Panadapter Signal During Transmit

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Is the signal displayed by the panadapter during transmit in the Flex 6700 a sample of the signal that goes out over the air or is it a sample at some point prior to the final amplifier transistors?

Ed, K0KC
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Ed, K0KC

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Posted 5 years ago

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Dale KB5VE

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Ed I have asked this question twice in the past and have not got a answer. If you notice the skirts are down about -125dbm.
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Ed, K0KC

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Although the signal may look clean on the panadapter, I would like some assurance that it it looks this good when it reaches the antenna. I do not have the equipment to properly monitor it at that point.

Ed, K0KC
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Steve - N5AC, VP Engineering / CTO

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When we first built the radio, we realized that much of the time the transmitter does not overload the receiver during transmit and you can see your on-air signal as it comes back into the receiver. This seemed like a good way to see what was going on, so we left it. Later we found that some mixing of on-air signals and other signals in the receiver result in some visible spurs or other anomalies that are not actually on-air. We discussed several options for how to best present the data. In PowerSDR, we just FFT the TX data (show you what we HOPE is going out over the air) and show that to you, but with the 6000, you could be looking at 14MHz of bandwidth and so we would have to build this FFT that adjusts for how much bandwidth you are looking at, where it is offset in the panadapter (since the receiver is no longer always in the center of the panadapter), etc. All of this seemed like a lot of work when we could be working on things that helped operators with better reception, more features etc.

In the end, after several discussions, we decided that we would leave it as it is and explain that all of what you see is not always going out over the air, but in general what you see in the filter passband during transmit is very representative of what is going out over the air. As engineers, everyone in our office preferred the radio to behave this way -- we felt better seeing what was going out over the air in our receiver, even if we had to learn that some things are "glitches" that are not real on-air. We decided that if there was much fussing, we could hide all of the offending signals by FFTing the transmit signal at a later date and showing you what is sent to the PA rather than what comes out of the PA. This solution, which is employed by most radios, hides anything that happens in the PA that is bad. For example, if you had a real issue in your PA, you would never be able to tell from what you see in the panadapter.

For me, personally, this is a lesser of two evils solution. In engineering, sometimes there is not a solution that makes everything do exactly what you want. Having said this, I believe this is the only amateur radio that let's you actually see through the receiver what it is transmitting on air and I like this solution. I'd be interested how others feel.
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Ken - NM9P, Elmer

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I like it showing what is actually being transmitted, even with the internally generated spurs outside the TX passband.

The next step would be to unmute the receiver and direct a reduced level audio to the monitor so we could actually hear what it sounds like in our own receiver. (I assume this is possible even on the 6500?) Or would that cause too many problems, such as messing up the very hard-earned excellent QSK performance on CW? (In which case I would nix the idea.)
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Dale KB5VE

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Great response Steve. I asked this same question about a month ago and it was never addressed. Now if I transmit on slice A on3720.000 I will be seeing my recieve when I key the mic and talk correct. And if I go to slice b on a another pan adapter and set it to recieve 3720.00 I will see the same thing on both panadapters , is that what you are saying? I have been doing this for a while and it appears the second panadapters is a little behind the transmit pan.

I like what I see and now as I understand it the spurs on the side of the transmit is actually othe noise on the frequency.
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Ed, K0KC

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Steve,

Thanks for the info...I, for one, like the current solution!

Ed, K0KC
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George Molnar, KF2T, Elmer

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Is there a way to drop the RX sensitivity during transmit to clean up the display and maybe correlate the peaks to the top of the panadapter?
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Mike - N1MD

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Steve,
Thanks so much for this insight and explanation. This has helped me understand what I am looking at during transmit and has increased my enjoyment of this amazing radio.

Personally, I am always impressed when you write to us collectively. Your explanations are concise, readable and with a minimum of jargon. You get to the point and answer the question without a tap dance.

I my world you would be "a great doc".

Mike
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philip.theis, Elmer

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I agree with George, a little less gain if that is possible.
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Bob Wright, N7ZO

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Steve,  Good explanation, but isn't there a tap on the output of the PA that was meant for adaptive pre-distortion (in the future)?  Why would that not be a good place to sample the signal for transmission monitoring?
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K4EAR

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I'm all for such an approach! I think the 5000 had this with a 80db pad on the HRFIO board and enabling TX monitor routed padded PA rf to a RX in.
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Steve - N5AC, VP Engineering / CTO

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Yes, Bob, there is a -80dBc tap point.  It is enabled in the software lower levels, but we have not yet taken advantage of this for transmit monitoring.  We originally added it just for pre-distortion, but you are correct we could also use it for transmit monitoring.  You will see some more dynamic range in v1.2 over what you have now in v1.1 so what you see in the panadapter should be more representative even with out this.  But we will probably look into using this switch for this in the future.
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Bob Wright, N7ZO

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One of the things that attracted me to the 6000 series was its precision - mathematically clean modulation, demodulation and filtering.  Counting on leakage to monitor the transmit signal seems so...un-Flexworthy.  :-)





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K4EAR

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My +1 bump for this monitoring ability. Let's make it happen sooner, not later.
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K4EAR

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Trying to monitor actual TX audio in my 5000 has too much delay to follow the 6700.
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Tim Ellam

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"Later we found that some mixing of on-air signals and other signals in the receiver result in some visible spurs or other anomalies that are not actually on-air. "

Is this still the case today? I have some transmit spurs on either side of my passband (in wide mode)which I thought were due to RFI. I gather that is not the case and such spurs are "normal". I am in the process of reducing some RFI when I moved to the 6700 (thanks to Al NN4ZZ for his website)and has assumed I would be able to eliminate the spurs.

73

Tim VE6SH
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DrTeeth

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Tim, I think it would be a good idea if someone could produce an annotated screenshot showing the spurs. I would find it handy to know exactly what one looks like so I can tell if what I see is okay. I was bemused the other day when I deliberately over-drove my TX (AGC slamming into the red) as a test to see the panadaptor showing a clean signal.
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Steve - N5AC, VP Engineering / CTO

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The deal here is this: the ability to see your transmit signal is a nice benefit, but it should not be trusted to tell the "whole story."  Due to some coupling in the radio (which varies by band), you will see things that are not representative of the actual on-air signal.  This display (pan during TX) is good to get an idea for what your signal looks like on the given mode and band and then recognize when something is very different.  In this case (something different), you should then look on a RF monitor (independent, spectrum analyzer, etc) to determine what is really wrong.
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Tim Ellam

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Thanks Steve. That confirms your earlier comment. I had thought this was a by-product of some RFI, but obviously not the case (and yes on closer inspection it only occurs on some bands).

Thank you for your response!

73

Tim VE6SH