Stereo Encoder for FM transmitters

Posted on Aug 10, 2012

This stereo encoder is a halfway between analogue and digital processing. It combines the best from both domains to provide high-quality and easy to build device. The sampling frequency used in this stereo encoder is 97 times (!!!) higher than the pilot tone frequency. This makes very easy to reject all spectral residues around the sampling frequency without affecting the main signal characteristics. Using of a microcontroller allows to build this stereo encoder with reduced part count and get excellent results in real operation.

Stereo Encoder for FM transmitters
Click here to download the full size of the above Circuit.

This stereo encoder advisedly does not contain any preemphasis circuit. Remember the key fact: a compressor/limiter/clipper device must be always present between the preemphasis circuit and the stereo encoder or modulator. Only this configuration ensures loud sound without exceeding the maximum frequency deviation limit (75 kHz). The stereo encoder is designed to provide really good sound. This always needs to use the compressor/limiter/clipper device where the preemphasis is precisely assured. The Pira CZ Compressor/Limiter/Clipper is highly suitable for this task. Characteristics: Signal overshooting prevention (overshooting max. 10 %) Low noise and distortion No adjust (only pilot tone level and output level) High channel separation and spectral purity in common operation RDS input and pilot sync. output makes easy to connect any RDS encoder Microcomputer controlled, hex file provided for free All parts are easy to buy (incl. crystal) Supply voltage: 9-16 V (stabilized) Quiescent supply current (12 V): 34 mA Audio 19 kHz rejection: 40 dB Channel separation at 1 kHz: >55 dB Subcarrier rejection: >60 dB Pilot sampling frequency: 1.843 MHz (19 kHz x 97) Subcarrier sampling frequency: 1.843 MHz (38 kHz x 48.5) Pilot sync. output: TTL Max. audio input voltage: 5 V pp (1.75 V rms) Pilot tone level: linear adjustable 0-0.5 V pp Output voltage gain: linear adjustable 0-1.5 Audio input impedance: 2000 ohm...

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