This NBFM VHF transceiver was orginally designed for a work collegue, who wished me to design a simple radio mic transmitter and receiver circuit. The first design resembled a WBFM bug and 7000 RX IC which was very unstable and drifted every time the antenna was waved. I then designed this simple NBFM crystal controlled transmitter with a AF bandwidth of about 14KHz and picked the signal on a converted Air band receiver with the audio output being fed into a Kareoke machine. This worked fine untill the requirement spanned to 4 radio mics.
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The solution was to build 4 receiver circuits and mix them into one amplifier. As the range was to only be a few metres, the receivers could be simple. I used a NE612 mixer IC for the XTAL Oscillator mixer and 455KHz I/F take off. A ready made ceramic 455KHz filter was handy and the O/P fed almost directly to the audio mixer/Output IC. Even though the bandwidth seems limited after being fed into the kareoke machine, it was quite good. The crystals were robbed from an old 5 Channel toy walkie-talkie and the transmitters were built into large handheld dynamic microphones with the insides empied out. The on/off switches perfectly placed. 9V batteries would not fit inside, but 12V car alarm batteries did and lasted about 4 hours use. The supply for the reciever is just a 300mA Plug in the wall adapter. If you use a 1.5metre telescopic antenna with the reciver and 10cm wire antennas for the transmitters 10 to 20 Metres range shoud be reached. I have only made this unit for 4 channels, more channels could be added but keep them well spaced as I noticed one transmitter gave me a 20KHz deviation.
This page was updated on 31st July
2000.