FM receivers with PLL
Radio 1985, 12
This article describes some simple FM direct conversion radio receivers with phase-locked loop (PLL). This receivers uses the method of locking the local oscillator frequency with the input signal [1].
All this FM receivers are based on the circuit shown in fig. 1. This is a combined oscillator and mixer, and it works as a synchronous detector. The input tank circuit L1C2 tuned to the frequency of the received signal, and the local oscillator circuit L2C6 tuned to a frequency less than input frequency in two times. The conversion occurs at the second harmonic of the local oscillator frequency, so the resulting frequency will be in audio frequency range. The transistor VT1 provides the frequency control of the local oscillator, the output conductance (it shunts the resonant tank circuit L2C6) of the transistor depends on the collector current, therefore it depends on the output signal of the receiver.
Fig. 1
VT1 - GT311E (an old USSR Germanium high-frequency transistor with hfe=15...80 (at DC: Uke=3 V, Ie=15 mA), hfe=2.5 (at f=100 mHz, Ukb=5 V, Ie=5 mA), Ft = 250)
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. From the wide band resonant tank circuit L1C2 the input signal is applied to the base of the transistor VT1. The resonant circuit L1C2 tuned to the middle of the VHF FM band (70 MHz). The local oscillator tuned to the frequency range of 32.9...37 MHz, so the frequency of its second harmonic lies in the frequency range of the VHF broadcasting band (65.8...74 MHz - this is the first FM band in Eastern Europe).
The efficiency of the detector depends on the level of the second harmonic of the local oscillator in the collector current of the transistor VT1. To increase the amplitude of this component, the capacitance of the capacitor C7 in the positive feedback loop is used in 2...3 times greater than required for the oscillating at the frequency of the first harmonic.
The transistor VT1 is configured as a common-base circuit to work as a synchronous detector. This transistor provides a sound amplification of the audio frequency (this is the low IF), the gain is approximately equal to the ratio of resistors R2/R3. The network R2C3 blocks the RF frequency across the local oscillator, and this network is the load of the synchronous detector. The time constant of this circuit makes it possible to pass the entire frequency range of the complex stereo signal. The capacitance of the capacitor C3 can be increased to get a standard value of the time constant 50 μs in case if the receiver will receive only monophonic signal. The signal voltage at the output of the receiver is approximately 10...30 mV (this is enough to listen to the radio with headphones connected to the circuit instead of the resistor R2) and this signal voltage is independent of the level of received radio signal.
The sensitivity of this receiver is not worse than a sensitivity of a super-regenerative radio receiver, and this receiver don't have a "noise" sound when there is no signal of a radio station. At the moment of tuning of the local oscillator to the half of the frequency of a radio station, the frequency locks and we hear a "click" in headphones, and now the receiver tracks the frequency (in some range) and carrying out synchronous detection of a signal. Because of the PLL and a good isolation between the input resonant circuit and the local oscillator resonant circuit (they has different resonance frequency) the receiver produces very low radiation in the antenna, so the receiver don't need an RF amplifier. The shortcoming of the receiver is that the locking range getting wider when a powerful radio signal received, and in this case AM detection may happen, but this is the common problem of all FM direct conversion receivers with PLL.
A silicon transistor (e.g., KT315V or any low power HF n-p-n type transistor) can also be used in this receiver. Coils L1, L2 are wound on a former with a diameter of 5 mm, the winding step is 1 mm. 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 the same wire.
A schematic diagram of a loudspeaking pocket radio is shown in Fig. 2. This circuit uses a loop antenna WA2 tuned with a capacitor C2 to the middle of the VHF FM band (65.8...74 MHz). The coil L1 is used to couple the loop antenna to the RF stage, based on the transistor DA1.1. The capacitor C8 is used for tuning. An audio pre-amplifier is composed of the transistor DA1.2, and a power amplifier is composed of transistors VT1-VT3. The output power at 8 ohms load is 50 mW (if the circuit is powered with two battery R10 (332): 2*1.5=3 V ). It is recommended to use an external antenna WA1 plugged into connector X1 for receiving a weak signal.
