Step attenuator linearity
Posted: Thu Feb 27, 2020 6:12 pm
This is a re-post of an analysis I posted to the old Yahoo Group back in December 2019:
Summary:
After some extensive business travel in December, I've finally had an opportunity to revisit this subject and to try to reproduce the results that Paul reported earlier in the month. I was unable to do so, and can find no evidence of any non-linear responses in the receive signal path at any reasonable level of PureSignal feedback input power level.
Recap of Paul's Results:
Back at the beginning of December, Paul, W9AC, proposed that it was possible to overdrive the step attenuator and thereby cause non-linear behavior that PureSignal could not reconcile, thereby causing significant degradation of PureSignal linearization performance.
Paul, using a home built coupler with a reported coupling factor of 50dB (and reporting step attenuator values from the auto-attenuate algorithm commensurate with a 50dB coupling factor so there is good confidence in it), experienced very poor PureSignal linearization performance at 1.85MHz at power levels above 1KW into a dummy load. As a result, he added 3dB of additional attenuation, and then 6dB, obtaining much better performance at each step. His original spectrum analyzer plots are attached.
My Analysis:
After some research and correspondence with Mini-Circuits engineers, Figure 1 in the latest step attenuator (Mini-Circuits DAT-31A-SP+) data sheet was identified as showing both the 0.2dB compression point vs. frequency as well as being the maximum safe input level vs. frequency for the step attenuator. Mini-Circuits further advised that this data is valid for both the older and now obsolete DAT-31-SP+. Figure 1 is attached.
As the figure shows, the curve at 1.85MHz gives approx. +13dBm as the max. allowable, 0.2dB compression point input signal level to the step attenuator.
Under Paul's test conditions, at 1KW the output of the coupler would be 60dBm - 50dB, i.e. 10dBm. This is 3dB less than the equivalent point on Figure 1, therefore it does not seem reasonable that there should be a problem at this input power level.
As a result of this I postulated that perhaps there was some other non-linearity at play in Paul's configuration at feedback input. And I wanted to try some similar measurements for myself to see if I could duplicate Paul's results and perhaps identify what else might be causing the issue.
My Experimental Results:
I used an original version AC2IQ coupler with a claimed coupling factor of approx. 44dB. This same coupler is now available commercially from HRO as the Xtronic XDC-4SO. The amplifier is a KPA-500 running at 500W into a dummy load. The RF unit is an ANAN-8000DLE. Firmware is Protocol 2 "pre1". Software is Thetis 2.6.9 (b6). I measured my coupler and it has an actual coupling factor of approx. 47dB, not unreasonable given that it is a essentially a prototype of the commercial product.
At 500W, i.e. 57dBm, the output of the coupler is 57dBm - 47dB, i.e. 10dBm, which is exactly commensurate with the level Paul was achieving. And, as with Paul's example, the step attenuator value arrived at via the auto attenuation algorithm confirmed the 47dB coupling factor.
However, under these conditions the achieved PureSignal linearization performance was quite good, as shown in the attached file "0dB atten warm.JPG". I repeated these measurements with an additional 6 and 10dB of attenuation (results attached) provided by inserting a JFW rotary adjustable attenuator inline. Step attenuation values changed by the proper amount of dB each time. There was no significant difference in performance at any of the three feedback signal levels as measured by the "DUP" display mode on the radio.
Note that the word "warm" appears in the filenames as it was necessary to allow the amplifier to warm for 20 to 30 seconds in order to remove thermally related memory effects. I.e. the FETs in the KPA-500 need to obtain a reasonably stable thermal state to get good, consistent results. This issue should not be overlooked with other amplifiers or other testing. It is important to have a good dummy load that can absorb the necessary power for the necessary amount of time!
Paul further proposed that the DUP display of the radio could not be trusted to provide proper results. This assertion is also not consistent with my results. Using an HP 8560 spectrum analyzer on the second forward output port of the AC2IQ coupler, and a tablet mirroring my main station computer display, one can see the results are quite comparable (photo attached). Note that the 8560 is using an un-attenuated output with levels exactly as you would expect (each tone 3dB down from the 10dBm total power output level) while the radio was still seeing an additional 10dB of attenuation. Also, I did not bother to match up the resolution bandwidths of the two measurement systems. No doubt had I bothered to match levels and match measurement settings the two displays would most likely match almost exactly. Regardless, there are no gross differences between the measurements, and no reason to distrust the DUP display mode of the radio. Indeed, the radio is probably a superior instrument compared to a nearly 25 year old spectrum analyzer!
