Detectors for PLL receivers

Radio 1987, 5

The detectors (Fig. 1 and 2) for FM direct conversion receivers with PLL (phase-locked loop - see [1]) are made on the basis of a similar device described in [2]. However, unlike the prototype, this two detectors have better sensitivity. In addition, the detector circuit shown in Fig. 2 has better selectivity.

The FM demodulator circuit shown in Fig. 1 is a frequency converter in combination with an local oscillator. The FM demodulator also works as a synchronous detector. The input resonant tank circuit L1C2 is tuned to the frequency of a received signal, and the local oscillator circuit L2C5 is tuned to the frequency less than input frequency in two times. The conversion occurs at the second harmonic of the oscillation frequency, so the intermediate frequency will be brought to an audio frequency. The transistor VT1 itself controls the frequency due to changes in the collector junction capacitance, this capacitance depends on the output signal of the detector. The use of inductive coupling in the feedback path of the local oscillator (L3) has allowed to get rid of the feedback resistor in the emitter lead of the transistor VT1 [2], so, this provides higher amplification of the detector in audio frequency range, i.e., it increases sensitivity of the circuit up to 50...100 μV. The gain of the cascade at this frequency
Ku = (R3 + Rin)Se, where
Rin - an input impedance of an amplifier, which is connected to the detector,
Se - an equivalent slope of the transistor VT1.
When the frequency conversion occurs the second harmonic of the local oscillator frequency, the value of Se reaches its maximum when the cut-off angle equal to 60°. This angle is set by adjusting the coupling coefficient between the coil L2 and L3. To keep a relatively stable frequency tracking when receiving very weak signals, the cutoff frequency of a low pass filter formed by the capacitor C4 and the equivalent input impedance of the transistor VT1, must be lower than the minimum frequency range of the detected audio signal. This condition is satisfied if the capacitance of the capacitor C4 is not less than 50 microfarads.

FM demodulator circuit for DCR PLL radio

Fig. 1
VT1 - GT313B (USSR germanium p-n-p transistor, hFE = 20...80, ft ≥ 450 MHz);
C1 - 6.2pF; C2, C5 - 6...25pF; C3 - 15nF; C4 - 50μF x 6V; C6 - 5μF x 10V;
R1 - 120K (tweak this value); R2 - 9.1K; R3 - 2K;
L1C2 is tuned to the F (65.8...73 MHz), L2C5 is tuned to the F/2 (32.9...36.5 MHz); L2, L3 - coils with the brass slug.

In the detector can be used transistors GT313 (p-n-p) and GT311 (n-p-n) with any letter index (to use the transistor GT311 (n-p-n) it is necessary to change the polarity of the power supply and the electrolytic capacitors). Coils are wound on a cardboard formers with a copper wire 0.27 mm (AWG 30), the coils are close wound. The cardboard formers of the coil L1, L2 has the diameter of 6 mm and for the coil L3 the cardboard former has 7 mm in diameter. The coil L1 has 5 turns and is tapped at two turns from the lower end, the coil L2 has 15 turns and the coil L3 has 5 turns. A brass slug (a screw with thread M5 placed inside the frame of the coil L2) is used for tuning the resonant tank circuit of the local oscillator. An antenna is a piece of wire 1 meter long.

Before aligning the circuit, replace the resistor R1 with a potentiometer with a value of 200...300 kilohms, make the maximum coupling between the coils L2 and L3 (move the coil L3 to the coil L2). After that, adjust the potentiometer to get a voltage at the collector of the transistor VT1 of 0.8...0.9 volts (measure the voltage across the capacitor C3). Then connect the detector to an audio amplifier with loudspeaker, use the capacitors C2, C5 and the slug of the coil L2 to tune the circuit to a powerful VHF radio station. By changing the position of the antenna WA1 get the best of reception. Next, reduce the coupling between the coils L2 and L3 and while maintaining the constant voltage at the collector of the transistor VT1, again tune to the same radio station. Repeat this manipulations to get the locking range of a radio signal as wide as possible. After that, use the capacitor C5 to set up the desired range of the tuning (the tuning range should be matched to the broadcast VHF band of 65.8...73 MHz), and use the capacitor C2 to tune the input resonant circuit to the middle of this broadcast VHF range.

Along with the high sensitivity, this detector has significant drawbacks: low selectivity and a significant variation of the gain across the tuning range due to the change of the operation mode of the transistor VT1.

The detector circuit shown in Fig. 2 is designed to receive the only one radio station. The selectivity and sensitivity of this device has increased due to the introduction of a positive feedback. The coil L2 is connected in this feedback loop and has one turn of copper wire of diameter 0.27 mm (AWG 30). This coil L2 can be moved along the coli L1 (all the coils is the same as in the previous circuit shown in Figure 1). While aligning this circuit, the coil L2 should be moved away from the coil L1, then tune the circuit to a powerful VHF radio station and move the coil L2 to the coil L1 while there is no distortion. If distortion appears, then move the coil L2 just a little back. Using the method described earlier, adjust the detector with the feedback, and one more time try to reduce the distance between the coils L2 and L1. Repeat it again to minimize the distance between them.

Improved FM demodulator circuit with PLL

Fig. 2
VT1 - GT313B (USSR germanium p-n-p transistor, hFE = 20...80, ft ≥ 450 MHz);
C1 - 6.2pF; C2, C4 - 6...25pF; C3 - 50μF x 6V; C5 - 15nF; C6 - 5μF x 10V;
R1 - 120K (tweak this value); R2 - 2K; R3 - 9.1K;
L1C2 is tuned to the F (65.8...73 MHz), L3C4 is tuned to the F/2 (32.9...36.5 MHz); L4, L5 - coils with the brass slug.

To avoid microphonics it is necessary to make the construction very rigid and axis of the coils should be perpendicular to each other. Keep in mind that this detectors is very sensitive to the ripple voltage, so if self oscillation occurs, then use a power supply with voltage regulator.

S. Chekcheev

REFERENCES

  1. Polyakov V. T. FM Radio broadcast receivers with PLL. M., Radio and Communications, 1983.
  2. A. Zakharov. FM receivers with PLL. Radio, 1985, 12

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