LONGWAVE_TRF_TUNER

Published:2009/7/17 2:29:00 Author:Jessie | From:SeekIC

This TRF tuner was intended for reception of Radio 4 AM broadcast signals in the United Kingdom on 198 kHz, with the best possible audio fidelity. It can be used as an IF amplifier or for reception of LW or MW AM broadcasts in other areas. The aerial wire is directly connected, via SK1, to the input tuned circuit (coil L1 and capacitors VC1 and C3) via a small-value capacitor C1. The second tuned circuit (L2, VC2, and C8) is similarly connected to the output from transistor TR2. The output from transistor TR3 is taken to the op-amp demodulator (IC1a) and also, via capacitor C9, to the diode-pump circuit D1/D2. The pump is used to generate a small negative AGC. For AM detection, if a diode is used as the rectifier, the tuner will not work very well on input volt-ages less than 0.5V peak. However, if a pair of diodes are hooked around an op amp, IC1a, as shown in the figure, the rectification threshold is reduced to a few millivolts, which increases the demodulator sensitivity by a few hundred times. The other half of the dual op amp, IC1b, is a straight AP amplifying stage, whose gain is adjustable by potentiometer VR1. Resistor R13 and capacitor C10, on its noninverting input (pin 5), act as a simple RE filter to remove any residual 198-kHz signal voltage from the audio output.



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