73,
Scott
Summary:
After some extensive business travel in December, I've finally had an opportunity to revisit this subject and to try to reproduce the results that Paul reported earlier in the month. I was unable to do so, and can find no evidence of any non-linear responses in the receive signal path at any reasonable level of PureSignal feedback input power level.
Recap of Paul's Results:
Back at the beginning of December, Paul, W9AC, proposed that it was possible to overdrive the step attenuator and thereby cause non-linear behavior that PureSignal could not reconcile, thereby causing significant degradation of PureSignal linearization performance.
Paul, using a home built coupler with a reported coupling factor of 50dB (and reporting step attenuator values from the auto-attenuate algorithm commensurate with a 50dB coupling factor so there is good confidence in it), experienced very poor PureSignal linearization performance at 1.85MHz at power levels above 1KW into a dummy load. As a result, he added 3dB of additional attenuation, and then 6dB, obtaining much better performance at each step. His original spectrum analyzer plots are attached.
My Analysis:
After some research and correspondence with Mini-Circuits engineers, Figure 1 in the latest step attenuator (Mini-Circuits DAT-31A-SP+) data sheet was identified as showing both the 0.2dB compression point vs. frequency as well as being the maximum safe input level vs. frequency for the step attenuator. Mini-Circuits further advised that this data is valid for both the older and now obsolete DAT-31-SP+. Figure 1 is attached.
As the figure shows, the curve at 1.85MHz gives approx. +13dBm as the max. allowable, 0.2dB compression point input signal level to the step attenuator.
Under Paul's test conditions, at 1KW the output of the coupler would be 60dBm - 50dB, i.e. 10dBm. This is 3dB less than the equivalent point on Figure 1, therefore it does not seem reasonable that there should be a problem at this input power level.
As a result of this I postulated that perhaps there was some other non-linearity at play in Paul's configuration at feedback input. And I wanted to try some similar measurements for myself to see if I could duplicate Paul's results and perhaps identify what else might be causing the issue.
My Experimental Results:
I used an original version AC2IQ coupler with a claimed coupling factor of approx. 44dB. This same coupler is now available commercially from HRO as the Xtronic XDC-4SO. The amplifier is a KPA-500 running at 500W into a dummy load. The RF unit is an ANAN-8000DLE. Firmware is Protocol 2 "pre1". Software is Thetis 2.6.9 (b6). I measured my coupler and it has an actual coupling factor of approx. 47dB, not unreasonable given that it is a essentially a prototype of the commercial product.
At 500W, i.e. 57dBm, the output of the coupler is 57dBm - 47dB, i.e. 10dBm, which is exactly commensurate with the level Paul was achieving. And, as with Paul's example, the step attenuator value arrived at via the auto attenuation algorithm confirmed the 47dB coupling factor.
However, under these conditions the achieved PureSignal linearization performance was quite good, as shown in the attached file "0dB atten warm.JPG". I repeated these measurements with an additional 6 and 10dB of attenuation (results attached) provided by inserting a JFW rotary adjustable attenuator inline. Step attenuation values changed by the proper amount of dB each time. There was no significant difference in performance at any of the three feedback signal levels as measured by the "DUP" display mode on the radio.
Note that the word "warm" appears in the filenames as it was necessary to allow the amplifier to warm for 20 to 30 seconds in order to remove thermally related memory effects. I.e. the FETs in the KPA-500 need to obtain a reasonably stable thermal state to get good, consistent results. This issue should not be overlooked with other amplifiers or other testing. It is important to have a good dummy load that can absorb the necessary power for the necessary amount of time!
Paul further proposed that the DUP display of the radio could not be trusted to provide proper results. This assertion is also not consistent with my results. Using an HP 8560 spectrum analyzer on the second forward output port of the AC2IQ coupler, and a tablet mirroring my main station computer display, one can see the results are quite comparable (photo attached). Note that the 8560 is using an un-attenuated output with levels exactly as you would expect (each tone 3dB down from the 10dBm total power output level) while the radio was still seeing an additional 10dB of attenuation. Also, I did not bother to match up the resolution bandwidths of the two measurement systems. No doubt had I bothered to match levels and match measurement settings the two displays would most likely match almost exactly. Regardless, there are no gross differences between the measurements, and no reason to distrust the DUP display mode of the radio. Indeed, the radio is probably a superior instrument compared to a nearly 25 year old spectrum analyzer!
73,
Scott