Agilent PN 8510-18 Testing amplifiers and active devices with the Agilent 8510C Network Analyzer Product Note
Table of Contents 2 3 Introduction 4 Amplifier parameters 5 Measurement setup 7 Linear measurements 11 Power flatness correction 13 Nonlinear measurements 15 Appendix A—High power measurements 17 Appendix B—Accuracy considerations 19 Appendix C—8360 series synthesized sweepers maximum leveled power (dBm) 20 Appendix D—Optimizing power sweep range
Introduction The Agilent Technologies 8510C microwave network analyzer is an excellent instrument for measuring the transmission and reflection characteristics of many amplifiers and active devices. Scalar parameters such as gain, gain flatness, gain compression, reverse isolation, return loss (SWR), and gain drift versus time can be measured. Additionally, vector parameters such as deviation from linear phase, group delay, and complex impedance can also be measured.
Amplifier parameters Parameter Equation Definition Gain Vtrans The ratio of the amplifier’s output power (delivered to a ZO load) to the input power (delivered from a τ = ______ ZO source). ZO is the characteristic impedance, in this case, 50 Ω. Vinc Gain (dB) = –20log10|τ| For small signal levels, the output power of the amplifier is proportional to the input power. Small signal gain is the gain in this linear region.
Measurement setup Before making an actual measurement it is important to know the input and output power levels of the AUT and the type of calibration required. Setup 1. Select input power levels Selecting the proper stimulus settings at the various ports of the AUT are of primary concern. If the small signal gain and output power at the 1 dB compression point of the amplifier are approximately known, the proper setting for the input power level can be estimated.
3. Power meter calibration (optional) The 8510C network analyzer provides leveled power at the test set port with a specified variation of less than 2.1 dB at 50 GHz. The power meter calibration feature is available to provide more accurate settable power when required and can also serve to remove the frequency response errors of the cables and adapters between the test set and the AUT. If a power meter calibration is performed it should be done prior to a measurement calibration.
Linear measurements Measurements in the linear operating region of the amplifier can be made with the 8510C by using the basic setup shown in Figure 2. Care must be taken when setting the input power to the AUT so that it is operating within its linear region. 1. Configure the system as shown in Figure 2. Return the 8510C to a known state of operation. 3. Perform a full two-port calibration. If attenuators are used on the output of the amplifier they should be included in the calibration.
3. Measure the gain flatness or variation over a frequency range using the following sequence. First, set the appropriate start/stop or center/span frequencies over which the flatness is to be measured. Then perform an appropriate calibration over this frequency range. Then perform the following to see a direct readout of the peak-to-peak difference in the trace. [MARKER] {MARKER 1} {MORE} {MARKER TO MINIMUM} [PRIOR MENU] {MARKER 2} {MODE MENU} {REF=1} {MORE} {MARKER TO MAXIMUM} 2.
2. Place a marker in the center of the band and activate the electrical delay feature, [MARKER] {MARKER 1} {12 GHz} RESPONSE [MENU] {COAXIAL} OR {WAVEGUIDE} depending upon whether the media exhibits intrinsic linear or dispersive phase shift. {AUTO DELAY} 3. {ELECTRICAL DELAY} is now the active function. Use the knob, STEP keys, or numeric and units to fine tune the electrical delay for a flat phase response near the center of the passband.
Return loss, SWR, and reflection coefficient Complex impedance Return loss (RL), standing wave ratio (SWR) or reflection coefficient (rho) are commonly specified to quantify the reflection mismatch at the input and output ports of an AUT. Because reflection measurements involve loss instead of gain, power levels are lower at the receiver inputs. Therefore, it may be necessary to increase power levels for reflection measurements. Alternatively, the noise levels can be reduced by increasing the averaging.
Power flatness correction The power flatness calibration feature of the 8510C network analyzer provides a more precise power level to the AUT. A 437B or 438A power meter and an appropriate power sensor such as the Agilent 8481A, 8485A or 8487A are required. The power sensor is attached to the desired test port, after any cables or adapters leading up to the point where the AUT will be connected, and a single power calibration sweep is performed.
