| Author |
Message |
    
Drifter_8291
Junior Member
Username: Drifter_8291
Post Number: 18
Registered: 10-2004
| | Posted on Friday, November 11, 2005 - 1:01 pm: | 
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Hi I was wondering about slowing down the AGC in a cobra 148 gtl....What resistor is it in the radio and what resistor value would you recommended replacing it with?
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Hollowpoint445
Advanced Member
Username: Hollowpoint445
Post Number: 892
Registered: 6-2004
| | Posted on Friday, November 11, 2005 - 4:28 pm: |
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You can increase the value of C23 to get the desired action. Or you can parallel another capacitor on a SPST switch so you can have both fast and slow AGC. Or if you want to go nuts you can use a single pole rotary switch and add a different capacitor value for each throw. |
Outkast
Member
Username: Outkast
Post Number: 93
Registered: 9-2005
| | Posted on Saturday, November 12, 2005 - 3:48 am: |
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Hmm I feel really dumb but what is AGC I thought that was the Adjacent channel rejection?
Outkast
CEF 650 |
Hollowpoint445
Advanced Member
Username: Hollowpoint445
Post Number: 897
Registered: 6-2004
| | Posted on Saturday, November 12, 2005 - 7:53 am: |
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Automatic Gain Control. It's part of the receiver and it adjusts the gain automatically to keep the receiver from being overloaded and making the annoying chugging sound that you may have heard on AM BCB when receiving a DX station.
Usually you want faster AGC on AM and slower on SSB, but it's really a personal preference. The speed refers to the decay time when a signal drops in strength. Good receivers will usually give you some kind of option - usually fast/slow. CBs have one AGC speed that is a compromise. |
Kid_vicious
Advanced Member
Username: Kid_vicious
Post Number: 829
Registered: 9-2004
| | Posted on Saturday, November 12, 2005 - 11:13 pm: |
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am i correct in my layperson's understanding of AGC?
it is what makes a weak signal sound louder, and a strong signal sound quieter, so that you can keep the volume control at one spot for communication with different signal levels.
do i get it or not?
matt |
Hollowpoint445
Advanced Member
Username: Hollowpoint445
Post Number: 899
Registered: 6-2004
| | Posted on Sunday, November 13, 2005 - 2:02 am: |
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Not quite. AGC automatically decreases the RF gain to keep the receiver from overloading - that's all it does. It doesn't make it more sensitive, it makes it less sensitive.
AGC activates when a strong signal is present, and when the strong signal goes away, the AGC decay eventually returns the receiver gain to wherever the RF gain knob is set until another strong signal activates the AGC again.
It has nothing directly to do with volume although to some extent it does control the volume of the audio by decreasing the gain of stronger signals.
If you want to listen to AGC work, then listen to the AM broadcast band after dark. Find a DX signal and listen to it for a little while. Eventually you'll hear it vary in strength due to changing conditions. The radio will try to deal with the changing signal strength. It'll go from where you can barely hear the audio because of very little signal to it overloading the receiver. You'll hear it chug as the receiver is overpowered until the AGC cuts the gain and the audio clears up. When the signal drops in strength you'll hear the background noise get louder as the AGC decays and the receiver returns to maximum gain.
If you were listening on a receiver that has different AGC speeds you could change to fast AGC and see the difference it makes. Fast AGC decays faster to return the receiver to maximum gain. Basically you would hear the background static grow much faster after the strong signal goes away. It can deal with the changing signal strength a little better. |
Hatchet
Junior Member
Username: Hatchet
Post Number: 45
Registered: 7-2004
| | Posted on Sunday, November 13, 2005 - 5:23 am: |
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Yes you are correct Kid |
Kid_vicious
Advanced Member
Username: Kid_vicious
Post Number: 837
Registered: 9-2004
| | Posted on Sunday, November 13, 2005 - 11:09 pm: |
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thanks for the explanation!
my reason for asking is that i read a spec and that's what it led me to believe.
(AGC)
LESS THAN 10db CHANGE IN AUDIO OUTPUT FOR INPUTS FROM 10 TO 100,000 MICROVOLTS.
matt |
