Building a complete stero transmitter would be easier than I thought. I discovered a number of IC types that contain a complete stereo encoder and even a PLL-controlled FM transmitter. These chips are used in large quantities in the tiny mini FM transmitters you can buy to play the music from your MP3-player on your car stereo.
I found out that ELV are offering an FM signal generator kit, “SUP1”, based upon one of those chips. It is using the BH1415 chip from Rohm. This ic contains a stereo multiplexer with two filter pre-amps. The 19 and 38 kHz signals are derived from a 7.6 MHz crystal oscillator. The crystal oscillator is also the reference for the PLL frequency synthesizer that has a serial input to set the frequency multiplication factor. The frequency can be set anywhere from 87 to 108 MHz.
My kit arrived by mail a few days later. It was very easy to assemble: ELV have already soldered all the SMD components to the PCB, leaving only the wired components and switches and connectors for me to solder.
Now I wanted to hear some music. The guys from ELV are warning that it is illegal to connect the generator to an antenna. So I used a balun to connect the generator directly to the antenna input of my Philips B8X44A FM stereo receiver.
Now something odd happened. When I tuned to the generator frequency, I only heard white noise. But when I pulled the balun from the antenna input, I got strong reception of the carrier. Then I connected an MP3 player to the input and heard the music. After some experimentation, I concluded that the signal from the test generator was just too strong for the radio. The strong signal stops the local oscillator of the FM tuner, so that no frequency conversion takes place and only the noise of the IF amplifier can be heard. I tried this on another Philips receiver that has a similar FM tuning stage, my A5X83A AM/FM tuner and got the same result: noise if the generator is connected to the antenna input, and reception when it was disconnected. So I needed to use an attenuator between the test generator and the radio.
I tried the test generator on a few FM stereo radios to judge its performance. I found it works ok, but the sound level is somewhat weak. In stereo mode, distortion and hissing noises can be heard. Channel separation is acceptable, but if you connect a tone to one channel and listen to the other, you hear a weak tone with distorsion and noise.
Well, after all, this is only a simple stereo encoder. Doing some more research, I found that people who really care about putting a decent FM stereo signal on the air, such as the Pira.CZ pirate radio site, hate the BH1415's performance and prefere a more complex DYI stereo encoder.
Albeit simplistic, this test generator looks like it will be useful for checking FM tuners and stereo decoders. But the signal wasn't good enough to set the channel separation of the Grundig decoder. Anyway, nice try.