6. When the calibration is complete, activate flatness correction. [PRIOR MENU] {FLATNESS ON} 7. Verify the constant power level at the test port by using the 437B to measure the test port power at CW frequencies. As the power is manually measured, the user must enter each test frequency on the 437B so that the correct calibration factor will be used. 8. The analyzer will automatically store the correction table into register 1 of the source. 9. Remove the power sensor.
Nonlinear measurements The Agilent 8510C has the capability to make measurements of amplifiers operating in their nonlinear region. A swept-frequency gain compression measurement locates the frequency at which the 1 dB gain compression first occurs. A swept-power gain compression measurement shows the reduction in gain at a single frequency as a power ramp is applied to the AUT.
Swept-power gain compression By applying a fixed-frequency power sweep to the input of an amplifier, the gain compression can be observed as a 1 dB drop from small signal gain. The power sweep should be selected such that the AUT is forced into compression. The S21 gain will decrease as the input power is increased into the nonlinear operating region of the amplifier. The 8510C network analyzer has a power sweep range as defined earlier in Table 1.
Appendix A—High-power measurements Custom test set configurations Special test set configurations The Agilent 85110 test set provides the greatest flexibility for the testing of high-power amplifiers which often require custom test set configurations. The 85110 test set is an open architecture which allows amplifiers to be added to the RF path of the test set. External test set components (amplifiers, couplers, isolators, attenuators, etc.
Jumper Jumper RF input High Power Load ▲ ▲ ▲ ▲ ▲ Port 2 ▲ ▲ ▲ ▲ Four way splitter ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ (5 Watts Port Power) ▲ ▲ ▲ Port 1 ▲ ▲ ▲ 30 dB Couplers ▲ ▲ Figure 14. 85110 simplified block diagram ▲ Port 1 Port 2 (Safely handles 500 watts CW or 5 KW peak) Figure 15.
Appendix B—Accuracy considerations Error correction can be applied to the measurements discussed in this note to reduce the measurement uncertainty. A full two-port calibration was used for the measurement examples (except where noted) to provide the best measurement accuracy of both transmission and reflection measurements of two-port devices.
Reflection measurements The uncertainty of a reflection measurement such as return loss, SWR, reflection coefficient and impedance is affected by directivity, source match, load match, and reflection tracking of the test system. With a full two-port calibration, the effects of these factors are minimized. A one-port calibration can provide equivalent results if the amplifier has sufficient isolation to reduce the effects of the load match.
Appendix C—8360 series synthesized sweepers maximum leveled power (dBm) Frequency 83620A/ 83621A 83623A 83631A 83651A 20 GHz 26.5 GHz 40 GHz 50 GHz +10 — — — +17 — — — +10 +4 — — +10 +4 +3 0 When power levels from the AUT are such that external attenuation is not practical or when the source cannot deliver enough power to properly drive the AUT, it may be necessary to construct a custom test set.
Appendix D—Optimizing power sweep range Power sweep range will be reduced if a power flatness correction is used in combination with power sweep. If flat test port power is required, there is no way to avoid this. If the source has step attenuators installed, and a power flatness correction is used with power sweep, the available sweep range will be further reduced. This reduction is due to the source setting the attenuator for the optimum ALC (automatic leveling control) range.
The following example program demonstrates this method. 350 360 10 20 30 370 380 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 ! RE-SAVE “POW_OFFSET” ! ! This program calculates and removes the average amplitude correction ! factor from the 8360 flatness correction array. It then sets and ! activates this average amplitude as a constant offset to the 8360 ! power output.
700 710 720 730 740 750 760 770 780 790 800 810 820 830 840 850 860 870 880 890 900 910 920 930 940 950 MAT Diff= Flat_on-Flat_off ! Diff(*) = source flatness corr array Offset=SUM(Diff)/(Bytes/16) MAT Diff(*,1)= Diff(*,0) ! Diff(*,1) = flatness amplitudes Freq_increment=(Stop_freqStart_freq)/ (Points-1) Freq=Start_freq FOR I=1 TO Points Diff(I,0)=Freq ! Diff(*,0) = flatness frequencies Diff(I,1)=Diff(I,1)-Offset ! remove offset from flatness amplitudes Freq=Freq+Freq_increment ! next frequency NEXT I OUTP
