http://elecraft.85.s1.nabble.com/AGC-Circuit-Explanation-tp390978p390979.html
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
the chip from the +8 volt rail. I have no idea how much effect the lowered
splendid idea. I would also recommend that you turn the Pre-Amp off when
> -----Original Message-----
> From:
[hidden email]
> [mailto:
[hidden email]]On Behalf Of Darrell Bellerive
> Sent: Thursday, June 15, 2006 11:18 AM
> To:
[hidden email]
> 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:
[hidden email]
> 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
>
>
Subscriber Info (Addr. Change, sub, unsub etc.):