Fig. 2.
DA1 - K159NT1E (a pair of transistors, hfe>=80) can be replaced with a 2N2222;
VT1 - KT315G; VT2 - MP38A (germanium); VT3 - MP42B (germanium);
VD1 - D9E (germanium).
The circuit can be housed in any suitable plastic box. The loop antenna (one turn of insulated hookup wire of diameter 0.3...0.5 mm (AWG 28..24)) is placed around the perimeter of the plastic box and glued. The approximate dimensions of the loop antenna are about 100x65 mm. The coupling coil L1 is formerless, the coil contains 2...4 turns, and it wound on a mandrel with diameter of 5 mm, a step of winding is 1 mm. The coil L2 can be used the same as in the previous radio receiver (fig. 1). However to avoid microphone feedback, which may occur due to acoustic feedback between the coil L2 and the loudspeaker BA1, it is better to wind the coil L2 turn to turn on a former with a ferrite slug. In this case the coil contains 9 turns of enameled copper wire with diameter of 0.27 mm (AWG 30). An air trimmer capacitor can be used as a tuning capacitor C8.
Adjustment of the circuit. It starts from the check of operating point of transistors. The voltage on the emitters of the transistor VT2, VT3 should be about half of the power supply voltage, tweak the resistor R11 value to adjust it. Next, short the resonant tank L2C6 and feed an audio signal of several millivolts to the emitter of the transistor DA1.1 to be sure of the signal passes through the stages of the receiver. Tweak the resistor R1 value to setup the operating point of the local oscillator. Tweak the capacitor C7 value to adjust the level of the second harmonic of the local oscillator, to get the maximum tracking bandwidth of the signals of received radio stations.
Fig. 3.
The switch SA1 is intended for switching ON/OFF stereo mode
Fig. 3 shows a schematic diagram of a simple stereo FM receiver for the stereo signal with polar modulation (OIRT 66-74 MHz). The serial resonant tank circuit L3C7 tuned to the middle of the UHF band is connected to the positive feedback loop of the transistor DA1.1 to obtain maximum sensitivity. A variometer L2 is used for tuning purposes. The time constant of the R2C3 circuit allows to pass the frequency of the complex stereo signal, the decline in the frequency of 46.25 kHz is less than 3 dB. The amplifier based on the transistor DA1.2 passes the frequency band of the sub-carrier frequency of 31.25 kHz. The load of this amplifier is the resonant tank circuit L4C8 connected in series with the resistor R5. The resonant impedance of this resonant tank circuit chosen to be able to fully recover the sub-carrier frequency to the level of 14...17 dB. (As described in [2], the Q factor of the recover circuit may differ from the standard value. This does not lead to nonlinear distortions at the detection process. The reduction of crosstalk at frequencies below 300 Hz for the stereo has virtually no effect).
The buffer stage based on the transistor VT1, this stage is directly coupled to the previous stage. It has a low gain (about two), high input impedance and doesn't shunt the recovery sub-carrier frequency circuit.
A polar-modulated signal from the collector of transistor VT1 goes to the volume control R8 and further to the polar detector based on the diodes VD1, VD2. To simplify the design of the receiver the volume control inserted before the detector. The inductor L5 and the capacitor C17 provides a tone compensation for low and high frequencies respectively. The polar detector is loaded with the R9C11 and R10C12 networks that compensates for the pre-distortion of a stereo signals. When receiving mono signal, the polar detector can be shorted with the switch SA1.
A stereo amplifier is based on the transistor VT2-VT5. The output stage operates in class "A". The output power of the amplifier at the load with impedance of 8 ohms is 1..2 mW, the consumption current is 7...8 mA. The amplifier can be loaded with a stereo headphones with impedance of 100 ohms.
Fig. 4.
