Accuracy re fidelity on/with the TX display while using PS

AB2EZ
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Re: Accuracy re fidelity on/with the TX display while using PS

Postby AB2EZ » Tue Feb 20, 2018 7:53 pm

Rob

I think your latest posted measurement results are very helpful, but:

You wrote

"The third-order IMD results were not as good as seen when running the transmitter into the dummy load where the signal strength into the receiver ranged between 20dB to 30dB over S-9. I think I would agree with Gary that the amount of signal that might leak out of the ANAN while running ~30 watts would be quite small compared to the power being dissipated and radiated by the dummy load. ..."

I don't think a dummy load, connected to an external amplifier with coaxial cable, is necessarily any more efficient a radiator than the coaxial connection from the output of the transceiver to the input of the amplifier. Maybe it is... and maybe it isn't... depending upon the details of the various cables being used, the mechanical design (shielding) of the dummy load, the mechanical design (shielding) of the external amplifier, etc.

If the amplifier is producing 700W and the transceiver is producing 30W... then there is no certainty (without confirmation via measurements) that the input power to the auxiliary receiver produced by the transceiver is more than 13.7 dB below the input power to the auxiliary receiver being produced by the amplifier.

That is not a great enough contrast for an accurate measurement of IMD at the output of the external amplifier for cases in which the external amplifier's output IMD... with Pure Signal on... is more than 13.7dB below the external amplifier's output IMD with Pure Signal off.

I have previously posted the mathematical analysis to prove this assertion... but I don't expect too many people to read it.

Stu
Last edited by AB2EZ on Wed Feb 21, 2018 3:23 pm, edited 1 time in total.
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Re: Accuracy re fidelity on/with the TX display while using PS

Postby W2PA » Tue Feb 20, 2018 11:47 pm

I read it, Stu (I read nearly everything ;-). Your math makes sense.

I think your point, put another way, is that PS corrects for net IMD through the entire transmit chain, exciter plus amplifier. This means the output of the exciter alone has not been corrected optimally. One wants to make sure that those IMD products aren't superimposed on the (probably much better) net spectrum, in the presence of 13.7 dB gain in the amp. Yes?
73,
Chris, W2PA
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Re: Accuracy re fidelity on/with the TX display while using PS

Postby AB2EZ » Tue Feb 20, 2018 11:54 pm

Chris

Yes... well stated

73
Stu
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Re: Accuracy re fidelity on/with the TX display while using PS

Postby W1AEX » Wed Feb 21, 2018 10:00 pm

Stu and Chris,

I have to admit that the math is well beyond my skill level (my wife looks for my handwriting whenever the checkbook doesn't balance) but between both of your explanations the light went on and I see what Stu is driving at with the verification of the (a) and (b) scenarios outlined previously in the thread. Thank you for the clarifications!

73,

Rob
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Re: Accuracy re fidelity on/with the TX display while using PS

Postby W1AEX » Sat Feb 24, 2018 3:43 pm

Stu,

Yesterday, I had a chance to run the scenario you outlined (in italics below) and found a bit less than 20dB difference in the signal level between setup conditions a and b. It looks like running the auxiliary SDR receiver on the same computer as the ANAN adds all kinds of potential for ground loops and other RF paths that make a mess of things. Until I figure out a setup that is reliable and gives repeatable results I'm going to go back to using the K3FEF webSDR page to get a visual of my OTA signal and also to listen to what it sounds like in the voice modes. The propagation when I made the screenshots below was not great as a 500W two-tone was not enough power to make the 3rd order peaks show up, but at least it confirmed that my S-9 + 10dB signal looked clean as a whistle.

73,

Rob W1AEX

"To do this right, the output of the external amplifier should be attached to the regular transmitting antenna... and the auxiliary pickup loop should be located far enough from the transceiver so that one can verify that the signal being picked up by the auxiliary receiver

a) when the amplifier is on, is at least 30dB greater than

b) when the external amplifier is off, and the output of the external amplifier is connected to a dummy load."


http://k3fef.com/

500 watt two-tone
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500 watt SSB signal
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AB2EZ
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Re: Accuracy re fidelity on/with the TX display while using PS

Postby AB2EZ » Sat Feb 24, 2018 5:36 pm

Rob

Yes... doing this measurement with an auxiliary receiver in the same location as one's transceiver/amplifier is much more complicated than it appeared to be when we first thought about it.

