A squelch (English: Squelch) is used in communication receivers to hide noise, especially noise during transmission breaks ( noise reduction method ).
Evaluation of the signal strength
In amplitude modulation, a comparator compares the output voltage of the envelope curve with a preselected value and outputs the LF signal only above free. If there is no or weak signal is not output. The comparison circuit has to have a hysteresis, so that the squelch is not " flapping ". For single-sideband (SSB ) no carrier signal is present, which is why a squelch thus constructed does not work.
Evaluation of the noise component
With frequency modulation, this method is useless, because the gain of the intermediate frequency amplifier is such that the amplitude is limited, and no input signal when the signal reception can not be larger. Instead, a character of each FM demodulator is exploited: the reception field strength falls below the FM threshold, the demodulator provides very intensive high frequency noise. Once a critical signal strength is exceeded, this noise is greatly reduced or eliminated altogether. When the transmitter is frequency modulated, provides the low-frequency demodulator with significantly lower frequencies.
Therefore the squelch circuit comprises a high pass filter with a cutoff frequency of about 20 kHz and an amplifier followed by a rectifier. This provides significantly less noise at a few volts, at modulation with speech or music. A downstream Schmitt trigger generates the switching signal.