RDS with Ramsey FM100B Transmitter

I modified Doug Hall's FM100B to connect to the Pira32 RDS encoder. The FM100B uses the same FM stereo and transmit signal chip (BH1415F) as the FM25B, but in a slightly different configuration. Therefore, I was able to use the knowledge I gained during my FM25B modification. The FM100B, however, is housed in a much larger, metal case, which makes it easier to build a clean modification.

I redesigned the interface circuit a bit, to this:

Circuit Description

The Pira32 specifications state that the pilot tone (MPX signal to RDS encoder) must be 100mV minimum. The pilot signal available on pin 5 of the BH1415F is much lower than that. With careful ajustment of the Pira32, it actually works, but I added the op-amp to make it more reliable. The op amp also acts as a buffer, preventing unwanted signals from being inserted. The 47k, 220p, and 100k resistors and capacitors act as a high-pass filter to let the 19kHz pilot tone through. The 4.7k and 100k resistors set the gain so that the 19kHz signal available is about 200mV.

The RDS input circuit have the resistor higher and the capacitors lower than suggested on the Pira32 site because the Ramsey transmitters seem to have more sensitivity in that area. This also provides less loading to the existing FM circuitry.

Construction

All modifications are connected to the top (component side) of the main PC board. There are solder pads available to find ground, +12V, and -12V. The components for the RDS input are soldered directly to the existing component leads, in order to keep the leads short. It is important to keep the leads of the resistor connected to D2, and the 1pF grounding capacitor short.

The MPX signal (from where the Pira32 derives the pilot tone) comes from pin 5 of the BH1415F. This is connected to R6, where it was easy to solder a wire.

The rest of the components were mounted on a scrap piece of copper-clad perf board.

Some hot glue made the line 2 level control into a good mounting point.

Next, it was time to get this circuit hooked up to the outside. I found a good location for the BNC jacks, drilled some holes, and mounted the jacks. The cables were connected to a 4-pin disconnect to still allow the cover to be removed.

Here's the back of the FM100B, with the new RDS jacks installed:

Testing

I listened to the audio before and after I did these modifications. It seems to sound the same to me. The RDS data was received correctly on the radio in my Toyota Prius.

Next

I did an even easier modification to a EDM LCD transmitter.