IM distortion on 6 meters

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  • Problem
  • Updated 4 days ago
  • (Edited)
Hello
While operating on 6 Meters I'm seeing lots of IM distortion. This is into a 50 Ohm resistive load. I have tried various mic and input settings to no avail. Anyone else seeing this?
Ver 3.1.11
Flex-6600  SN 1120-1088-6600-4670 
See attached photos

USB


FM


George N1NAZ
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George Moranian

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Posted 2 weeks ago

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Tim - W4TME, Customer Experience Manager

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First, upgrade to SmartSDR v3.1.12 at your earliest convenience.  Second, what is your power level in the screenshots?  if it is relatively low, then open a HelpDesk ticket.
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George Moranian

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Tim power level on both screens is at 100 watts, tried at lower power level and same results seems to peak at about 20 watts but just as bad at 100 watts
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Steve G1XOW

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George, I think there is something wrong with the PAN display software when using higher pre-amp settings on 6m. I see you are using +24dB. So, I see exactly the same crud on the screen when on 6m with either 16 or 24dB preamp engaged. See my screen grab attached. This is tune power only of about 10w no amp on-line and the antenna is about 20m away and over 20m AGL.


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Steve G1XOW

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Geroge, here is another screen, with the PAN about the same width and settings as you have it. Looks pretty repeatable to me. v3.1.12 in use here.

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George Moranian

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Thanks for verifying this Steve, even though the IM is less its still there at lower to no preamp. Hope its software and not hardware related.

George N1NAZ

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Bill -VA3WTB

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The panadapter is not a very good representation of our TX signals. Can you show the TX on the water fall?
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Steve G1XOW

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Bill, it's a TUNE carrier at 10w o/p, see below.

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Chris DL5NAM

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George, forget what YOU see at your PAN or Water if you transmit.
Search for a Ham who is close to you to check your signal on a SDR with his Pan - if you not have a own Spec Analyzer.
And not forget: if you TX your TRX create also local noise, you will see on your Pan but you not transmit this noise too over air.
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KD0RC

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If you want to see what your signal looks like, NM9P has a great video on the subject on YouTube. Basically, you put the rig in full duplex mode, transmit on the xverter port, while listening on a different port. With the few mW produced by the xverter port, you will not be overdriving the reciever. I think you will see a very clean sig. If not, then it is time to ask Flex for help.
73,
Len, KD0RC
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Bob KC9RF

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Hi George
Just put up a 6 meter antenna and noticed the same thing. I too did the dummy load test and found almost the same results on my 6600. I have a 6400 so hooked up my Maestro to the 6400 while the 6600 was transmitting into a dummy load. As you see there is almost nothing on received on the 6400 see picture #1
Picture #2 is using Muti Flex More harmonics show up on the Maestro but not as bad as the transmit slice of the 6600. Guess we need a explanation from Flex about this phenomenon.
I tried the same thing on 20 meters and transmit display looks much better.                                       
The bottom line is it's ok to transmit on 6 after test with second receiver                         
Picture 1 6600 on left 6400 on the right with Maestro                                                                          

  Picture 2 using Multi flex 

 
(Edited)
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Steve G1XOW

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Bob, almost exactly the same as I saw. Am pretty sure the real RF output IMD is fine as I checked on another RX with dummy load, but the display software is nasty and needs fixing PDQ.
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George Moranian

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Good to know, Thanks

Thanks everyone for their reply's. Hope the display gets cleaned up.
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Bill -VA3WTB

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This is why I suggested to look at the water fall while on TX. It shows much better what is going on. The panadapter is working as designed. What your seeing is everything going on inside the transmitter. Your also seeing the TX signal coming back into the receiver and being mixed with the TX signal. Most companies take steps to hide this, so everything looks clean. Flex decided to leave everything as is so we can see everything being mixed. Also this way if there was something going wrong with the transmitter it would show up,,you won't miss that, it is really messy. On other radios the TX could run into trouble but the operator would never know because their radio will not display the TX problems. Bottom line, the panadapter does not represent what is actually going out over the air.
(Edited)
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Steve G1XOW

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"and the cow jumped over the moon". Bill please do not belittle yourself by trying to explain this away as an intentional feature. You know isn't, and so do most other experienced long term users.

