SL623 AGC Generator

AM Detector, AGC Amplifier and SSB Demodulator

SL623C AGC Generator Plessey


  • Negligible Distortion
  • Easy Interfacing
  • Fast Response Time
  • AM SSB Receivers

Quick Reference Data

  • Test Equipment
  • Supply Voltage: 6V
  • Maximum Operating Frequency: 30MHz
  • Ambient temperature: -55°C to +125°C


The SL623C is a silicon integrated circuit combining the functions of low distortion AM detector and AGC generator with SSB demodulator. It is designed especially for use in SSB/AM receivers in conjunction with SL610C, SL611C, and SL612C RF and IF amplifiers. It is complementary to the SL621C SSB AGC generator.
The AGC voltage is generated directly from the detected carrier signal and is independent of the depth of modulation used. Its response is fast enough to follow the most rapidly fading signals. When utilised in a receiver comprising one SL610C and one SL612C amplifier, the SL623C will maintain the output within a 5dB range for a 90dB range of receiver input signal.
The AM detector, which will work with a carrier level down to 100mV, contributes negligible distortion up to 90% modulation. The SSB demodulator is of single balanced form. The SL623C is designed to operate at intermediate frequencies up to 30MHz. Also, it functions at frequencies up to 120MHz with some degradation in detection efficiencies.

Input Conditions
The input impedance is about 800 ohms in parallel with 5pF. The connection must be made to the input via a capacitor to preserve the DC bias. An input of about 125mV RMS is required for satisfactory carrier AGC performance and 20mV RMS for SSB detection. Generally, the AGC will cope with this variation, but in an extreme case, a receiver using an SL623C and having the same gain to the detector in both AM and SSB modes will be some 10dB less sensitive to AM.

AGC Generator

Pin 3, the AGC amplifier phase correction point should be decoupled to ground by one microfarad capacitor (C4), keeping leads as short as possible. The value of C4 is quite critical, and should not be altered: if it is augmented the increased phase shift in the AGC loop may cause the receiver to become unstable at low frequency and, if it is reduced, the modulation level of the incoming signal is reduced by fast-acting AGC.
The AGC output (Pin 4) will drive at least two SL610/11/12 amplifiers. The AGC output is an emitter follower similar to that of the SL621C. Hence the outputs of the two devices may be connected in parallel when constructing
AM/SSB systems.
Less signal is needed to drive the SSB demodulator than the AM detector. In a combined AM/SSB system, therefore, the signal automatically produces an SSB AGC voltage via the SL621C as long as a carrier (BFO) is present at the input to the SSB demodulator of the SL623C. The AGC generator of the SL623 does not contribute in such a configuration. 
For AM operation the BFO must be disconnected from the carrier input of the SSB demodulator. In the absence of an input signal, the SL621C will then return to its quiescent state. To switch over a receiver using the SL623C from SSB to AM operation it is, therefore, necessary to turn off the BFO and transfer the audio pick-off from the SSB to the AM detector. 
Neglecting to disconnect the SSB carrier input during AM operation can result in heterodyning due to pick-up of the carrier on the input signal. In some sets different filters are used for AM and SSB; these will also need to be switched.
The 10kΩ gain setting preset potentiometer is adjusted so that a DC output of 2 volts is achieved for an input of 125mV RMS. There will then be full AGC output from the SL623C for a 4dB increase in input. A fixed resistor of 1.5kΩ can often be used instead of the potentiometer.

SSB Demodulator

The carrier input is applied to Pin 6 via a low-leakage capacitor. It should have an amplitude of about 100mV RMS and low second harmonic content to avoid disturbing the DC level at the detector output.
Pin 8 is the SSB output and should be decoupled at RF by a 10nF capacitor. The output impedance of the detector is 3kΩ, and the terminal is at a potential of about +2V which may be used to bias an emitter follower if a lower output impedance is required. The input to the audio stage of a receiver using an SL623C should be switched between the AM and the SSB outputs; no attempt should be made to mix them. Since the SL621C is typically used in circumstances where low-level audio is obtained from the detector, the relatively high SSB audio output of the SL623C must be attenuated before being applied to the SL621C. This is most easily done by connecting the SL623C to the SL621C via a 2kΩ resistor in series with a 0.5 microfarad capacitor.