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eHam.net Forum : Digital : AGC off w/digital modes?

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AGC off w/digital modes?	Reply
by N0ZLD on May 29, 2007	Mail this to a friend!
All, Is it a good idea to turn AGC completely off when running digital modes? I normally run 'slow' AGC, but saw a post about a guy who runs his AGC off. 73, N0ZLD

RE: AGC off w/digital modes?	Reply
by K4SC on May 29, 2007	Mail this to a friend!
I realize the following may not answer your exact question, but I infer your issue is with agc pumping. I ususally notice the practice detailed below improves the % copy, often dramatically for weaker signals. I have found I get best results once I have identified a signal I wish to copy by tuning the VFO such that the signal of interest is centered in the I.F. passband (1500 Hz on an IC-746PRO). You can experiment to find the center of your passband by observing the waterfall display. Take the difference between the upper and lower frequencies in the displayed band of audio. Divide the difference by two and add the result to the lower frequency. The result should be very close to the center of your passband. After centering the signal, narrow your I.F. passband as much as your radio will accomodate through use of filters, e.g. if you have a CW filter, it will provide the narrowest passband. I have I.F. DSP and have my narrowest filter is preset for 250 Hz. When I establish contact, I usually invoke that 250 Hz. filter and have no problems with agc pumping. I usually run in "Fast" agc. If there is another strong signal within the 250 hz passband, I then adjust the IF shift control to eliminate that signal. Hope this might be of some help.

RE: AGC off w/digital modes?	Reply
by AA4PB on May 29, 2007	Mail this to a friend!
I use MixW with the PRO and have macros automatically do the radio tuning and filter switching. I use a 50Hz filter for PSK31 and it works great. Turning off the AGC will result in overloading the receiver on the strong signals. I find it best to run slow AGC.

RE: AGC off w/digital modes?	Reply
by N0ZLD on May 29, 2007	Mail this to a friend!
I currently run slow AGC and when runnning PSK ran a 50hz filter but find it does have a little bit of DSP 'ring' to it. I opened it up 100hz and its a little better. I am not having any issues with digital copying, I was just kind of curious if the AGC could help. When set to 2.8, I get all sorts of AGC pumping. Filtering really helps, but I was wondering if I could use AGC to help copy weak PSK (or whatever digi-mode) even more. Sounds like slow it is. Thanks N0ZLD

RE: AGC off w/digital modes?	Reply
by N0SYA on August 11, 2007	Mail this to a friend!
You could always turn it off and adjust the if gain manualy to a level suitable to present rx condx.

RE: AGC off w/digital modes?	Reply
by K5MBV on October 15, 2009	Mail this to a friend!
If you can't turn off the AGC, then I believe "Fast" is the next best thing. This is from my own experience and your rig might perform different, so there doesn't seem to be a firm answer. Ken K5MBV

RE: AGC off w/digital modes?	Reply
by AA4PB on October 15, 2009	Mail this to a friend!
No, you don't want to use fast AGC. The issue is not AGC pumping from strong signals in the passband. It is that fast AGC can follow the PSK waveform and flatten the peaks of PSK signals (signal distortion) causing the IMD to deteriorate on stronger signals. Slow AGC has a decay time longer than the PSK waveform so it sits at a steady average level and doesn't distort the waveform. I've done some testing with this on an IC761. With an S5 or better PSK signal I could see a copy of the PSK signal with a scope on the AGC line with fast AGC and the displayed IMD level increased by as much as 10dB. Poor IMD is the result of distortion anywhere in the signal chain between the transmit sound card and the receive sound card.

RE: AGC off w/digital modes?	Reply
by K5MBV on October 15, 2009	Mail this to a friend!
I agree 100% with your last sentence. The prior statement relates your results using a different radio with posssibly a different AGC time constant, so that the outcome is subjective. I would expect the audio to follow the PSK signal, but I'm surprised that IF AGC would do the same as the signal level from a clean PSK source shouldn't be changing. Ken K5MBV

RE: AGC off w/digital modes?	Reply
by VA7CPC on October 15, 2009	Mail this to a friend!
PMFJI -- PSK31 _doesn't_ have a constant signal level. The signal drops to 0 during phase transitions -- that's how the bandwidth is kept narrow. I can watch my digital power output display (FT-450) jitter around when I'm transmitting. It stays below whatever "peak power" I've set, but it spends a lot of time below that. I'm one of the "AGC off" people. The _only_ reason I turn off AGC is because my IF filter is too wide to give good results with AGC ON/SLOW. If I had a narrow IF filter -- so that adjacent signals didn't affect the IF gain -- I'd run AGC ON/SLOW. There's no reason to do anything else. Charles

RE: AGC off w/digital modes?	Reply
by K5MBV on October 16, 2009	Mail this to a friend!
"PSK31 _doesn't_ have a constant signal level. The signal drops to 0 during phase transitions -- that's how the bandwidth is kept narrow." You're right, it does drop to 0 during phase transitions. At that time, the signal is not present at all. Otherwise, when it is on, there is no variation in signal level. The on/off transition is causlng the fluctions in audio readings and this happens pretty fast. I don't know if the AGC time constant ratio for "Slow", "Medium", and "Fast" are the same for different radios, so the setting for best decoding might depend on the radio. Ken K5MBV

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