adjusted from this value to avoid problems caused by the aliasing of interfering signals into the output passband.
Chapter 6, COMPUTER OPERATION FRQ[.] Reference frequency meter If the lock-in amplifier is in the EXT or EXT LOGIC reference source modes, the FRQ command causes the lock-in amplifier to respond with 0 if the reference channel is unlocked, or with the reference input frequency if it is locked. If the lock-in amplifier is in the INT reference source mode, it responds with the frequency of the internal oscillator. In fixed point mode the frequency is in mHz. In floating point mode the frequency is in Hz.
Chapter 6, COMPUTER OPERATION 17 18 19 20 21 22 23 24 25 10 s 20 s 50 s 100 s 200 s 500 s 1 ks 2 ks 5 ks The TC. command is only used for reading the time constant, and reports the current setting in seconds. Hence if a TC 11 command were sent, TC would report 11 and TC. would report 1.0E-01, i.e. 0.1 s or 100 ms.
Chapter 6, COMPUTER OPERATION AXO Auto-Offset The X and Y output offsets are turned on and set to levels giving zero X and Y outputs. Any changes in the input signal then appear as changes about zero in the outputs. EX [n] Output expansion control Expands X and/or Y outputs by a factor of 10. Changes meter, CH1 and CH2 outputs full-scale to ±10 % if X or Y selected.
Chapter 6, COMPUTER OPERATION XY[.] X, Y outputs Equivalent to the compound command X[.];Y[.] MAG[.] Magnitude In fixed point mode causes the lock-in amplifier to respond with the magnitude value in the range 0 to 30000, full-scale being 10000. In floating point mode causes the lock-in amplifier to respond with the magnitude value in the range +3.000E0 to +0.001E-9 volts or +3.000E-6 to +0.001E-15 amps. PHA[.
Chapter 6, COMPUTER OPERATION ENBW[.] Equivalent noise bandwidth In fixed point mode, reports the equivalent noise bandwidth of the output low-pass filters at the current time constant setting in microhertz. In floating point mode, reports the equivalent noise bandwidth of the output low-pass filters at the current time constant setting in hertz. This command is not available when the reference frequency exceeds 60 kHz. NN[.
Chapter 6, COMPUTER OPERATION Caution: Check that the computer program does not automatically add a carriage return or carriage return-line feed terminator to the * command, since these characters will slow down communications. 6.4.06 Internal Oscillator OA[.] [n] Oscillator amplitude control In fixed point mode n sets the oscillator amplitude in mV. The range of n is 0 to 5000 representing 0 to 5 V rms. In floating point mode n sets the amplitude in volts OF[.
Chapter 6, COMPUTER OPERATION FSTEP[.] [n1 n2] Oscillator frequency sweep step size and type The frequency may be swept either linearly or logarithmically, as specified by parameter n2. The step size is specified by parameter n1. Log sweep n2 = 0 In fixed point mode, n1 is the step size in thousandths of a percent. In floating point mode n1 is in percent. The range of n1 is 0 to 100.00 % Linear sweep n2 = 1 In fixed point mode, n1 is the step size in millihertz. In floating point mode n1 is in hertz.
Chapter 6, COMPUTER OPERATION BYTE [n] Digital output port control The value of n, in the range 0 to 255, determines the bits to be output on the rear panel digital output port. When n = 0, all outputs are low, and when n = 255, all are high. 6.4.08 Auxiliary Inputs ADC[.] n Read auxiliary analog to digital inputs Reads the voltage appearing at the rear panel ADC1 (n = 1) and ADC2 (n = 2) inputs.
Chapter 6, COMPUTER OPERATION TADC parameter 0 1 2 3 4 5 6 7 8 9 Effect on CBD parameter none none automatically set to 32 automatically set to 96 automatically set to 32 automatically set to 96 automatically set to 32 automatically set to 96 automatically set to 32 automatically set to 96 The maximum sampling rate depends on the number of ADC inputs used and whether the sampling is timed or simply runs as fast as possible.
