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Author Topic: PSK31 and AGC Action  (Read 11621 times)
N3HKN
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Posts: 243




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« on: February 15, 2001, 07:52:49 AM »

One of my perceived problems with PSK31 is the reaction of my FT-847 AGC to strong PSK31 signals within the passband. I have no way of disabling the AGC and thus see weak signals drop out when AGC is raised with an S9+ signal - RTTY or PSK.

Fortunatly the operators are keeping their power within reasonable limits - less than 50W. In fact have worked several with 2 watts. However, the human spirit of "more is better" will eventually prevail and my AGC will keep 2 watt signals invisible.

A possible answer is this months's QST article on a PSK31 direct conversion transceiver. Without the burden of AGC it should allow "all" PSK31 signals to be visible to the DSP sound card. For $45.00 the kit makes real sense.

Dick N3HKN
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NB6Z
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Posts: 550


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« Reply #1 on: February 15, 2001, 02:08:17 PM »

My experience with direct convertion receivers is that you will still need some front end attenuation on real strong sigs, otherwise they can distort and that could cause problems for the sound card programs.
I have the FT-840 and no way to turn off AGC. The best thing you and I and all digi hams can do is use the narrowest IF filter we can buy for our XCVRs. I have learned to use the optional 500 Hz CW filter along with the split VFO feature to TX in SSB mode and then RX in CW mode with the filter ON. I then use the IF shift control to position the pass band so that interferring signals are not seen in the water fall (hopefully). For side band specific modes (like MFSK16) I may have to use the program's INVERT feature (USB/LSB) depending on which band I am operating in.
73...
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N3HKN
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Posts: 243




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« Reply #2 on: February 21, 2001, 11:55:15 AM »

I agree...

After looking at the schematic for the PSK xceiver in QST I would also add that drift is a concern. With both stations "netted" you could become QRM to others.

I have also found that the IF-Shift to be of value despite my use of the stock SSB filter. I am encouraged to see that the 500hz filter is of value, and adding in use of IF-Shift, as you say, is about the best we can do.

The only other answer is the new Kenwood TS-2000 which is the FT-847 with a 2m/440 xceiver plus IF-DSP. It is the IF DSP that would make improvements in PSK. I can't give up SSB on 2M & 440 in hopes that AO-40 will eventually work in some manner.

Dick  N3HKN
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