What is Squelch
This page describes squelch basics and mention function of squelch circuit.
The term " squelch " means suppress or crush completely. The concept is applied in receiver where in cudio part is turned off completely until RF signal is appears at the input.
As we know if receiver in the mobile phone is kept on continuously, it will receive noise in the absence of call. This noise level is usually high and will also get amplified and generates annoying noise. Most of the people do not like to listen such noises. Hence squelch circuit has been developed which turns on the receiver only when useful call is present at the input and keeps the receiver off when noise signal is present. Let us see how squelch circuit functions as explained below.
Function of Squelch circuit
As depicted in the figure, there are two main parts in a squelch circuit viz. high pass filter and audio amplifier. There are two transistors Q1 and Q2. Q1 acts as squelch gate which drives the base of the Q2 transistor.
As we know most of the audio frequencies are below 4KHz. The high pass filter is designed to pass the frequencies above these audio frequency range. Hence when audio is present, no squelch voltage is developed at the receiver output. Here Q1 transistor is cut off and hence Q2 transistor gets biased as usual. As a result audio signal is passed through the audio power amplifier and to the speaker.
Let see how squelch circuit functions when noise is present.
As we know most of noise signal is of high frequency, it is amplified by two transistor stages and later rectified into DC control voltage
by rectifier-voltage double circuit(made of D1, D2, C2, C3).
This rectifier output will drive Q1 transistor to saturation region. The base current of Q2 transistor is shunted away from Q1. Hence no audio amplification takes place and receiver will be quiet. This squelch circuit is widely used in communication receiver. It is also known as mute circuit.
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