Chapter 6, COMPUTER OPERATION 6.4.09 Output Data Curve Buffer CBD [n] Curve buffer define Defines which data outputs are stored in the curve buffer when subsequent TD (take data) or TDC (take data continuously) commands are issued. Up to 16 curves, or outputs, may be acquired, as specified by the CBD parameter. The CBD is an integer between 0 and 65,535, being the decimal equivalent of a 16-bit binary word. When a given bit in the word is asserted, the corresponding output is selected for storage.
Chapter 6, COMPUTER OPERATION and HC commands. It also interacts with the LEN command and affects the values reported by the M command. LEN [n] Curve length control The value of n sets the curve buffer length in effect for data acquisition. The maximum allowed value depends on the number of curves requested using the CBD command, and a parameter error results if the value given is too large.
Chapter 6, COMPUTER OPERATION HC Halt curve acquisition Halts curve acquisition in progress. It is effective during both single (data acquisition initiated by TD command) and continuous (data acquisition initiated by TDC command) curve acquisitions. The curve may be restarted by means of the TD or TDC command, as appropriate. M Curve acquisition status monitor Causes the lock-in amplifier to respond with four values that provide information concerning data acquisition, as follows.
Chapter 6, COMPUTER OPERATION The computer program’s subroutine which reads the responses to the DC command needs to run a FOR...NEXT loop of length equal to the value set by the LEN (curve length) command. Note that when using this command with the GPIB interface the serial poll must be used. After sending the DC command, perform repeated serial polls until bit 7 is set, indicating that the instrument has an output waiting to be read.
Chapter 6, COMPUTER OPERATION etc, where and are the delimiter and terminator characters respectively. The computer program’s subroutine which reads the responses to the DCT command needs to run a FOR...
Chapter 6, COMPUTER OPERATION bit number 0 1 2 3 4 bit negated data + parity = 8 bits no parity bit even parity echo disabled prompt disabled bit asserted data + parity = 9 bits 1 parity bit odd parity echo enabled prompt enabled GP [n1 [n2]] Set/read GPIB parameters n1 sets the GPIB address in the range 0 to 31 n2 sets the GPIB terminator and the test echo function according to the following table: n 0 1 2 3 4 5 Terminator [CR], test echo disabled [CR], test echo enabled [CR,LF], test echo disabled [CR
Chapter 6, COMPUTER OPERATION byte is accessed by performing a serial poll. Bit 0 Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 N Command complete Invalid command Command parameter error Reference unlock Overload New ADC values available after external trigger Asserted SRQ Data available Report overload byte Causes the lock-in amplifier to respond with the overload byte.
Chapter 6, COMPUTER OPERATION 6.4.12 Front Panel LTS [n] Lights on/off control The value of n controls the front panel LEDs and LCD backlights according to the following table: n 0 1 Selection All lights off Normal operation 6.4.13 Default Setting ADF Default Setting command This command will automatically set all the instrument controls and displays to the factory set default values.
Chapter 6, COMPUTER OPERATION The controlling program would send a new output command each time a new reading were required. Note that a good “rule of thumb” is to wait for a period of five timeconstants after the input signal has changed before recording a new value. Hence in a scanning type experiment, the program should issue the commands to whatever equipment causes the input signal to the lock-in amplifier to change, wait for five time-constants, and then record the required output. 6.5.
Chapter 6, COMPUTER OPERATION 6.5.04 X and Y Output Curve Storage Measurement In this example, the lock-in amplifier is measuring a current input signal applied to the B input connector and the measured X output and Y output are recorded for 10 seconds at a 100 Hz sampling rate. The acquired curves as read back to the computer are required in floating point mode.
