[ARC5] A discussion on AGC.

[Richard Knoppow]

    Think for a minute of the difference between an amplifier and mixer. 
If we take a favorite hi-fi amplifier we want to be able to put two 
signals into it say 50hz and 10,000hz, and get only those two 
frequencies out. If any others show up it is due to non-linearity. 
Depending on the order of the nonlinearity we will get harmonics out or 
sum-and difference frequencies out (intermodulation).  In a triode or 
pentode amplifier the distortion is a matter of the grid bias and other 
characteristics.  Older books will show several types of AM detectors 
using tubes other than diodes. Diodes are rectifiers, which by 
definition are not linear.  Further, the ratio of the input voltage to 
the output voltage of the half wave that is passed, is not linear but 
depends on the input voltage and characteristics of the diode. For 
instance, in an RF voltmeter using a single diode rectifier, the 
response varies from RMS for small signals, for which the diode has a 
square law characteristic, to peak response for large voltages where the 
response approaches being linear in the sense that the ratio of voltage 
of the half wave passing through the diode is linear to the half wave 
going in. The overall response is still non-linear because only half the 
input wave is passed through.
    In receivers using a diode detector for CW the injection level of 
the BFO becomes important because too low a level requires a low level 
of RF signal thus limiting the sensitivity and too high a level will 
desensitize the receiver by either creating some bias in the detector or 
(more usually) by causing the AVC to operate. There are receivers with a 
separate path for the AVC, as has been mentioned earlier in this thread. 
In them the BFO voltage has no effect on the AVC. Receivers with 
"exhalted carrier" operate this way.
     In a receiver with a product detector the output of the detector 
contains (ideally) only the sum and difference of the two input 
frequencies, which in most cases are easy to separate with filters.
     Note that diodes can be used as "mixers" and were quite popular for 
this application in early microwave equipment. They were quieter than 
any of the more elaborate arrangements.

On 5/10/2017 9:23 AM, Kenneth G. Gordon wrote:
> On 10 May 2017 at 9:17, Ian Wilson wrote:
>
>>
>> A triode has enough nonlinearity to be able to demodulate AM. The various
>> flavours of triode detector are biased to choose a region of curvature that
>> provides reasonable detection efficiency versus distortion of the AM envelope.
>
> Yes. It is all in the "biasing".
>
> Witness the "Pullen Mixer".
>
> Ken
>
> ---
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL