Friday, May 08, 2009

PSK31 with the AGC off

I'd been planning to write about this for a couple of days, but originally decided to leave it for later as I've already made two posts today. But a comment in W9OY's blog made me decide to write about it now.

W9OY wrote: "I especially don't like PSK31. The dynamic range of that mode is so bad that if you run more than 30W you blow out everyone's receiver and mess up the throughput." What nonsense!

For years I've worked PSK31 with the AGC on. Unlike many people, who have grown accustomed to the "Digipan" style of operation where you look at a 2KHz or more slice of spectrum on the waterfall and point and click to select your frequency of operation, I've always tuned the station I want to work to the "sweet spot" and used narrow filters to block out any strong nearby stations that could be depressing the gain and making the one I want to work hard to copy. I especially enjoyed the razor-sharp selectivity of the K3 that allows me to narrow the receiver down to allow just one PSK31 signal through, if necessary.

In the last few days, to allow my station to report received PSK31 activity to PSK Propagation Reporter, I've been operating with the K3 filters opened up as far as they would go, which is 4KHz in data mode at the moment. It's obvious from the waterfall that when a strong signal comes on, it depresses the receiver gain affecting the strength of everything in the passband. The solution, of course, is to turn off the AGC, which on the K3 is as easy as pressing one button.

Normally, if you turn off the AGC, you are in danger of damaging your eardrums. But when working PSK31 you don't really need the sound on at all, so it causes no problem. The only thing you need to watch is that you don't overload the computer sound card, or the receiver line out audio stages. Yesterday evening I noticed multiple traces evenly spaced across the waterfall when a very strong signal came on. The solution was to back off the RF gain a bit. Depending on where the overloading occurred, it might be better to reduce the line output level (something that's also easy to do with the K3.) You can then view a full 3 or 4KHz of spectrum and point and click Digipan style, without any problems caused by loud stations. Assuming their signal is clean, of course - but that's a different matter.

Turning off the AGC and managing the receiver gain by hand is a technique used by "old hand" DXers to copy weak CW stations in the presence of close-by strong ones. If it works for one narrow-band mode, why not use it for another?

PSK31 has poor dynamic range? I don't think so!
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