Chapter 6, COMPUTER OPERATION NC LEN 20000 TADC 6 Clear and reset curve buffer 500 ms recording time at 40 kHz = 20,000 points Set ADC1 sampling to burst mode, fixed rate (≈ 40 kHz), external trigger, and arm trigger As soon as a trigger occurs, the acquisition starts. Once it completes the acquired data may be transferred to the computer using:DC. 5 Transfers ADC1 values in floating point mode The input routine of the program must be prepared to read and store 20,000 responses to this command. 6.5.
Chapter 6, COMPUTER OPERATION NC CBD 49180 LEN 100 STR 100 Clear and reset curve buffer Stores Magnitude, Phase, Sensitivity and Frequency (i.e. bits 2, 3, 4, 14 and 15) Number of points = 100 Store a point every 100 ms - must match SRATE parameter The data may now be acquired by issuing the compound command: TD; SWEEP 1 Starts sweep and curve acquisition Note that the order of these two commands is important.
Specifications Appendix A Measurement Modes X Y R θ In-phase Quadrature Magnitude Phase Angle Noise The unit can simultaneously present any two of these as outputs Harmonic 2F or 3F Noise Measures noise in a given bandwidth centered on frequency F Displays Two LED backlit, two-line, 16-character alphanumeric dot-matrix LCDs giving digital indication of current instrument set-up and output readings. Edge indicating analog panel meter. Menu system with dynamic key function allocation.
Appendix A, SPECIFICATIONS Current Input Mode Full-scale Sensitivity Low Noise Wide Bandwidth Dynamic Reserve Frequency Response Low Noise Wide Bandwidth Impedance Low Noise Wide Bandwidth Noise Low Noise Wide Bandwidth Gain Accuracy (midband) Low Noise Wide Bandwidth Line Filter Grounding Low Noise or Wide Bandwidth 20 fA to 10 nA in a 1-2-5 sequence 20 fA to 1 µA in a 1-2-5 sequence > 100 dB (with no signal filters) -3 dB at 500 Hz -3 dB at 50 kHz < 2.
Appendix A, SPECIFICATIONS Orthogonality Drift Acquisition Time Internal Reference External Reference 90º ±0.0001º < 0.01º/ºC below 10 kHz < 0.1º/ºC above 10 kHz instantaneous acquisition 2 cycles + 50 ms Reference Frequency Meter Accuracy 120 kHz > F > 40 kHz ±4 Hz 40 kHz > F > 400 Hz ±0.8 Hz at F = 40 kHz improving to ±0.008 Hz at F = 400 Hz 400 Hz > F > 1 mHz ±0.040 Hz at F = 400 Hz improving to better than ±0.
Appendix A, SPECIFICATIONS Amplitude Range Setting Resolution 1 mV to 500 mV 501 mV to 2 V 2.001 V to 5 V Accuracy 0.001 Hz to 60 kHz 60 kHz to 120 kHz Stability Output Impedance 1 mV to 5 V 1 mV 4 mV 10 mV ±0.3 % ±0.5 % 50 ppm/ºC 50 Ω Auxiliary Inputs ADC 1 and 2 Maximum Input Resolution Accuracy Input Impedance Sample Rate ADC 1 only ADC 1 and 2 Trigger Mode Trigger input ±10 V 1 mV ±0.
Appendix A, SPECIFICATIONS Signal Monitor Amplitude Impedance Aux D/A Output 1, 2 Maximum Output Resolution Accuracy ±0.
Appendix A, SPECIFICATIONS General Dimensions Width Depth Height With feet Without feet Weight A-6 432 mm (17 ") 415 mm (16.4 ") 74 mm (2.9 ") 60 mm (2.4 ") 7.4 kg (16.
Pinouts Appendix B B.1 RS232 Connector Pinout 5 4 9 3 8 2 7 1 6 Figure B-1, RS232 and AUX RS232 Connector (Female) PIN FUNCTION COMMENT 2 3 5 7 RXD TXD GND RTS Data In Data Out Signal Ground (Always at +12 V) All other pins are not connected. B.2 Preamplifier Power Connector Pinout Figure B-2, Preamplifier Power Connector PIN FUNCTION 1 2 3 -15 V GROUND +15 V All other pins are unused. Shell is shield ground.
