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 Post subject: AGC Circuit
PostPosted: Tue Nov 27, 2012 10:04 am 
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Joined: Fri Aug 03, 2012 5:35 pm
Posts: 36
I'm receiving 10 and 12 meter SSB signals very well (as well as my K2 transciever). However, I'm a little concerned that the S meter doesn't respond to anything except the RF Gain control (no response to even the strongest signals).

I measure 2.03 v on the AGC line when the RF Gain control is max'ed, rock steady, no discernible fluctuation.

As an experiment, I disconnected the AGC line from the DAC and rigged a manual "AGC" control. To my ear, gain increases as voltage on the AGC line increases until a point of diminishing return is reached at about 2.1 v. Greater than 2.25 v there's definitely no noticeable increase in gain ( and maybe a small decrease above about 3 v).

So, the 2.03 v that the DAC gives when RF Gain is max'ed, is about max AGC. But the S meter and AGC do not react at all to any RF signals, not even the strongest.

Maybe there's something wrong with my compilations of the PIC and/or DSP code.

Any ideas? (Thanks in advance.)


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 Post subject: Re: AGC Circuit
PostPosted: Tue Nov 27, 2012 10:51 am 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
The AGC signal is used to decrease the gain in the IF so if it's always at max gain it means that the DSP isn't seeing enough signal to reduce the gain for some reason. The only real way to tell where the problem is is with a variable level signal source. There are audio signal generators apps for the sound card that you could use to inject a signal directly into the A/D and see what levels get some AGC action.

If that works you'll need to work back to the IF so you'll need a 10.7 MHz signal to see if you can trigger AGC action.

I doubt that it's something in the software IF the S meter works with the RF gain pot. It's more likely that there's a chunk of gain missing somewhere.

73's
Jim WA2EUJ


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 Post subject: Re: AGC Circuit
PostPosted: Tue Dec 04, 2012 12:30 pm 
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Joined: Fri Aug 03, 2012 5:35 pm
Posts: 36
Jim:

I'm still looking for missing (or lost) gain. The ESC spec sheet for the crystal filters shows filter pairs connected back-to-back (pin 1 to pin1). I connected the pairs back-to front (pin 3 to pin 1) per DSP-610 schematic. Could that account for missing gain?

Thanks for your help.


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 Post subject: Re: AGC Circuit
PostPosted: Tue Dec 04, 2012 8:58 pm 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
I'm not sure if the filter orientation makes a difference. A signal generator would certainly help. You need to work backwards from the DSP, make sure it can hear the 2nd IF, check the gain then move back to 10.7 IF the mixer should have gain and the IF amps should have LOTs of gain.


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 Post subject: Re: AGC Circuit
PostPosted: Fri Feb 22, 2013 4:26 pm 
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Joined: Fri Aug 03, 2012 5:35 pm
Posts: 36
You can refer to the Trouble thread for more discussion, but the bottom line solution to the AGC problem is to replace L8 and L9 with 390 nH parts, per Jim's suggestion. (I used Newark P/N 20T1119.)


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