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THE AUTOMATIC GAIN CONTROL OR (AGC)

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    K0YNE
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    VERY EARLY IN THE YEARS OF RADIO, STRONG STATIONS WERE A PROBLEM, WITH TOO MUCH AMPLIFICATION. FINALLY, A CIRCUIT WAS DESIGNED TO REDUCE THE AMPLIFICATION WITHIN THE RADIO, ELIMINATING DISTORTION. THIS CIRCUIT ALSO WAS A BIG STEP FORWARD IN KEEPING ADJACENT STATIONS FROM OVERRIDING ONE ANOTHER COMING DOWN THE IF BANDPASS.

    IN AMATEUR RADIO, AND CONSIDERING THE SSB MODES, THE IF BANDPASS WILL BE  FROM 3000 HZ TO 3500 HZ, JUST WIDE ENOUGH TO ACCOMODATE A CLEAR SINGLE SIDEBAND AUDIO BANDWIDTH.
    THE MORE SOPHISTICATED AGC CIRCUITS CONTROL THE GAIN OF THE RF AMPLIFIER AND THE IF AMPS AS WELL.  CONSIDER THAT YOU HEAR NOTHING IN THE “BANDPASS” OF THE RADIO NOW, AND SO THE
    AGC CIRCUITRY IS TELLING ALL AMPLIFIERS TO RUN AT OR NEAR MAXIMUM GAIN.  NOW, A SIGNAL ENTERS THE BANDPASS OF THE “IF'S”. A VERY STRONG SIGNAL CAUSING NEARLY FULL QUIETING OF THE RECEIVER. SINCE THE AGC CIRCUIT IS TO DETECT A CHANGE OF VOLTAGE WITHIN THE “IF” BANDWIDTH,
    A TRANSISTOR COULD BE USED TO MEASURE THIS CHANGE OF GAIN IN VOLTS OR TENTHS OF VOLTS.
    THIS DETECTED DIFFERENCE COULD BE ELECTRICALLY COUPLED TO THE RF AMPLIFIER AND THE IF AMPLIFIERS, FEEDING A REVERSE BIAS VOLTAGE TO THEM AND CAUSING A REDUCTION OF THESE AMPS AS TO THE GAIN.  IN OTHER WORDS, THE STRONGER THE INCOMING SIGNAL IS, THE MORE REVERSE BIAS VOLTAGE IS SENT TO THE IF CHAIN OF AMPLIFIERS, THUS REDUCING ALL THE GAIN NECESSARY,
    BUT ENOUGH GAIN NEEDED TO SUSTAIN GOOD RECEPTION. THERE CAN BE A SMALL DOWNSIDE TO THIS AND THAT WOULD BE ON PSK31 FOR EXAMPLE, WHEN YOU ARE TRYING TO COPY A VERY WEAK STATION AS YOU SEE IT AS A VERY LIGHT SIGNAL ON THE WATERFALL. IF A STRONG SIGNAL APPEARED ANYWHERE IN THE BANDPASS , ITS OVERWHELMING SIGNAL STRENGTH WOULD CAUSE THE AGC TO LOWER GAIN AND POSSIBLY ENOUGH TO SEE YOUR STATION IN THE WATERFALL DISAPPEAR AND LOSE IT ENTIRELY. MOST RADIOS HAVE BANDPASS FILTERS THAT CAN BE TUNED , SO TO HELP ELIMINATE THE STRONG SIGNAL FROM THE BANDPASS AND GIVING  THE STATION YOU ARE LISTENING TO A BIG BOOST. SO WHEN YOU HAVE A CHANCE TO PRACTICE STUDYING THIS ON A WATERFALL, YOU CAN ALSO TRY TURNING OFF THE AGC AND SEE THE RESULTS.
    AGC….. GREAT THING FOR RADIOS..PAUL K0YNE

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