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Author Topic: Transmitting Power with PSK31  (Read 2100 times)
KB3MDT
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Posts: 269




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« on: July 08, 2005, 09:41:56 PM »

Hi,
   I made my own computer / transceiver interface using audio transformers for isolation.   It seems to work well with Win Warbler.  

   I have been monitoring my signal on a separate PC using an inexpensive Multiband Shortwave Radio.  I found my signal looks pretty bad on the waterfall (splatter) if I try to drive my transceiver more than a few watts.

   I haven't had any luck getting response to CQs on PSK31.   I'm not sure if this is because my transmit power is low or what.   I'd like to have someone independently check my signal when I try to feed the transceiver more audio.  

    In any case, should I be able to drive a 100 watt transceiver (ICOM 718) between 25 and 50 watts with a PSK31 signal without getting distortion?  If so, it sounds like my homebuilt interface may not be doing the job.

    Thanks.

KB3MDT
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AA4PB
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Posts: 15022




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« Reply #1 on: July 09, 2005, 08:03:57 AM »

Distortion anywhere in the transmit/receive chain can cause IMD to go up. Even switching from fast to slow receiver AGC can make a difference. It is quite possible that when you run more than a few watts you are overdriving the second receiver and that is where the distortion is occuring. Normally you should be able to run 25-30 watts with a 100W radio with a good IMD. The interface won't normally cause an increase in IMD unless you are overdriving some really tiny transformers.
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Bob  AA4PB
Garrisonville, VA
VA7CPC
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Posts: 2829




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« Reply #2 on: July 10, 2005, 03:22:47 PM »

There's a chance you're overdriving the front end of the cheap receiver, and your IM measurement is faulty.  

If not, you'll have to find a combination of
   a) sound card output level, and
   b) transceiver "mic gain"

that gives you little or no ALC on the transceiver, and enough RF output (around 25 watts) to get a good signal out.

You'll get to recognize the signals of stations running too-high sound card output.  Their PSK31 signals have a "wide start-up spectrum" on the waterfall -- distorted for a few milliseconds until the ALC takes over and reduces the modulation level in the audio/RF mixer.

Many PSK31 stations operate around the 25-watt level -- it's plenty of power.
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KI4NQ
Member

Posts: 25




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« Reply #3 on: July 28, 2005, 06:03:02 AM »

I am running an Icom 718, and to set up my audio, I set a transceiver up 2 feet away from my 718 with no antenna on the recieving radio, and reduced the power on the 718 to 1 watt (while connected to an external antenna).

I used mmsstv for the calibraion because I am familiar with the sounds of the sstv audio.
I sent a cq, and listened on the reciever, and adjusted the audio for the interface for the best natural sound. If you have audio to low, you will get a buzz. if too high you will also get a buzz. otherwise it will be clean audio. after settin up this way, I raised the power, and sent a cq on 20 meters with about 40 watts, and got an immediate contact. I have tried from 1 - 100 watts on the Icom 718 with no changes in quality of signal. normally I run about 20 watts.

Good Luck
73's
Ki4nq
Gary
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AA4PB
Member

Posts: 15022




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« Reply #4 on: July 28, 2005, 08:42:47 AM »

ALC varies considerably from one radio model to the next. A few radios have a long enough ALC decay time that they will simply reduce the peak power without distorting the PSK waveform. Most decay times are fast enough that the ALC will follow the PSK waveform to some degree and cause distortion. With a scope you can actually see the PSK waveform on the ALC line. The safe thing to do is to reduce the power so that there is no ALC action at all.

I did some experimenting with a K2 and found that I could increase the ALC decay time so that it didn't distort the PSK signal.
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Bob  AA4PB
Garrisonville, VA
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