I am hoping I can induce some nice person to spend some time explaining the K2 AGC circuit to me. I've been trying to tweak the AGC threshold potentiometer, and in the process trying to learn about this circuit.
>From the datasheet of the MC1350P it looks like pin 5, the AGC control voltage, works on a range of 5 to 7 volts. Yet when I measure it with my 10 Megaohm input DMM, it ranges from about 3.7 to 4 volts from no signal to a 50 microvolt signal from the XG1. The AGC seems to work just fine. I would have expected a higher voltage on this pin. Also, the tweak procedure is to listen to the receiver generated noise without antenna connected and adjust the pot (R1) so there is no difference in noise level with the AGC on or off. I found that the control was quite broad and not easy to find an exact spot. So I decided to use Baudline to help me out. Baudline is an audio spectrum analyzer computer program for Linux similar to Spectrogram for Windows. What I found was that as the AGC voltage was varied by adjusting R1, the total power level of the noise would vary, as expected. What I found interesting was that the AGC voltage is always different between AGC on and off, and similarly the total power level of the noise was always different between AGC on and off. What I did find was that the difference between noise power levels decreased as the voltage decreased. Note that I am measuring the AGC voltage at pin 5 of U12 (MC1350P) and not pin 5 of U2 (LM833) where the AGC threshold voltage is measured. So the voltage readings below are the actual AGC voltage and not the AGC threshold voltage as measured at U2. Here's my measurements: Case 1: AGC off: Volts: 3.706 Noise: -22.68 dB AGC on: Volts: 3.763 Noise: -23.22 dB Difference: Volts:0.057 Noise: 0.54 dB Case 2: AGC off: Volts: 3.821 Noise: -22.75 dB AGC on: Volts: 3.872 Noise: -23.69 dB Difference: Volts:0.051 Noise: 0.94 dB Case 3: AGC off: Volts: 3.867 Noise: -23.07 dB AGC on: Volts: 3.907 Noise: -24.65 dB Difference: Volts:0.040 Noise: 1.58 dB So since a change of less than 1 dB is not noticeable to the human ear then perhaps this might be a good point for the AGC threshold? Comments? Is what I am seeing normal behavior in a K2? Everything seems to work just fine. Darrell VA7TO K2#5093 -- Darrell Bellerive Amateur Radio Stations VA7TO and VE7CLA Grand Forks, British Columbia, Canada _______________________________________________ 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
