FM radio with PLL

Radio, 1986, 05

This radio receiver with PLL is capable of receiving FM signals in Russian VHF band of 65.8 MHz to 73.0 MHz. This band can be changed to the standard band of 87.5 MHz to 108.0 MHz by reducing number of turns of coils L1, L2. The receiver has low power voltage (6 V) and improved thermostability.

The circuit diagram of the FM radio receiver with PLL

Fig. 1
C1, C2 - 15 pF; C3, C7, C10 - 1 nF; C4, C5 - 10 nF; C6 - 100 pF;
C8 - 18 pF; C9 - 9...360 pF; C11 - 150 pF; C12 - 47 nF; C13 - 2.2 nF;
R1, R3, R6 - 1.5k; R2 - 1.2k; R4* - 10k; R5 - 220 Ω; R7 - 47 Ω;
R8, R12, R13 - 6.8k; R9 - 120k; R10 - 2.2k; R11 - 62k;
L1 - wound on a resistor of 1 Watt with resistance ≥ 100 kOhms, 8 turns of AWG 24 (0.5 mm);
L2 - 9 turns of AWG 29 (0.3 mm), on a former of 5..8 mm in diameter;
VD4 - KVS111A - variable capacitance double diodes with Cd = 30 pF at Vrev = 4 V, f = 1 MHz; Q = 200 at Vrev = 4 V, f = 50 MHz; Cd (4 V)/Cd (30 V) = 2;
VT1, VT2 - KT316B, si n-p-n, Ff = 800 MHz, hFE ≥ 40;
VT3 - KT342B, low noise si n-p-n, hFE ≥ 200;
VT4 - KT361B, si p-n-p, hFE ≥ 100;
VD1 - KD521V, VD2, VD3 - KD503A - si HF diodes.

The circuit diagram is shown in Fig. 1. The signal, received by the aerial telescopic antenna WA1, goes to the input resonant tank L1C1C2, it is tuned to the middle of the FM band (69.4 MHz), and further through the network R2C3 the signal goes to a mixer, composed of two diodes VD2, VD3 in anti-parallel connection. The heterodyne is based on the multivibrator circuit with output signal stabilization by collector junctions of transistors VT1, VT2. The frequency of the heterodyne resonant tank L2C8C9VD4 is two times lower than the input frequency. For example, if we tune the receiver to the radio station of 70 MHz, so the heterodyne frequency is 35 MHz. A variable capacitor C9 is used to change the frequency of the heterodyne. A variable capacitance double diodes VD4, connected across the resonant tank, is used to provide auto-tuning.

The output of the heterodyne circuit is passed through a network R5C6 to the mixer circuit. The resistor R5 is used to avoid possibility of signal conversion by using harmonics of the heterodyne. It's greatly improves stability of the system mixer - heterodyne while tuning the radio across the band.

From the output of the mixer, the signal is feed through the resistor R6 to the input of the DC amplifier, the amplifier circuit is based on transistors VT3, VT4. The initial bias voltage across diodes of the mixer is corresponded to the bias voltage at the input of the DC amplifier. The diode VD1 and resistors R2, R4 provide this bias voltage. The resistor R12 works in the loop of the phase-locked system, the voltage from the output of the DC amplifier is feed through this resistor R12 to the variable capacitance double diodes. A zero voltage across the variable capacitance double diodes is provided by the resistor R8.

The ladder filter, based on R7, C7, R11, C11 and capacitance of the collector-base junction of the transistor VT3, provides stable work of the tracking system while amplitude of input signal changes.

From the output of the DC amplifier, the audio signal goes through a network R13C13 to the input of an audio amplifier.

A thermal stability of voltage in the node of the connection R2 and R4 is determined by a p-n junction of the diode VD1, in the system mixer - heterodyne it is determined by p-n junctions of diodes VD2, VD3 and transistors VT1, VT2, an input voltage of the DC amplifier is determined by the base-emitter junction of the transistor VT3, therefore all voltages are related to the balance of the DC amplifier will be changed in one direction with a temperature changes, so the system is stable enough. An additional stability is provided, as it was mentioned before, by the resistor R5 and negative feedback network R14 of the DC amplifier, the resistor R14 limits the gain to the value of 1000...2000.

The circuit is mounted on a 103x23 mm board (see Fig. 2.).

FM radio receiver with PLL mounted on a board

Fig. 2

The coil L1 is wound on a resistor of 1 Watt with resistance not less than 100 kOhms. The coil has 8 turns of AWG 24 (0.5 mm). The coil L2 has 9 turns of AWG 29 (0.3 mm), it is wound on a former of 5..8 mm in diameter. All transistors and diodes are silicon, VT1, VT2 - high frequency VHF transistors, VT3 - a low noise transistor with hfe ≥ 200, VT4 - a common transistor with hfe ≥ 100. VD1 - VD3 - high frequency diodes.

It's easy to adjust the circuit. Disconnect the antenna, match the resistor R4 to get the voltage of 2.5...3 V across the output of the DC amplifier. After that, connect the antenna, and set up the frequency of the heterodyne by changing space between turns of the coil L2.

Using this receiver In Sverdlovsk (in 1986 year), it was available to receive all FM radio stations of this region. Because of the very wide bandwidth, some radio station could be skipped while tuning. It is can be fixed by reducing a length of the telescopic antenna.

The circuit consumes 4.5 mA, the minimum operating voltage is 4.5 volts.

I. Pogartsev, Sverdlovsk

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