Even with the relatively low internal crosstalk levels in the more recent ANAN-XXX products, I remain skeptical of actual amplifier output IMD improvements (i.e. with Pure Signal on vs. Pure Signal off) that are much larger then 20dB. [Note that a 20dB IMD improvement is still very significant].

A recent measurement on 20m:

ANAN=10e output: 4W Elecraft KXPA100 amplifier output: 100W

Measurement 1: Amplifier on (100W output), ANAN output 4 W, amplifier feeding a dummy load, the Pure Signal input of the ANAN-10 is obtained from a home-brew coupler at the output of the amplifier . S-ATT reading: 26, Panadapter reading: +12dBm
Measurement 2: ANAN-10E output switched from the amplifier input to a separate dummy load using a DPDT coaxial relay. At the same time, the amplifier input is connected to a third (separate) 50 ohm termination, and the Pure Signal input to the ANAN-10E is connected to the same home brew coupler, at the output of the amplifier, as in measurement 1. S-ATT reading: 26, Panadapter reading : -18dBm.

Contrast = +12dBm - (-18dBm) =30dB

Stu
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Re: Accuracy re fidelity on/with the TX display while using PS

Postby K9RX » Sun Feb 25, 2018 2:53 pm

Per Scott and Rob's suggestion I got an SDRplay RSP1A. I have a 1' long loop connected through a 100 ohm resistor as an antenna (shorts back to connector shield).... I'm using an older "notebook" ... a pentium processor at 1.3Ghz ... it is running Win7. It seems to handle SDRuno ok with the processor loading going from about 35 - 55%.

When I try transmitting on 75M (I had started this thread - trying to get a confirmation that what I see is what is there) at 1300W out it seems clean ... although the whole noise floor jumps around a bit when I first start xmitting - smoothing out if I continue to talk... the need here was to now ascertain if the issue, splatter reported two different times on 75M by two "somewhat local - W4's, whether or not it was real or if I was being BS'd ... trying to see if there was an issue with rectification of antenna connections causing issues ... so far so good.

In so doing though that jumping around is a bit bothersome. Its difficult to really tell how clean it is - it seems to jump even more on say 15 meters where I really can't seem to see any improvement turning PS on and off ...

I am unsure what "Single CAL" means - Scott had said something about it earlier on I believe in this thread ... seems maybe it's a one time measurement and then it keeps these values. So I brought up the panel and clicked on it - sure enough no more adjusting on the AMP VIEW panel - didn't seem to help though overall... then I was unable to turn it off - i..e to go back to it automatically on the fly adjusting - it remained in single cal mode! I tried hitting the PS button off and then on - no change... couldn't get it to "come back" again. The single cal button seems to just get a colored outline when you click on it - no indication of "ON/OFF" status ... finally I got out of the "Linearity" panel all together - then turned PS OFF and ON and that worked ...

Long story short - is the jumping around on the noise floor around the signal on the SDRuno due to PS causing it to splatter as it adjusts ... or is this proximity effects? Note I get the same results : the notebook on its charger or off internal batteries. When doing the latter there's nothing but an isolated system re the RSP1A/notebook. The USB cable has 3 large clamp on ferrites with 2 turns in each - adding them didn't make any difference...

Gary
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Re: Accuracy re fidelity on/with the TX display while using PS

Postby w-u-2-o » Sun Feb 25, 2018 6:41 pm

Gary,

You are going to have to post a video of "jumping around", because I have no idea what you mean. If you use a constant signal, like two-tone, not voice, there should be no "jumping around", particularly if you have utilized single cal. mode.

73,

Scott
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Re: Accuracy re fidelity on/with the TX display while using PS

Postby W1AEX » Sun Feb 25, 2018 10:00 pm

...the whole noise floor jumps around a bit when I first start xmitting - smoothing out if I continue to talk...


Hi Gary,

If you are referring to what you are seeing with the SDRplay, the baseline noise floor as displayed in SDRuno will jump around a lot in the presence of a very strong local signal unless the AGC is turned off as shown in the screenshot below. Apparently, the AGC for the IF Gain level gets confused as it "hunts" for an optimal receive level when it's dealing with a very strong signal. Once you turn off the AGC you can set the RX level manually with the IF Gain slider.

73,

Rob W1AEX

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