Simple VHF FM radio

Posted on Feb 5, 2014

The RF stage of the receiver is based on a transistor VT1, this stage works as a mixer and a local oscillator and it works as a synchronous detector. A cable of the headphones works as an antenna. A signal received with this antenna is fed to the input resonant tank circuit L1C2 which is tuned to a center frequency 70 MHz of the VHF band (65. 8. 74.

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0 MHz - it`s the first FM band in Russia). From a tap of the coil L1 the RF signal goes to the base of the transistor VT1. The transistor VT1 is configured as a common-base circuit to work as the local oscillator, and in the same time the transistor VT1 is configured as a common-emitter circuit to work as a frequency converter. The local oscillator works in the frequency range of 32. 9. 36. 5 MHz, so the frequency of its second harmonic is within the range of the VHF band 65. 8. 73 MHz. The frequency of the resonant tank L2C5 is two times lower than the frequency of the input resonant tank L1C2, so the conversion occurs at the second harmonic of the local oscillator frequency, and because of this the resulting frequency will be in audio frequency range. The amplification of the audio frequency provides with the same transistor VT1 which is configured for the audio signals as a common-base circuit (the base is shunted by a capacitor C4). The audio amplifier of the receiver has two stages (VT2, VT3). A preamplifier stage is composed of the transistor VT2, a power amplifier is based on the transistor VT3. The load of the last stage is the headphones BF1 with resistance of 50 ohms. If the circuit is powered with a 1. 5 volts battery, then the output power is 30 mW at the load of 8 ohms. The current consumption of the circuit is less than 10 mA. The circuit can be mounted in any suitable housing. Coils L1, L2 are wound on a...

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