Instruction manual

37
Subject to change without notice
Signals captured and stored in storage mode can be called
via the built-in RS232 interface for documentation purposes.
For further information please note section “RS232 Interface
- Remote Control”.
Signal recording modes
Signals can be recorded and displayed in six different modes:
REFRESH mode (RFR LED lit, readout indicates RFR)
ENVELOPE mode (ENV LED lit, readout indicates ENV)
AVERAGE mode (AVM LED lit, readout indicates AVM)
SINGLE mode (SGL LED lit, readout indicates SGL)
ROLL mode (ROL LED lit, readout indicates ROL)
XY mode (RFR LED lit, readout indicates XY)
Except ROLL and XY mode, a signal recording in all other
modes requires a trigger signal.
In REFRESH, ENVELOPE and AVERAGE modes the ins-
truments behaves like an analog oscilloscope. The trigger
circuit starts a recording, overwriting the previous recording
from the left to the right side of the screen. After the recording
has been finished, the next trigger event starts the same
procedure. This can also be caused in automatic trigger mode
without an applied signal by the automatic circuitry. Then only
the trace (Y-POS. setting) is recorded.
In contrast to automatic trigger mode, in normal trigger mode
the automatic system is switched off and consequently only
a trigger signal can start a recording. Unlike analog mode
where the screen is dark until a trigger signal starts the time
base, in store mode the last recorded signal remains visible
as long as no new recording is triggered by an input signal.
AVERAGE and ENVELOPE are REFRESH sub-modes and
described in section “Controls and Readout” under item
STOR. MODE (9).
SINGLE mode (SGL) enables the capture of one-time events,
started by a suitable trigger signal. It is recommended, to
select input ground (GD) condition and set the trace on a
graticule line which is then used as the 0 Volt position
(reference) line. After SINGLE (SGL) mode has been selected,
the trigger point symbol should be set above or below the 0
Volt position line, according to the expected voltage of the
event to be captured. Whether the slope selection is set for
a rising or falling slope depends on the measurement task.
After this procedure AC or DC input coupling must be selected
and the signal capture started after pressing the RESET
pushbutton.
The following example is for additional explanation.
If a voltage drop on a +5 Volt line is expected, the 0 Volt line
may be set to the horizontal center graticule line by using the
Y-POS. control. Then the trigger point symbol may be set 2
division above the 0 Volt position using the LEVEL control.
If the deflection coefficient is set to 200mV and DC input
coupling selected, using a x10 probe, the trigger point is at
+4 Volts. A voltage drop from + 5 Volts below +4 Volts then
is used for triggering, if negative slope setting is selected. If
the slope selection is positive the end of the event (voltage
drop ended) is used for triggering.
For explanations regarding ROLL mode, please note this item
(9) in section “Readout and Controls”.
Vertical resolution
The dot density in each operation mode is 8 bits = 2
8
= 256
dots displayed over a height of roughly 10 divisions. The
instrument is adjusted for 25 dots per division. This eases
processing and cursor measurement.
Insignificant differences between the (analog) screen display
and the (digital) data are unavoidable.
This concerns signal height as well as the position. The trace
position is defined in respect to the following horizontal
graticule lines:
Center line = 10000000 (binary) = 80 (hex) = 128 (dec).
Top line = 11100100 (binary) = E4 (hex) = 228 (dec).
Bottom line= 00011100 (binary) = 1C (hex) = 28 (dec).
In contrast to analog mode with its theoretically unlimited
resolution, the vertical resolution has 25 possible trace
positions per division.
If the signal is superimposed by noise or a critical Y-POS.
setting is used, the least significant bit (LSB) may change
continuously. This additionally reduces the vertical resolution
in storage mode, but is unavoidable. In contrast to the
expensive flash A/D converters used in this instrument, other
converters such as CCD cause more noise.
Horizontal resolution
The maximum number of signals to be displayed simulta-
neously is four. Each signal consists of 2048 (2
11
) byte
(samples). Referred to the horizontal raster, the resolution is
200 samples per division.
Only pure digital oscilloscopes with VGA monitor type CRTs
offer only 50 samples per division. If LCD displays are used
the current resolution is 25 samples per division. For a given
time base setting this B&K instrument samples at a 4
(compared to VGA) or 8 ( referred to LCD) times higher
sampling rate. The higher number of samples/div results in a
shorter sampling interval. For the following example it must
be kept in mind, that the time base setting is related to the
signal period duration and consequently should enable the
display of one complete signal period. If e.g. a 50Hz signal
has to be displayed the time base should be set to 2ms/div.
The maximum signal frequency of a superimposed sine wave
signal, which must be sampled with at least 10 samples per
period, depends on the horizontal resolution:
samples/div sampling interval sampling rate max frequency
200 2ms : 200 = 10µs 100kS/s 10kHz
50 2ms : 50 = 40µs 25kS/s 2.5kHz
25 2ms : 25 = 80µs 2.5kS/s 1.25kHz
During X magnifier mode the record length is still 2048 byte.
The CRT displays a tenth of the recording (20 byte/division)
plus 180 byte/div calculated in linear interpolation operation
by a RISC processor. The smallest available time coefficient
then is 10ns/div instead of 100ns/div without the X magnifier.
Maximum signal frequency in storage mode
The highest recordable signal frequency cannot be exactly
defined, since it depends to a large extent on the waveform.
With the start of each recording, the signal voltage at the
input(s) of the analog-to-digital converters is briefly measured
(sampled), converted to an 8-bit value and written to an ad-
dress in RAM. The next sampled value is converted in the
same way, but stored at the next RAM address.
The maximum sampling rate is 200MS/s. This yields a sam-
pling interval of 5ns. Assuming that 10 measurements
(samples) per signal period are sufficient for a sine wave
Storage mode