VD3 - KVS111A (C = 33pF at the reverse voltage of 4 V and F = 1 mHz; KC overlap >= 2.1)
The design of the variometer L2 is shown in Fig. 4, a. It is made of fluoropolymer, threaded inside with M5 thread (5 mm in diameter). The retaining clip 2 is made of copper wire with 0.5 mm in diameter, the rod with thread 3 is made of brass. The knob 4 is used any one available or homemade. In Fig. 4, the 5 is the box of the receiver, 6 - printed circuit board.
The variometer coil L2 has 16 turns of enameled copper wire with diameter of 0.56 mm (AWG 23), the coils L1 and L3 is formerless with the diameter of 5 mm, a step of winding is 1 mm. L1 has 6 turns (the coil is tapped at the midpoint), L2 has 10 turns of the same wire. The coil L4 has 155 turns of enameled copper wire with diameter of 0.2 mm (AWG 32), it has a movable former placed on a ferrite rod with a diameter of 8 mm and with a length of 20 mm, the ferrite rod has permeability of 400. The choke L5 has 500 turn of the wire with diameter of 0.1 mm (AWG 38) wound on a former which is mounted on a core made of permalloy E-shape plates 3x6 mm. All stages based on the IC DA1 is shielded. With this receiver can be used an antenna made of steel wire of the diameter of 1..1.5 mm and length of 200..300 mm. The far end of the antenna should be bend in shape of a circle.
To the receiver you can add an electronic tuning (Fig. 4, b). In this case the potentiometer R18 is used for tuning. The bias voltage from the moving contact of the potentiometer goes to the varicap VD3. The potentiometer is connected directly to the power supply. With the power supply voltage of 1.5 V the electronic tuning can overlap about half of the VHF band. The second half of the VHF band can be overlapped by applying a forward bias to the varicap (the switch SA2 is in the left position on the circuit diagram of Fig. 4, b). This electronic tuning can be used with the circuit diagram shown in Fig. 2, the supply voltage must be fed through the bypass filter R19C20 (Fig. 4, b), and delete the switch SA2 from the circuit.
Adjustment of the circuit. It starts from the the setting of operating points of output stage transistors. Tweak the resistors R11, R14 to get the current of 5..8 mA at the collectors of transistors VT5, VT6. Next, check the response curve of the stereo demodulator circuit. To do this, short the coil L2, fed to the emitter of the transistor DA1.1 an audio signal with the voltage of several millivolts. The output signal checks at the potentiometer R8, set it to the maximum volume, the switch SA1 must be open. The decline in the response curve at the frequency of 46.25 kHz should be less than 3 dB (adjust it by tweak the capacitor C3), and the rise in the response curve at the frequency of 31.25 kHz (if the resonant tank circuit L4C8 is tuned to the frequency of the input signal) must be at least 14 dB (5 times).
You can adjust the stereo decoder by using the received stereo signal. To do this, connect across the switch SA2 a high-impedance millivoltmeter and move the ferrite core of the coil L4 to get the maximum DC voltage at the output of the polar-modulation detector. If the resonant tank is tuned, the voltage must be about 0.25..0.3 V, and the voltage is about 0.05 V if the resonant tank is detuned or shorted. If necessary, tweak the resistor R7 value to have the maximum dynamic range of the cascade based on the transistor VT2.
The Fig. 5 shows a diagram of FM adapters for the transistor radio "VEF-202" [3] (in the brackets are shown components of the "VEF-202" circuit). The adapter is mounted in the rotary switch on the PCB for the range of 52..75 m. A one section of the variable capacitor C3 is used for tuning. The telescopic antenna is used for receiving.
Fig. 5.
The coil L1 has 3+2 turns, the coil L2 has 9 turns of enameled copper wire of 0.31 mm in diameter (AWG 28). The coils use a 5..8 mm plastic former. The adapters power supply voltage is 3.5..4.5 V
REFERENCES
- V. Polyakov. FM Radio broadcast receivers with PLL. - M.: Radio and Communication, 1983.
- L. Kononovich. Stereo radio broadcast - M.: Communications, 1974.
- I. F. Belov, E. V. Dryzgo Handbook of transistor radios, radiogram, turntables. Part I. Portable radios and radiograms. - Moscow: Soviet Radio, 1976.
A.Zakharov