Older receivers (like my HRO-5) used a beat frequency oscillator (BFO) operating on the same frequency as the I.F., offset only by the audio beat note required.
That meant that a huge local signal (the BFO) was blasting into the I.F. strip. AGC was derived from the RF signal at the "second detector" (where the I.F. was demodulated to audio). So the BFO would cause AGC to crank down the gain to minimum when it sensed the BFO signal, making the receiver totally deaf. It was essential to turn the AGC off and ride the RF gain manually with the BFO running. By the late 50's Hams were tinkering with various ways to keep the AGC active with the BFO on. Some use "audio" AGC. That is the AGC was derived from the audio signal, not the I.F. But that made the AGC slow since it took a certain number of cycles of signal to build up the AGC voltage and that happened relatively slowly when it was taken by rectifying an audio signal compared to the I.F. signal. The most successful solutions (before SDR technology took over) were to split the I.F. with a second mixer that was far removed from the main I.F. the second I.F. was used to generate the AGC voltage and, since it was on a wholly different frequency from the main I.F. the BFO didn't trigger the AGC. In fact, the Elecraft K2 uses that technique quite successfully. 73, Ron AC7AC -----Original Message----- From: Elecraft [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Jim Brown Sent: Tuesday, October 27, 2015 6:02 PM To: elecraft@mailman.qth.net Subject: Re: [Elecraft] K3 - PSK31 - AGC/No AGC On Tue,10/27/2015 3:50 PM, Wes (N7WS) wrote: > I never turn AGC off on any mode. Never understood why anyone would > want to but I've only been licensed for 57 years, so maybe I missed > something. I started three years before you, so maybe I remember more from back then. :) In those older days, it was quite common for CW ops to turn down the RF gain and turn up the AF gain. In effect, this more or less disables AGC. It can also help with listener fatigue with lots of static QRN. BUT -- this can be taken to extremes, and doing so is, I think, responsible for those hams who complain of the K3 being "hissy" and having "clicks" when switching bandwidth. One of the first important things we had to learn in the world of pro audio is setting audio gains properly to minimize both distortion and noise. For every gain stage, there is a Goldilocks-like "sweet spot" to keep the signal away from clipping (distortion) and the noise floor (hiss, clicks, etc.). 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to r...@elecraft.com ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
