Squelch
A squelch (English: Squelch ) is in communication receivers used to noise, especially noise during transmission pauses hide ( noise reduction process ).
Evaluation of the signal strength
With amplitude modulation , a comparator compares the output voltage of the envelope demodulator with a preselected value and only releases the LF signal when it is exceeded. If the signal is missing or too weak, there is no output. The comparison circuit must have a hysteresis so that the squelch does not "flutter". With single sideband modulation (SSB), there is no carrier signal, which is why a squelch built in this way does not work.
Evaluation of the noise component
In the case of frequency modulation , this method cannot be used because the gain of the intermediate frequency amplifier is so high that the amplitude is limited even without an input signal and cannot increase when a signal is received. Instead, a peculiarity of every FM demodulator is used: If the reception field strength falls below the FM threshold , the demodulator delivers very intense high-frequency noise. As soon as a critical signal strength is exceeded, this noise becomes considerably quieter or disappears completely. As soon as the transmitter is frequency-modulated, the demodulator delivers low frequencies with significantly lower frequencies.
The squelch therefore consists of a high-pass filter with a cut-off frequency above about 20 kHz and an amplifier with a subsequent rectifier . This delivers a few volts with noise, significantly less with modulation with speech or music. A downstream Schmitt trigger generates the switching signal.
literature
- Hans Lobensommer: Handbook of modern radio technology. Principles, technology, systems and practical applications. Franzis Verlag GmbH, Poing 1995, ISBN 3-7723-4262-0 .
- Stratis Karamanolis: All About CB. A manual for the CB radio operator. 2nd Edition. Karamanolis, Putzbrunn 1977.
- H. Meinke, FW Gundlach: Pocket book of high frequency technology. Volume 3: Systems. 5th edition. Springer Verlag, Berlin / Heidelberg 1992, ISBN 3-540-54716-9 .
- Frank-Werner Albrecht: PA technology. Theory and Practice, 3rd edition, expert Verlag, Renningen 1995, ISBN 3-8169-0424-6 .
Individual evidence
- ^ Otger Neufang: Lexicon of Electronics . Friedrich Vieweg & Sohn Verlagsgesellschaft, Wiesbaden 2002, ISBN 3-528-04099-8 .