Surfing the web, I found a small interesting FM transmitter, the RDVV 300mW in a shareware version, this could be the basis for a fine little FM test transmitter. The transmitter frequency is controlled by a Philips TSA5511 PLL IC through I2C communication, very clever engineered.
We also wanted to be able to transmit in stereo and with RDS, and found the pira.cz website, which has a very fine high quality stereo encoder and they also sell programmed RDS IC's, we choose the MRDS192. Combining these three parts has resulted in our test stereo FM transmitter with RDS and we are very pleased with the finished result.
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The front of our finished FM transmitter. It is very easy to operate: '-' or '+' to decrease or increase the transmitter frequency with 100KHz, then 'sel' (select) to tune the transmitter to the frequency on the display and 'sto' (store) to save the frequency on the display as a new power on start frequency.