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FMsynchroPLL
07-28-2017, 02:10 AM
Post: #1
FMsynchroPLL
based on A.Zakharov fm synchro pll receiver

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.

Base circuit diagram of VHF FM receivers with PLL

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.


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08-01-2017, 07:43 PM
Post: #2
RE: FMsynchroPLL
Let say better tested in simulator CM show as
method of mixing and demodulation (slope) of FM
signal .
As we can see sensitivity is not very high,
if we have input signal 30-40uV on antenna then we can expect just
4 mV of AF output,which means that receiver need
simple amplifier to amplify signal to 10 times and get
cca 40mV on output,which is fine.
original circuit have polar-capacitor(10uF) -which make problems
because act as low freq reactance and cose motorboating
of receiver.On this way coupling is direct.
RF signal is grounded trough 1nF capacitor.
L1-C2(33pF) are oscillator tank of 44Mhz,
bypass cap 27pF lead second harmonic(2x->88Mhz) to emiter of transistor
and synchronize with incoming signal,mixing is presented
on transistor collector as we can see.
RF signal is grounded with C3-5nF and we get AF voltage swing
as reulting demodulated signal.
C2 can be fixed capacitor of 33pF but then we must use
aluminum tube (as slug) to tune receiver.
This slug decrease inductance of coil and that way change frequency
of oscillator.
Another method would be using varicap diode and change frequency.


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