Low power FM Stereo Radio Station

Improved FM Stereo Modulator (Rev D)

New builders should go straight to the Rev E (2008) version of this circuit.

This page was last modified on April 14, 2008.

The following circuitry is used for generating a high quality FM stereo multiplex signal that is suitable for driving mono FM transmitters such as the Free Radio Berkeley "No Tune" PLL synthesized unit.

Be sure to download the FRB transmitter modifications file below.

Construction of this circuit should not be attempted without the use of a decent oscilloscope and an audio signal generator.

It is the operator's responsibility to run the transmitter in accordance with the frequency regulating authority of their country, it may be necessary to use an output attenuator on some transmitters.

Rev D Design

The Rev D FM stereo multiplex generator is the most recent design in a series of stereo multiplex circuits that have appeared on this web site. The older designs are now considered obsolete. The design philosophy for this circuit was centered on replacing all of the op-amp circuitry found in the earlier circuits with discrete transistors and LC filters. The previous circuits had many (high quality) op-amps in series, this resulted in audible distortion, especially on "ess" sounds. The discrete transistor amplification stages in this design produce an audio signal with much lower levels of distortion.

To make alignment of the stereo modulator easy, first build this Stereo Test Tone Generator, or generate equivalent waveforms with standard test equipment. The circuit produces a number of waveform combinations that are necessary for adjusting this circuit for proper operation.

The design phase of the new rev D FM Stereo Modulator circuit is about complete. The preliminary tests indicate that the circuit produces excellent quality stereo modulation.

This circuit has been tested with the FRB 1W No Tune Xmitter, the FRB 1W No Tune Xmitr Modifications are required to use this circuit with that transmitter.

Here is a photo of the hand-wired Rev D prototype circuit, The left board is the audio peak meter, the center board is the audio filter and the right board is the oscillator/multiplexer board.

One reader suggested this modification for the transistor gain setting circuits, the modification keeps the transistor's DC bias from changing as the gain is changed. He also suggested removing the 10K resistors from the inputs of the 15Khz low pass filters. These suggestion were all used in the Rev E circuit.

Here are the alignment instructions for the Rev D circuit.

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