WA6BFH wrote: "don't believe the old wives tale about 6 dB's per S-unit. It is the rare radio indeed where that is true!..."
---------------------------------------------- Back when the "old wife" was a YL 6 dB/S-unit was largely true. I tested a number of receivers from the 40's and 50's including my HRO-5 and several Hallicrafters units of the period and the S-meter calibration tracked within a dB of 6 dB/S-unit across the range. Those old receivers simply monitored the AGC voltage produced by rectifying some of the signal at the "second detector" and used the d-c level produced to bias the i.f./r.f. amplifier tubes to reduce their gain. As a result the AGC could be used only on amplitude modulated phone. The AGC had to be turned off for SSB or CW because the BFO signal would be rectified and would shut down the gain of the receiver, rendering it 'deaf'. Newer rigs are something else. When SSB became the dominant voice mode on HF, things started to change with more exotic AGC systems that could handle both CW and SSB. Many rigs use extra amplifiers in the AGC. Some rigs, like the Elecraft K2, use a completely separate and different i.f. from the main signal i.f. to drive the AGC detector. Having a dedicated AGC i.f. avoids getting the BFO signal into the AGC rectifier and rendering the receiver deaf. I've never bothered to measure any "modern" receivers to see how close they track, but I'd not be surprised to find a large range of behavior. On the other hand, that behavior may be more controllable. 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