Appendix B, PINOUTS B.3 Digital Output Port Connector Figure B-3, Digital Output Port Connector 8-bit TTL-compatible output set from the front panel or via the computer interfaces; each line can drive 3 LSTTL loads. This connector mates with a 20-pin IDC Header Plug. The pinout is as follows.
Demonstration Programs Appendix C C.1 Simple Terminal Emulator This is a short terminal emulator with minimal facilities, which will run on a PC-compatible computer in a Microsoft GW-BASIC or QuickBASIC environment, or can be compiled with a suitable compiler. 10 'MINITERM 9-Feb-96 20 CLS: PRINT "Lockin RS232 parameters must be set to 9600 baud, 7 data bits, 1 stop bit and even parity" 30 PRINT "Hit key to exit" 40 OPEN "COM1:9600,E,7,1,CS,DS" FOR RANDOM AS #1 50 '..............................
Appendix C, DEMONSTRATION PROGRAMS 100 B$=B$+CR$ 110 GOSUB 180 120 GOSUB 310: PRINT Z$; 130 IF A$="?" THEN GOSUB 410: GOSUB 470 140 150 WEND 160 ' 170 ' 180 '...output the string B$.............. 190 ON ERROR GOTO 510 200 IF LOC(1)>0 THEN A$=INPUT$(LOC(1),#1) 210 ON ERROR GOTO 0 220 FOR J1%=1 TO LEN(B$) 230 C$=MID$(B$,J1%,1): PRINT#1,C$; 240 WHILE LOC(1)=0: WEND 250 A$=INPUT$(1,#1) 260 IF A$<>C$ THEN PRINT”handshake error” 270 NEXT J1% 280 RETURN 290 ' 300 ' 310 '....read response..................
Appendix C, DEMONSTRATION PROGRAMS C.3 GPIB User Interface Program GPCOM.BAS is a user interface program which illustrates the principles of the use of the serial poll status byte to coordinate the command and data transfer. The program runs under Microsoft GW-BASIC or QuickBASIC on a PC-compatible computer fitted with a National Instruments IEEE-488 interface card and the GPIB.COM software installed in the CONFIG.SYS file. The program BIB.
Appendix C, DEMONSTRATION PROGRAMS 290 V%=11: CALL IBTMO(DEV%,V%) 300 '....set status print flag........................ 310 INPUT "Display status byte y/n "; R$ 320 IF R$="Y" OR R$="y" THEN DS% = 1 ELSE DS% = 0 330 '....main loop....................................
Cable Diagrams Appendix D D.1 RS232 Cable Diagrams Users who choose to use the RS232 interface to connect the model 7220 lock-in amplifier to a standard serial port on a computer, will need to use one of two types of cable. The only difference between them is the number of pins used on the connector which goes to the computer. One has 9 pins and the other 25; both are null modem (also called modem eliminator) cables in that some of the pins are cross-connected.
Appendix D, CABLE DIAGRAMS Figure D-2, Interconnecting RS232 Cable Wiring Diagram D-2
Alphabetical Listing of Commands Appendix E ACGAIN [n] AC Gain control Sets the gain of the signal channel amplifier. Values of n from 0 to 9 can be entered corresponding to the range 0 dB to 90 dB in 10 dB steps. ADC[.] n Read auxiliary analog to digital inputs Reads the voltage appearing at the rear panel ADC1 (n = 1) and ADC2 (n = 2) inputs. In fixed point mode the response is an integer in the range -12000 to +12000, corresponding to voltages from -12.000 V to +12.000 V.
