Model 10811A/B44Figure 8-6. EFC8-20. AUTOMATIC GAIN CONTROL (AGC)8-21. The output for the AGC (and output amplifiers, discussed later) is taken across capacitor C10,and is applied to Q3. Since C10 is effectively in series with the crystal, the current passingthrough the crystal also passes through C10. The voltage across C10 is therefore proportional tothe current through the crystal. As the output of the oscillator changes, the output of the peakdetector circuit changes. This change in the AGC voltage changes the voltage applied to the baseof Q1 and stops the impending output voltage change. A peak detector circuit formed by C12, CU,CR4 and CR5 is used to develop a dc voltage to control the crystal current. This negative controlvoltage forms the lower half of a voltage divider for the base of Q1 (R6 and R7). Controlling thebias current and the gain of Q1. Thus AGC action controls the output signal level. (See Figure 8-7).Figure 8-7. Automatic Gain Control (AGC)8-22. By adjusting the AGC voltage with R6 the amplitude for the output (at the base of Q3) can be set.R5 sets the AGC limit when R6 is at its minimum resistance.
