Vest Pocket VHF FM Test Generator Schematic Circuit Diagram

After licensing restrictions were relaxed in many countries for VHF FM band transmitters with 50 NW transmit power, several small, inexpensive FM transmitter modules appeared on the market. In the author’s view, such a module forms a good basis for a small FM-band test generator. This only requires a sine-wave modulation signal, which can come from an existing audio generator. If you don’t have a suitable audio generator available, you can build the Wien bridge oscillator described here.

FET T1 provides amplitude stabilization in order to keep the distortion low. The generated signal is fed to the transmitter module via a 3.5-mm stereo headset socket, which mates with the usual 3.5-mm stereo plug of the FM transmitter (the left and right terminals of the socket are wired together). Adjust the output level of the audio oscillator with potentiometer P1 to avoid overdriving the transmitter.

In the transmitter module used by the author, the HF stage is built around a Rohm BH1418FV IC. You can easily find the datasheet for this IC with a Google search, and it can help you identify the RF output on the transmitter circuit board. You can then use a length of coax cable to tap off the FM signal and feed it to the antenna connector of the receiver under test. Here you must pay attention to the maximum rated input level of the receiver and impedance matching, and if necessary you should use an attenuator at the receiver input. You can use an oscilloscope to track the signal in the receiver and analyze the receiver output signal.