Appendix E, ALPHABETICAL LISTING OF COMMANDS AXO Auto-Offset The X and Y output offsets are turned on and set to levels giving zero X and Y outputs. Any changes in the input signal then appear as changes about zero in the outputs. BURSTRATE [n] Sets the burst mode sampling rate for ADC1 and ADC2 n sets the sample rate for the Variable Rate burst modes according to the following equations: When storing only to ADC1: (i.e.
Appendix E, ALPHABETICAL LISTING OF COMMANDS Bit 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Decimal value 1 2 4 8 16 32 64 128 256 512 1024 2048 4096 8192 16384 32768 Output and range X Output (±10000 FS) Y Output (±10000 FS) Magnitude Output (±10000 FS) Phase (±18000 = ±180°) Sensitivity setting (4 to 27) + IMODE (0, 1, 2 = 0, 32, 64) ADC1 (±10000 = ±10.0 V) ADC2 (±10000 = 10.0 V) Unassigned DAC1 (±10000 = 10.0 V) DAC2 (±10000 = 10.
Appendix E, ALPHABETICAL LISTING OF COMMANDS CH n1 [n2] Analog output control Defines what outputs appear on the rear panel CH1 and CH2 connectors according to the following table: n2 0 1 2 3 4 5 6 Signal X %FS Y %FS Magnitude %FS Phase 1: +9 V = +180°, -9 V = -180° Phase 2: +9 V = 360°, -9 V = 0° Noise %FS Ratio: (1000 × X)/ADC 1 n1 is compulsory and is either 1 for CH1 or 2 for CH2 CP [n] Input coupling control The value of n sets the input coupling mode according to the following table: n 0 1 Coup
Appendix E, ALPHABETICAL LISTING OF COMMANDS The computer program’s subroutine which reads the responses to the DC command needs to run a FOR...NEXT loop of length equal to the value set by the LEN (curve length) command. Note that when using this command with the GPIB interface the serial poll must be used. After sending the DC command, perform repeated serial polls until bit 7 is set, indicating that the instrument has an output waiting to be read.
Appendix E, ALPHABETICAL LISTING OF COMMANDS etc, where and are the delimiter and terminator characters respectively. The computer program’s subroutine which reads the responses to the DCT command needs to run a FOR...
Appendix E, ALPHABETICAL LISTING OF COMMANDS EX [n] Output expansion control Expands X and/or Y outputs by a factor of 10. Changes meter, CH1 and CH2 outputs full-scale to ±10 % if X or Y selected.
Appendix E, ALPHABETICAL LISTING OF COMMANDS FSTART[.] [n] Oscillator frequency sweep start frequency Sets the start frequency for a subsequent sweep of the internal oscillator frequency. In fixed point mode, n is in millihertz. In floating point mode n is in hertz. FSTEP[.] [n1 n2] Oscillator frequency sweep step size and type The frequency may be swept either linearly or logarithmically, as specified by parameter n2. The step size is specified by parameter n1.
Appendix E, ALPHABETICAL LISTING OF COMMANDS HC Halt curve acquisition Halts curve acquisition in progress. It is effective during both single (data acquisition initiated by TD command) and continuous (data acquisition initiated by TDC command) curve acquisitions. The curve may be restarted by means of the TD or TDC command, as appropriate. ID Identification Causes the lock-in amplifier to respond with the number 7220.
Appendix E, ALPHABETICAL LISTING OF COMMANDS LF [n] Signal channel line frequency rejection filter control In instruments manufactured prior to June 1996, the value of n sets the mode of the line frequency notch filter according to the following table: n 0 1 Selection Off On (i.e.
Appendix E, ALPHABETICAL LISTING OF COMMANDS LTS [n] Lights on/off control The value of n controls the front panel LEDs and LCD backlights according to the following table: n 0 1 M Selection All lights off Normal operation Curve acquisition status monitor Causes the lock-in amplifier to respond with four values that provide information concerning data acquisition, as follows.
