Darrell, If you look again at the MC1350 spec sheet you should see 2 graphs - first is the gain reduction Vs. AGC voltage and the second is Noise Figure vs. Gain Reduction. I would believe the results of these 2 graphs combined are part of what you are observing. As you increase the gain, the chip itself becomes noisier - plus there are noise sources ahead of the IF Amp in the K2 that are amplified by the MC1350 which further complicates the overall effect.
It is true that the graph of Gain Reduction shows that less than 5 volts should give 0 dB of gain reduction, but note also that the spec sheet says the supply voltage for these conditions is +12 volts while the K2 operates the chip from the +8 volt rail. I have no idea how much effect the lowered voltage has on the AGC operation of the chip, but for certain, the bias point of the AGC transistors in the 1350 schematic shown should change a bit with decreased supply voltage. Your best bet is to do as you have done and determine the proper AGC Threshold empirically, and watching the noise floor with Spectrogram is splendid idea. I would also recommend that you turn the Pre-Amp off when determining the optimum AGC Threshold - the optimum point is more easily observed without the added preamp noise to contend with. 73, Don W3FPR > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of Darrell Bellerive > Sent: Thursday, June 15, 2006 11:18 AM > To: elecraft@mailman.qth.net > Subject: [Elecraft] AGC Circuit Explanation? > > > 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 > > > -- > No virus found in this incoming message. > Checked by AVG Free Edition. > Version: 7.1.394 / Virus Database: 268.8.4/364 - Release Date: 6/14/2006 > > _______________________________________________ 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
