RF Circuits

1) Take both channels and low-pass-filter them at 15kHz, with steep rolloff;
2) Apply pre-emphasis. Depending on the part of the world, it should have either a 75µs or a 50µs time constant;
3) Strictly limit the audio level to ensure that overdeviation cannot happen;
4) Create a stable, clean 38kHz sine wave;
5) Subtract the right channel from the left channel, and multiply the result with the 38kHz carrier;
6) Create a clean 19kHz sine wave, phase-locked to the 38kHz one;
7) Add the left channel, right channel, the (L-R)*38kHz signal, and the 19kHz signal, with specific amplitudes.

There are several ways to implement this algorithm. Modern factory made transmitters often do the whole thing digitally, in a DSP. But it's still less expensive and simpler to do in the analog domain. That can be done in various ways too, and far too many transmitters these days use ultra cheap, mediocre methods like hard-switched multipliers based on CMOS switches. They do work, but are very noisy! My design instead uses a true, high quality analog multiplier for that task. As a result, the signal from my transmitter is as good as the very best signals I can receive locally, and MUCH better than the bulk of them!

Here is the schematic diagram. You probably won't be able to read it at this resolution, so better click on it, save it in full resolution, print it, and refer to it for the following explanation. If you have trouble opening the large version, right-click on the diagram, so you can save it to disk, then open it using IrfanView or any other GOOD image viewer. This is valid for all drawings on this page. The full resolution drawings are large, and depending on the amount of memory in your computer, some web browsers cannot open them and will report a broken link.

The two single-ended line-level audio signals enter through feedthrough capacitors, and are welcomed by an LC low-pass filter to get rid of any RF that could be on them. In each channel there is a buffer stage, and then a combined pre-emphasis and soft limiter stage. The advantage of doing the limiting and the pre-emphasis in one step is that it avoids overdeviating from loud treble sounds, or having loud bass sounds flatten out the treble, without the need of a multiband limiter. The gain of the non-limited portion of the audio signals is adjustable by means of trimpots. Then comes a six-pole low pass filter that removes signals above 15kHz.

A 74HC4060 chip derives the 38kHz and 19kHz signals, as square waves, from a custom-made quartz crystal. Two resonant circuits using ferrite pot cores turn these square waves into very clean, low noise sine waves. Trimpots allow to set the levels, while the adjustable cores of the inductors allow precise tuning. Jumpers allow to disable each of these signals for testing and adjustment purposes.

A rather old fashioned, but low-noise and low-distortion analog multiplier chip modulates the L-R signal, produced by an op amp differential amplifier, onto the 38kHz subcarrier. This circuit has three adjustments for balance. Its output level is adjustable too. The signals that are necessary only for stereo can be disconnected for testing by means of a jumper.

The output adder combines the L signal, R signal, (L-R)*38kHz signal, and the pilot tone. The first two signals are fixed at this stage, while the (L-R)*38kHz can be adjusted by its own trimpot, and the pilot tone by the trimpot before its LC circuit. Then there is a final level adjustment, used to set the deviation of the transmitter, and then a buffer stage with low output impedance, that drives the output through a resistor to avoid instability from capacitive loads.

There is an additional circuit which consists basically of a dual superdiode detector with a time constant and driver with adjustable output. This circuit picks up the complete multiplex signal just before the final level control, and produces a DC signal to directly drive a small meter, for deviation indication. This is a most important tool for the transmitter operator to set the proper audio level during routine operation!

Here is the printed circuit board. Click on it to get it in high resolution.... It's seen "through the board", so you can print it directly and place the ink in contact with the copper to get a correct sided copper pattern.


The entire circuit is built on this single-sided PCB. Only a few jumper wires are necessary, so it's not worthwhile making a dual sided PCB for this.


And this is a crude parts overlay, just to see where a part goes. Exactly which part goes where, is something you will have to work out with the schematic! Don't be lazy! Source: FM Transmitter 80W

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