Keith wrote: All this AGC talk makes me think TenTec's hang AGC should have been the ideal. In effect it would turn the RF Gain down when a big signal came in and keep it down until the signal went away, then would turn it back up rather quickly. But for some reason it just never caught on. Maybe because it was implemented in rigs with audio derived AGC?
--------------------------- Right on, Keith! Hang AGC filled a specific need. It goes way back - to the 50's at least. Its intent is to avoid turning the gain up immediately when the signal disappears. It keeps the gain turned down for a specific period of time. That's the "hang" time. At the end of that period it is then turned up very quickly. On most hang AGC systems, the "hang" time is adjustable from a few mS to several seconds. It solved a tough problem in early SSB/CW AGC systems: slow attack time. A big problem trying to create an AGC for a SSB or CW receiver was the BFO signal in the I.F. bandpass. The AGC detector would sense that huge BFO signal and turn down the gain accordingly, making the receiver deaf! So early SSB/CW receivers (and the more simple receivers today, including some of the Elecraft line) used audio-derived AGC instead of I.F. derived AGC. That is, instead of rectifying some of the I.F. signal to produce a d-c AGC voltage that corresponded to the strength of the signal, it rectified some of the resulting audio signal to produce the AGC signal. That eliminated the problem of the BFO getting into the AGC system, but it made the attack time slow. That's because, when a signal appears, the detector has to rectify several cycles to produce the AGC voltage. At R.F. that signal may be hundreds of kilohertz or even higher while the audio frequency is typically near 600 Hz. With audio-derived AGC it takes much, much longer to produce the d-c voltage from the AGC rectifier. For example, it would take an AGC detector working at 600 Hz about 200 times longer to produce the d-c voltage required than it does when the detector is using an I.F signal at, say 100 or 200 KHz. That delay to rectify the signal often resulted in a "pop" in the audio every time the signal appeared as it slammed through at full volume for an instant before the AGC voltage built up to turn the gain down. The first solution was to slow down the AGC action so that it only very gradually returned the gain after a strong signal went away. That way, there was less of a "pop" if it returned soon, but it meant that the receiver remained deaf to weaker signals longer. Hang AGC provided a better response by leaving the receiver desensitized for a specific period of time to prevent another "pop" if the station was still transmitting CW or SSB, but then quickly resumed full volume after the hang period elapsed. Hang systems started to disappear as ways were devised to use the I.F. signal for creating the AGC. This is most commonly done by using another mixer and producing a new I.F. just for the AGC far removed from the BFO frequency. The K2, for example, uses an AGC I.F. at about 200 kHz for this purpose. That takes more circuit complexity but the fast attack time allowed truly quick AGC without the "pops" or the delay of a hang time. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
