Question about spectrum display in tune with and without amp

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  • Updated 2 years ago

I recently had a report that my RTTY signal was broad and distorted. I figured I was doing something wrong with the new 6600. I wish I had paid more attention to the spectrum display while transmitting. I noticed that it didn't look as clean as I remembered it looking previously. I couldn't find anything wrong with the settings but I did notice something odd with the tune signal. Can anyone tell me if this is normal?

Here is what a 10 W TUNE signal looks like on my 6600 spectrum display WITHOUT the amp in line:

If I switch the amp in-line and set the tune power to anything between 1 and 70 watts I see this:

This is operating into a dummy load. Looks about the same on 80-40-20. Didn't try the other bands. The amp is an old Henry 2K . I can even detune the amp so that it isn't putting out any power (maybe a watt) and I still see this same display. And I have swapped out all the cables - RF and otherwise - to make sure that it isn't RF related. Everything is bonded to ground.

This signal doesn't look very clean to me - not compared to the first picture. I don't know where in the signal chain this spectrum is picked from but it is certainly coming from inside the Flex. 

How can merely connecting the Flex to the input of an amp affect what comes out of the Flex?

Is what I'm seeing normal?

My RTTY signal looks a lot like the second picture. Probably not good.

Thanks, Mark K5XH

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Mark K5XH

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Posted 2 years ago

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George Molnar, KF2T, Elmer

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Pretty normal and plenty clean. The TX panadapter doesn't show a perfect reproduction of the transmitted signal, unfortunately. Even so, consider the displayed "noise" is way down from your main signal. 

Does your signal sound okay in Monitor mode? That's a good indication.

If so, I suspect the report is due to AGC action or receiver overload at the other end.
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Mark K5XH

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Thanks George. I hope you are correct! I am just worried that something has gone wrong with my new radio!

I did listen in the second receiver and it looked about like the second picture. I am going to do some more testing this week using the second RX.

Allow me to ask this question to anyone who can explain this to me: Why is there a difference between the two pictures?

It seems to me that the display should be showing what is coming out of the radio and that should not matter if it is going into a dummy load or a matched input to an amp. What causes the radio to "know" it is driving an amp and causing the display to change? It can't be RF because I can detune the amp enough that I can't see the needle move on the 20 watt scale.

Maybe someone can duplicate the simple experiment. Check their display with and without their amp. If they get the same results then I'll be happy and just caulk it up to one of the many things I don't understand.  

I have been reluctant to actually get on the air on RTTY and try to make a contact to see if my signal is bad. It looks bad on the spectrum display from what I remember it looking like. Unfortunately there are no local hams that are knowledgeable enough about RTTY and have a Flex that can help me out.   

73, Mark K5XH

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I think the difference between the two pictures is a result of local RF being stronger in your shack when the amp is on.  You have an incredibly sensitive receiver there.  I too agree your signals both look fine.  When you transmit RTTY, take a look at the DAX control panel, perhaps you need to back off on the TX of the display.
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Mark K5XH

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Thanks Peter. My first thought was RF getting into something but....

Can't see how there can be too much RF in the shack when there is no RF coming out of the amp - or at least not more than a watt or so. (TUNE power set to 1 watt and amp detuned). And that's going into a dummy load. If that were the case then I should see the "bad" display when I turn up the TUNE power to 100 watts with no amp. But I don't.

The TX gain is set to provide -10 db on LEVEL meter when in RTTY mode.