I cannot say exactly when this nasty bug crept in to SmartSDR, but I do know with absolute certainty that it has not always been there. I've used SmartSDR since the first release of version 1 (when I was running 2 x 6500 side by side on 6m SSB DX). It certainly never showed then. I suspect we've only been observing this in the last 12-18 months max. There are many examples coming out of the woodwork recently. It is nothing to do with power output (also happens at just 1W O/P), it is nothing to do with SWR or return power, or antenna type/location. It happens at 1W in to a dummy load connected directly at the back of the radio. Therefore any "noise" being developed must either be internal noise, or fantom display aberrations.

(Edited)
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Bill -VA3WTB

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I took my comments from Eric at Flex where he explained what we see. It is a combination of internal noise and the return of the TX signal re entering the radio and mixing with the TX signal. This signal is usually coming back through the other ant input. All radios do this. But most companies design their radios to block this signal so the user can't see it. What you see on the panadapter is not being transmitted. 
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Bill -VA3WTB

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When I have time I will post Eric's article then you can argue with him..
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Bill -VA3WTB

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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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Steve G1XOW

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Hi Bill, thanks for the text. Gland to see it is like I said a display handing aberration, not RF IMD. Interesting info from Eric, and I feel this information should be communicated far and wide, is it even in the Radio manual?

As I am also an engineer of some 34 years experience (20 as a low-level comms programmer), I generally concur with the design principle of seeing an unadulterated signal "warts and all". However, where I cannot agree is presenting the user with a screen that is so far out from on-air reality. Misleading or false feedback instrumentation that suggests problems that are not real is not a wise move for any product, and moreover is of no value if the foreground imagery is so wildly distorted that you simply cannot see any "real" underlying RF issues that may be there. Flights tend to crash with a big bang if the pilot gets presenting with misleading information! I saw beyond the problem, less experienced users such as the original poster may not.
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George Moranian

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I am the original poster of this thread and I was unaware of what was going on with the panadapter on 6 meters when all the other bands are relatively clean, but I am fully aware of IM, harmonics, spurious, birdies and their possible detrimental effect on the transmitted signal.
How is one supposed to tell how well the transmitted signal is with all the other artifacts present? Real or not!
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Bill -VA3WTB

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As Eric points out, it can be a useful diagnostics tool to see if something is going wrong in the radio. If there were problems in the radio it would show big time. There have been times Flex would ask for a screen shot of the radio in TX mode to help determine what is wrong. After most people understand what they are seeing, most people just ignore it and move on.
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Conny N5HC

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This is exactly what wrote about in my post:

https://community.flexradio.com/flexradio/topics/tx-spectrum-waterfall

And I was asking specially if the pan adapter was showing was coming out of the PA or not. 

But I was told not to worry. And not to use the pan adapter for checking my transmission quality.

BTW I also have issues with unlinear sounding modulation on 20m up to 6m.

Few days ago i got a report that I was +/- 200 KHz wide on 6m. And my pan adapter (DSP receiver acting as a spectrum analyzer?!) showed this garbage too. So should I ignore a report that correlates to my pan adapter?

I also have worked with Radio communications since 1975 and would say have enough experience to rule out the most basic bonehead ideas.
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Bill -VA3WTB

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Depends on what their working conditions were, how strong were you? It is possible if you were over driving the PA.
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Steve G1XOW

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On my desk I have an expensive professional grade Marconi 2955 RF analyzer. If it presented misleading information I'd have it sent back to the factory for recalibration, or I may even buy a replacement from another manufacturer. 

When a BMW thunders past a pedestrian like a farting elephant on steroids will it make him want to buy one?  The designer knows why it sounds like that and thinks its really great, the pedestrian (aka prospective buyer) on the other hand may not. Moral is that designers and technicians don't always know what user expect.

This may prove to be such an elephant for Flex.