Electronics Projects

FM Stereo Transmitter

FM stereo transmitters projects based on Rohm BH1417F chip are published many times on the Internet, see some links below. I wanted to repeat one of them but faced with several problems. The schematics of each of those projects basically follows the one from the Rohm datasheet for BH1417F. However, several components there are either obsolete or very difficult to find. First of all, this concerns the 7.6MHz crystal. It is reported on the Electronics DIY web site that that crystal can be replaced with a 7.68MHz one which is much less difficult to find. I tried it and found the parameters unsatisfactory. The generated this pilot tone is 19.3KHz instead of 19KHz and this affects the transmission quality. Also, I had problems by receiving the signal with a digital FM receiver. The transmission frequencies were shifted quite a bit, so I was not able to catch them properly. Replacing this crystal with a proper one for 7.6MHz influenced tremendously on the sound quality and PLL stability.

Another problem is that the coil recommended in the datasheet is not manufactured any more. I replaced it with a 52.3nH coil from the MC122 series manufactured by Toko America Inc. (DigiKey part #TK2802-ND). And the last big problem was the varicap. The ones that were used in similar projects on the Internet are not available for me. I used MMBV2109LT1 manufactured by ON Semiconductor, which is very well spread out here, in the US. However, with those coil and varicap I was not able to cover the entire FM band without tuning the coil with its built-in ferrite core. One can easily cover either the lower or the upper part of the FM band without tuning the coil, but not both. This varicap requires different capacitors than the ones from the datasheet. I used 22pF cap which is connected parallel to the coil and a 100pH cap connected in series with the varicap. I have experimented with a lot of home-made coils, including the printed ones as on the C.K.RF-DESIGN'S web page. None of them covered the entire FM band without additional tuning. A part of the problem was in the used varicap. Its capacitance varies only in a factor of 2 in the voltage range 1 - 4.5V. This working range was selected in order keep the NPN Darlington transistor away from the saturation point 0.6V and also have a reliable offset form the chip voltage of 5V. Using other varicaps with the capacitance ratio of 10 brings other problems. Those varicaps have extremely non-linear characteristic, which results in an over-modulation and sound distortion. With those varicaps one needs to adjust the audio volume by tuning from one frequency to the other. So, I gave up covering the entire FM band and use only its upper part. The coil and varicap combination mentioned above appeared to be an optimal choice.

Surprisingly, all projects based that chip which I found use mechanical switches for tuning. It looks like the switches are used just once to select a free frequency in a local environment and then are buried inside a box. I used a tiny microcontroller instead of the tuning switches and this way the transmitting frequency can be easily selected by pressing a single button. The schematics below shows an interface with the PIC and the output filter. I did not put any adjustting pots. In my design I use the upper part of the FM band, so the frequencies vary from 106.7MHz to 107.9MHz with a 0.2MHz step (all together 7 fixed frequencies). My intention is to use this transmitter for playing MP3s over the car radio, so a possibility to operatively change the transmitting frequency is important if you travel long distances.

Schematic Side view Front view

The last part which was impossible for me to find is the filter at the output of the transmitter. I was unable to find the recommended one in the datasheet. However, fortunately I found an alternative solution of building the filter on discrete components. This design also includes an attenuator that reduces the output level down to a legal value. The above shown PCB was initially supposed to be a development board. So the filter was added later and it is mounted on a separate small PCB between the coil and the crystal. The three resistors belonging to the attenuator are mounted directly on the power connector. The transmitter is assembled in a standard 2.5x1.5x1.1" plastic box. The front panel carries an audio input jack and the tuning button. The back side has the power connector on in. The filter output is connected to the +12V terminal, so the power cable is used as a long antenna. The PCB is fastened with only one screw inside the box. However, several rigid wires connect the PCB with the power and audio jacks, so the entire design is pretty stable.

Power input Front panel Completed assembly

As a helpful addition I recommend to save the last set frequency in the PIC's EEPROM.


Last modified:Mon, Jan 23, 2023.