Appendix E, ALPHABETICAL LISTING OF COMMANDS \N n Address command When the 7220 is daisy-chained with other compatible instruments this command will change which instrument is addressed. All daisy-chained instruments receive commands but only the currently addressed instrument will implement or respond to the commands. The exception is the \N n command. If n matches the address set from the front panel the instrument will switch into addressed mode.
Appendix E, ALPHABETICAL LISTING OF COMMANDS OA[.] [n] Oscillator amplitude control In fixed point mode n sets the oscillator amplitude in mV. The range of n is 0 to 5000 representing 0 to 5 V rms. In floating point mode n sets the amplitude in volts. OF[.] [n] Oscillator frequency control In fixed point mode n sets the oscillator frequency in mHz. The range of n is 0 to 120,000,000 representing 0 to 120 kHz. In floating point mode n sets the oscillator frequency in Hz. The range of n is 0 to 1.2E5.
Appendix E, ALPHABETICAL LISTING OF COMMANDS RS [n1 [n 2]] Set/read RS232 interface parameters The values of n1 set the baud rate of the RS232 interface according to the following table: n1 0 1 2 3 4 5 6 7 8 9 10 11 12 Baud rate (bits per second) 75 110 134.5 150 300 600 1200 1800 2000 2400 4800 9600 19200 The lowest five bits in n2 control the other RS232 parameters according to the following table: bit number 0 1 2 3 4 RT[.
Appendix E, ALPHABETICAL LISTING OF COMMANDS SEN[.
Appendix E, ALPHABETICAL LISTING OF COMMANDS ST Report status byte Causes the lock-in amplifier to respond with the status byte. Note: this command is not normally used in GPIB communications, where the status byte is accessed by performing a serial poll.
Appendix E, ALPHABETICAL LISTING OF COMMANDS return or carriage return-line feed terminator to the * command, since these characters will slow down communications. STR [n] Storage interval control Sets the time interval between successive points being acquired under the TD or TDC commands. n specifies the time interval in ms with a resolution of 5 ms, input values being rounded up to a multiple of 5. The longest interval that can be specified is 1000000 s corresponding to about one point in 12 days.
Appendix E, ALPHABETICAL LISTING OF COMMANDS SYNCOSC [n] Synchronous oscillator (demodulator monitor) control This control operates only in external reference mode. The parameter n has the following significance: n 0 1 Effect Synchronous Oscillator (Demodulator Monitor) disabled Synchronous Oscillator (Demodulator Monitor) enabled When enabled and in external reference mode, the instrument’s OSC OUT connector functions as a demodulator monitor of the X channel demodulation function.
Appendix E, ALPHABETICAL LISTING OF COMMANDS The maximum sampling rate depends on the number of ADC inputs used and whether the sampling is timed or simply runs as fast as possible. In the modes above described as Fixed Rate, sampling runs at the maximum possible rate, nominally 20 kHz when sampling both ADC1 and ADC2 or 40 kHz when sampling ADC1 only. In the Variable Rate modes, the sampling speed is set by the BURSTRATE command. TC [n] TC.
Appendix E, ALPHABETICAL LISTING OF COMMANDS TDC Take data continuously Initiates data acquisition. Acquisition starts at the current position in the curve buffer and continues at the rate set by the STR command until halted by an HC command. The buffer is circular in the sense that when it has been filled, current data overwrites earlier points. VER Report firmware version Causes the lock-in amplifier to respond with the firmware version number.
Appendix E, ALPHABETICAL LISTING OF COMMANDS YOF [n1 [n2]] Y output offset control The value of n1 sets the status of the Y offset facility according to the following table: n1 0 1 Selection Disables offset facility Enables offset facility The range of n2 is ±30000 corresponding to ±300 % full-scale.
