Sjoerd, If you look at the MC1350 data sheet, you will find the AGC characteristics for operation wiht a 12 volt supply to the chip and that graph indicates that the gain changes almost linearly from 0 dB to 20dB with an AGC voltage change from 5 to 6 volts, or 10 dB for a 500 mV change, and the rate of change increases abruptly above the 6 volt level.
However, the K2 uses that IF amplifier chip at an 8 volt supply, so as a rough guess, the voltages should be multiplied by a factor of 2/3. I have not attempted to characterize this amplifier at an 8 volt supply. Fact 2 - the gain of the IF Amp is governed by the current rather than the voltage into the AGC pin, so you should conclude that there are several dependencies that will modify the actual voltage required for a given gain. I would suggest that you set the AGC Threshold somewhere between 3.7 and 3.8 volts as measured at Control Board U2 pin 5. You may end up with some slight reduction of gain between AGC on and AGC off, but lower voltage here will tend to produce problems with CAL S HI and CAL S LO settings. If you are willing to live with the consequences of using a voltage lower than 3.7 volts to produce less change between AGC ON/OFF, then you must accept the fact that you will have a weaker AGC gain reduction overall, and your S-meter may produce readings that are different from the norm. You may not have any problem at all, only a consequence of the AGC action produced by your high noise level. 73, Don W3FPR > -----Original Message----- > > Hello all, > > Just some added info abt my K2's problem. > > Measured Vagc at pin 5 of the IF amp: > > -No antenna connected AGC on: 3.83V > -no antenna connected AGC off: 3.82V > -40m antenna connected: 3.90V (my FT-1000D shows abt S5 > noise) > -XG2 connected : 3.99V (50uV level, should be > S9) > > Found a post from N6IZ in 2004 that stated that Vagc varies about > 100mV per > 10dB. ( > http://ac6rm.net/mailarchive/html/elecraft-list/2004-10/msg00085.html ) In > my case that says my S9 level is only about 16 dB above noise level... > > Is there anyone who measured Valc at some point and can tell me if these > values are way off? > > Thanks again, > > Sjoerd > PE2SVN > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
