Understanding Squelch Circuits: Suppressing Noise in Radio Receivers

The term “squelch” refers to the act of suppressing or completely crushing something. In the context of radio receivers, squelch is a circuit that mutes the audio output in the absence of a desired RF signal.

The main idea behind squelch is to avoid the annoying noise that a receiver produces when no signal is present. Imagine a mobile phone constantly outputting static in the absence of a call. Nobody wants that! Squelch circuits solve this problem by activating the receiver’s audio output only when a valid signal is detected.

Function of Squelch Circuit

Let’s delve into how a squelch circuit works.

Image alt: squelch circuit

As shown in the figure above, a squelch circuit typically consists of a high-pass filter and an audio amplifier, along with transistors Q1 and Q2. Q1 acts as a squelch gate controlling the base of transistor Q2.

When Audio is Present

Most audio frequencies fall below 4 kHz. The high-pass filter is designed to block these lower frequencies. Therefore, when a legitimate audio signal is present:

• No squelch voltage develops at the receiver output.
• Transistor Q1 is cut off, allowing transistor Q2 to be biased normally.
• The audio signal passes through the audio power amplifier to the speaker, and you hear the sound.

When Noise is Present

Noise signals generally contain higher frequencies. Here’s how the squelch circuit responds to noise:

• The high-frequency noise is amplified by the two transistor stages.
• This amplified noise is then converted into a DC control voltage using a rectifier-voltage doubler circuit (comprising D1, D2, C2, and C3).
• The DC voltage drives transistor Q1 into saturation.
• The base current of transistor Q2 is shunted away from Q1.
• As a result, no audio amplification occurs, and the receiver remains silent.

In essence, the squelch circuit acts as a gatekeeper, allowing audio signals to pass while blocking unwanted noise. This type of circuit is common in various communication receivers. You might also hear it referred to as a “mute circuit.”

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