FM receivers with PLL

  
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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. Th
FM receivers with PLL - schematic

e 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. 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...



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