Simple VHF FM radio

Radio 1990, 11

The design of this VHF FM receiver (see figure 1) is based on the circuit of the direct conversion receiver with PLL.

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...74.0 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).

Simple VHF FM radio circuit schematic

Fig. 1.
VT1...VT3 - KT315B (hfe = 50...350 at Uke=10 V and Ik=1 mA), (hfe = 2.5 at Uke=10 V and Ik=1 mA and F=100 mHz)

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 former with a 5 mm outside diameter. The coil L1 has 6 turns of wire of diameter 0.56 mm (AWG 23), the coil is tapped at the center point. The coil L2 has 20 turns of wire of diameter 0.56 mm (AWG 23). Coils L3, L3 has 200 turns of wire of diameter 0.06 mm (AWG 42), they are wound on a ferrite slug of diameter 2 mm, 10 mm long and a permeability of 400. Transistors KT315B can be replaced with 2N2222.

The adjustment of the circuit starts with the audio amplifier. Tweak the resistor R6 value to get the current of the transistor VT3 around 6..9 mA. Tweak the resistor R1 to adjust the operation point of the local oscillator. Tweak the capacitor C6 to adjust the level of the second harmonic of the local oscillator. Adjust the inductance of the coil L2 (by stretching and squeezing the coil) to set up the boundaries of the frequency band. Adjust capacitor C2 to tune the resonant tank L1C2 to the middle of the VHF FM band. The capacitor C7 is used for tuning.