Re: Question: Why do you call "AGC" a simple automatic antenna "attenuator" in this forum? #ubitx #ubitxv6

Bill Cromwell


Attenuating the signal at the receive antenna input is not gain control at all. Attenuation can reduce overwhelming signals that distort the receiver front end and make those very weak signals copyable again. It may seem counterintuituve to improve weak signal reception by increasing attenuation at the antenna but that is *exactly* what happens.

Automatic Gain Control may be applied to an RF amplifier immediately following the antenna input and before the first mixer but that is not attenuation. Some os us still like to have manual gain control applied in the RF and IF sections. A manual attenuator that can be switched out completely is an excellent choice and one that I prefer. Not a PIN diode.


Bill KU8H

bark less - wag more

On 11/18/20 5:11 AM, IW4AJR Loris wrote:
From my tests and from the measurements made, it appears to me that the best known "AGC" modification module is a very bad "ATTENUATOR" placed in series with the receiving antenna!
Although I have tried, measured and studied the scheme and compared it with the reality installed on my µBITX-V6, the theory of the principle used confirms what the practice shows:
Although an automatic attenuator placed in series with the antenna is excellent (try it yourself with the best PIN attenuators on the market) the residual attenuation of the circuit at the minimum attenuation will never be less than -3dB, often, even in the most refined circuits. , yes it gets a minimum of -5 / -6 dB! ... not to mention what two poor quality MOSFETs can do (compared to a PIN diode attenuator) ... the best measurement I got was -10 / -15 dB, which is equivalent to losing at least 2 S points on the reception!
It is totally unacceptable! and technically from "geeks" and not from radio amateurs!
From my tests, only one module acts correctly on the "gain" of the MF amplification and, even if a bit obsolete and bulky (it is not built in SMD), it would seem the most efficient module, not changing the "sensitivity" of the receiver (very important thing in DX also in QRP).
Why do you insist on referring to these "ATTENUATORS" as "AGC" "Automatic GAIN Control"?

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