Appendix E, ALPHABETICAL LISTING OF COMMANDS E-22
Default Settings Appendix F Default Setting Function The default setting function sets the model 7220’s controls and output displays as follows:Left-hand LCD Displays the AC Gain control on the upper line and the full-scale sensitivity control on the lower line. Right-hand LCD Displays the magnitude as a percentage of full-scale output on the left-hand side and the phase angle in degrees output on the right-hand side.
Appendix F, DEFAULT SETTINGS F-2 Line frequency rejection filter AC Gain control Time constant mode ADC trigger rate Front panel lights Display contrast Off Manual Sync 200 Hz On 0 RS232 interface settings Baud rate Data bits Stop bits Parity Prompt character Character echo Delimiter Address 9600 7 1 Even On On , (044) 1 GPIB interface settings Address Terminator SRQ mask byte Test Echo 12 CR,LF 0 Disabled Digital outputs Sample rate adjustment 0 (i.e.
Index Index * command 6-18, E-16 \N n command 6-28, E-12 2F reference mode 3-7, 5-4 8-bit programmable output port 3-8 90° Key 4-5 Absolute accuracy 3-8 AC Gain and full-scale sensitivity 3-9 and input overload 3-4 control 5-8, 5-22 description of 3-4 AC GAIN [n] command 6-12, E-1 AC/DC input coupling 3-2 Accuracy 3-8 Active cursor 4-3 ADC sample rate control 5-17 sampling frequency 3-4 trigger control 5-10 ADC[.
INDEX Computer control, sample programs 6-30 Contrast control 5-10 Control options menu 5-8 Control setup menu 5-17 CP [n] command 6-11, E-4 Current input mode 3-2, 5-2 Current to voltage converter 5-2 Current/voltage input mode selection 3-3 Curve storage 6-1 DAC[.] n1 [n2] command 6-20, E-4 DAC1 auxiliary output 3-8 connector 4-8 control 5-24 DAC2 auxiliary output 3-8 connector 4-8 control 5-25 DC[.
INDEX Input connector ground/float 5-3 connector selection 5-2 connector shell ground/float 3-2 coupling 3-2, 5-3 device selection 3-2, 5-3 float/ground control 3-2 impedance 3-2, 5-3 mode 5-2 mode selection 3-3 overload 3-4 overload indicators 4-1 selection 3-2 setup menu 5-2 Inspection 2-1 Installation 2-1 Internal oscillator 3-7 Internal oscillator frequency sweep 3-8 Internal reference 5-4 Internal reference mode 3-6 Key specifications 1-3 LCD contrast control 5-10 LED indicators 4-1 Left-hand LCD displ
INDEX Ratio display 5-32 RATIO output 5-7 Rear panel layout 4-6 REF IN connector 3-10, 4-2, 5-4 REF MON connector 4-7 REF TTL connector 4-7, 5-4 Reference channel DSP 3-6 Reference frequency changes 3-10 Reference frequency display 5-29 Reference harmonic 5-4 Reference harmonic number 3-7 Reference mode external 3-6 internal 3-6 Reference phase 3-10 Reference phase control 5-26 Reference setup menu 5-4 Reference source control 5-4 Reference unlock indicator 4-2 REFP[.
INDEX Update program 3-8 Vector magnitude 3-8 Ventilation 2-1 VER command 6-29, E-20 VMODE [n] command 6-10, E-20 Voltage input mode 3-2 What is a lock-in amplifier? 1-2 X % output 5-6 X %fs display 5-27 X & Y demodulation functions 3-7 X in volts/amps display 5-30 X output offset level control 5-34 X[.] command 6-16, E-20 XOF [n1 [n2]] command 6-15, E-20 XY[.] command 6-17, E-20 Y % output 5-6 Y %fs display 5-27 Y in volts/amps display 5-30 Y output offset level control 5-34 Y[.
INDEX Index-6
WARRANTY EG&G Instruments Corporation warrants each instrument of its own manufacture to be free of defects in material and workmanship for a period of ONE year from the date of delivery to the original purchaser.
