Step attenuator linearity

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w-u-2-o
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Step attenuator linearity

Postby w-u-2-o » 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

T44-160m-1KW-PSon-0 dB.jpg
T44-160m-1KW-PSon-0 dB.jpg (217.92 KiB) Viewed 3180 times


T44-160m-1KW-PSon-3 dB.jpg
T44-160m-1KW-PSon-3 dB.jpg (204.74 KiB) Viewed 3180 times


T44-160m-1KW-PSon-6 dB.jpg
T44-160m-1KW-PSon-6 dB.jpg (202 KiB) Viewed 3180 times


DAT-31A-SP+ Figure 1.JPG
DAT-31A-SP+ Figure 1.JPG (48.07 KiB) Viewed 3180 times


0dB atten warm.JPG
0dB atten warm.JPG (531.64 KiB) Viewed 3180 times


6db atten warm.JPG
6db atten warm.JPG (518.28 KiB) Viewed 3180 times


10dB atten warm.JPG
10dB atten warm.JPG (512.28 KiB) Viewed 3180 times


Analyzer vs Radio.jpg
Analyzer vs Radio.jpg (2.9 MiB) Viewed 3180 times
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w9ac
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Re: Step attenuator linearity

Postby w9ac » Thu Feb 27, 2020 8:07 pm

Scott,

My DUP display does track with the S/A until the non-linearity occurs and PureSignal can no longer linearize. You have not yet reached a point of non-linearity with the HP measurement. As such, I would expect your result.

It would be helpful if you varied coupling to the point where IMD is reached and PureSignal no longer linearizes. If that point nears or reaches the maximum allowable MCL input power so be it, but please measure it.

If you and I have reached different conclusions, then we need others to validate the testing and outcome.

Paul, W9AC
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w-u-2-o
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Re: Step attenuator linearity

Postby w-u-2-o » Thu Feb 27, 2020 9:52 pm

w9ac wrote:It would be helpful if you varied coupling to the point where IMD is reached and PureSignal no longer linearizes. If that point nears or reaches the maximum allowable MCL input power so be it, but please measure it.

I'm loathe to do this because I'm already banging up against the maximum safe input level of +13dBm at the rear panel (well above the level at which you say 160M IMD occurs in the feedback path).

If you and I have reached different conclusions, then we need others to validate the testing and outcome.

I believe both sets of measurements are correct. And I also believe that there is a condition in your station that causes this problem. What I don't agree on is that it has anything to do with the step attenuator.

You may have already done this, but if you have access to an Xtronic coupler I wonder if that would make any difference in the measurements?

Also, do you have return loss data on all three ports of the toroidal sampler? If so, what does it look like?

73,

Scott
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w9ac
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Re: Step attenuator linearity

Postby w9ac » Thu Feb 27, 2020 10:20 pm

w-u-2-o wrote:You may have already done this, but if you have access to an Xtronic coupler I wonder if that would make any difference in the measurements?


Yes, I have repeated the 160m results using three couplers:

Coupler #1: Xtronic XDC-1
Coupler #2: 60dB coupling coefficient capacitive voltage divider (later changed to 50 dB)
Coupler #3: 50 dB coupling coefficient T-44 toroid sampler.

The first notion that something was amiss occurred after constructing the toroid sampler. I initially blamed the inability to linearize (on 160m at 1KW) on core saturation of the T44 core. It wasn't until I inserted a Weinschel programmable attenuator that the couplers could be compared while looking at the IMD threshold where PureSignal won't linearize. I see zero difference in any of the couplers with a 56 dB total coupling coefficient. That means all samplers have fixed pads. The 50 dB couplers have 6 dB fixed pads, the Xtronic started with 20 dB and later replaced with a 10 and 3dB pad in series.

w-u-2-o wrote:Also, do you have return loss data on all three ports of the toroidal sampler? If so, what does it look like?


No, I did not measure return loss. only swept insertion loss (IL). I first measured the Weinschel's IL and discovered a bad cell. After replacement, I used my N2PK VNA in S21 mode and swept IL through 60 MHz at 0, 1, 3, 6, and 10 dB. I did this to prove that the attenuator was performing as expected. Like your attenuator, the Weinschel uses purely passive cells.

As Warren pointed out on the Apache list, it doesn't take much IMD at the receiver's input to destroy PureSignal's ability to linearize. That's why I would like others to repeat the test and get a comprehensive set of data. Your results and mine alone may not be enough to form an accurate conclusion. Here's Warren's response:

"I did want to point out that 0.2dB attenuator compression (compression from zero amplitude to maximum amplitude of the signal) is quite significant as far as linearization with PureSignal is concerned. 0.2dB is more than 2% voltage change which will be seen by PureSignal as additional non-linearity of the PA. When we’re trying to get IMD to, say, -60dBc, a 2% error is quite significant...It’s unfortunate that the non-linearity persists when the attenuator is set to 0dB – if it did not, at least there would be a complete workaround."

Finally, we shouldn't just depend on datasheets and math. It's a necessary start, but measurement is still required.

Paul, W9AC
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w-u-2-o
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Re: Step attenuator linearity

Postby w-u-2-o » Fri Feb 28, 2020 12:28 am

Would it be difficult to measure the return loss of your coupler? Just curious to see if there is an impedance problem lurking.
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w9ac
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Re: Step attenuator linearity

Postby w9ac » Fri Feb 28, 2020 3:53 pm

w-u-2-o wrote:Would it be difficult to measure the return loss of your coupler? Just curious to see if there is an impedance problem lurking.


I can do it as time permits, but keep in mind I've made IMD measurements using three different RF samplers that yield the same result. The only RL measurement that I had considered earlier is an S11 reflection sweep into the 7000's Bypass port during TX. For a problem to exist, it would mean all three samplers are going non-linear into what's predominantly a passive input circuit.

Concerning the MCL DAT-31A-SP+ datasheet and the "Figure 1" graph, what I believe is shown (and suggested by Don Jackson, W5QN) is the maximum safe input level. It does not mean that the device isn't going non-linear to some small degree at levels below the curve.

It would be interesting to conduct some out of circuit testing of the DAT-31A-SP+. I'm not motivated to do that level of testing at this time. Frankly, MCL should provide more detailed analysis of input power by frequency vs. IMD products on their datasheet.

Paul, W9AC

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