Compressor-Limiter-Clipper for FM transmitters

Posted on Aug 6, 2012

Audio signals as music or speech have big dynamic ranges. There are silent and loud sections. These audio signals aren't too good for a transmitter, which requires audio signal with constant level on the input. Limiter is a device, which weakens loud signals and intensifies silent signals. On its output there is signal with constant level. Signal clipping on the limiter output allows to increase the signal level without exceeding maximum frequency deviation limit 75 kHz. It's very suitable since preemphasis is used. If your transmission chain does not include any similar device, you should build this one. You will be surprised by its quality, efficiency and simplicity.

Compressor-Limiter-Clipper for FM transmitters
Click here to download the full size of the above Circuit.

Be careful if you want to buy any simple compressor/limiter board available on the market! Although a big list of features is mentioned, some of these toys have no signal overshooting protection and have no precise preemphasis with HF clipping option. With these devices it's not possible to keep loud sound AND meet the frequency deviation limit. So there is no reason why to pay for them. The device should guarantee basic technical characteristics of the modulation signal, nothing more - no equalizers and other disutilities. Characteristics: Absolutelly no signal overshooting (tested with PIRA75 FM Broadcast Analyzer) Low noise and distortion Simple to adjust Only a few parts to solder Precise preemphasis including HF clipping with no distortion audible Stereo version available here Supply voltage: 9-16 V Quiescent supply current (12 V): 15 mA Output voltage: linear adjustable 0-3.5 V p-p (0-1.2 V rms) Lower cut-off frequency (3 dB): input: 25 Hz, output: <2 Hz Upper cut-off frequency (3 dB): 14.5 kHz Min. input voltage: 0.6 V p-p (0.2 V rms) Input impedance: 5000 ohm Output impedance: 500 ohm Signal-to-noise ratio: >70 dB Part list: R1, R3 - 10k R2 - 1k R4, R5 - 1M R6 - 18k R7, R8, R15-R17, R19 - 33k R9 - 1M5 R10, R12, R14, R18 - 470R R11 - 270R R20, R23, R25 - trimmer 5k R21 - trimmer 5M R22 - trimmer 1k R24 - trimmer 500R C1 - 4n7 (EU) or 6n8 (USA), plastic C2 - 470n plastic C3 -...

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