BC-348 AVC

<< < (2/3) > >>

k4kyv:
If the receiver doesn't have a separate agc amplifier, but feeds the agc rectifier directly off the final i.f. transformer secondary, parallel to the diode detector, the agc must be disabled whenever the BFO is turned on.  If not done automatically, it must be turned off manually.  Otherwise, the agc rectifier makes no distinction between the BFO output and a strong signal, and the rectified BFO output generates enough agc voltage to nearly cut off the agc-controlled stages.  Turning off the BFO for CW and SSB is standard procedure with the classic single-conversion superhet where the i.f. feeds a diode detector and the BFO is coupled to the diode detector through a small capacitor.  In some  receivers, the same diode serves as detector and agc rectifier.

If the receiver has a  separate agc amplifier stage, I have found that slow agc is  best for cw and ssb, but for AM, fast agc is preferable, since it allows the agc to better track the carrier under QSB conditions.

Another problem with using agc with  cw or ssb is that under key-up condx or between ssb syllables, the receiver gain recovers and the background noise rises between cw characters or voice syllables.  Hence slow agc for cw or ssb.  If I use agc at all for those  modes, I use just enough to prevent the receiver from overloading on strong signal peaks, by turning down the rf  gain control.  Some ssb receivers attempt to get around this problem by using audio-derived agc.

Maybe you could find two pots mounted with concentric shafts to separate the af and rf  gain controls.  That used to be common with older tube type TV sets which often had concentric knobs to accomadate dual functions.  Just make sure you use pots with the proper taper.  The rf  gain should be linear taper, while the af gain should be audio, or logarithmic taper.  If a linear taper pot is used for the af gain control, all the audible signal variation will be squeezed into about the first 25 degrees of rotation of the pot.

w3jn:
Quote from: k4kyv on November 13, 2006, 10:14:46 AM

If the receiver doesn't have a separate agc amplifier, but feeds the agc rectifier directly off the final i.f. transformer secondary, parallel to the diode detector, the agc must be disabled whenever the BFO is turned on.  If not done automatically, it must be turned off manually.  Otherwise, the agc rectifier makes no distinction between the BFO output and a strong signal, and the rectified BFO output generates enough agc voltage to nearly cut off the agc-controlled stages.  Turning off the BFO for CW and SSB is standard procedure with the classic single-conversion superhet where the i.f. feeds a diode detector and the BFO is coupled to the diode detector through a small capacitor.  In some  receivers, the same diode serves as detector and agc rectifier.






THat's one of the reasons BFO signals are very lightly coupled to the IF.  You can improve SSB reception by increasing the coupling, but at the expense of decreased sensitivity due to the AVC action (resulting from the BFO signal) decreasing the gain of the receiver.

The other reason, as I noted, is to prevent the BFO from phase-locking to the incoming signal.

W1RKW:
Quote from: w3jn on November 13, 2006, 02:30:17 PM


The other reason, as I noted, is to prevent the BFO from phase-locking to the incoming signal.


What is the behavior of a BFO phase-locking to an incoming signal? What does one hear when this occurs? I've been messing around with an old buzzard rcvr where the BFO injection is extremely light but I get strange behavior when I increase the BFO coupling. One thing I'm definitely getting is reduced sensitivity.

w3jn:
You can't smoothly tune the BFO thru zero beat and up the other side.  The BFO will jump in to zero beat (or nearly so) at low audio frequencies.  The BFO could also try and phase lock itself to the audio creating an irritating FM effect. 

WU2D:
I'm still in sunny California and do not have the skizmatic with me. One trick that the BC-348 uses is to inject the BFO into the last IF stage instead of into the detector. That last IF has no AGC applied - it is the only fixed gain IF stage.  That stage amplifies the IF and also acts as a BFO amplifier. I am not sure how they isolate the BFO from getting rectified by the AGC detector. Perhaps they keep it low enough to not cause trouble with that trick.

As far as mods go, I did not see anything fishy and the unit never had a ham power supply - it had a dynamotor inside that looked original.

Thanks for the comments. I obviously have some troubleshooting to do with my generator and a voltmeter.

Mike WU2D

Navigation

[0] Message Index

[#] Next page

[*] Previous page