“BAT_BAND”CONVERTER

Published:2009/7/13 21:27:00 Author:May | From:SeekIC

The full circuit diagram of the bat-band converter is shown. All components, with the exception of the transducer (X1), the ON/OFF switch, and the ferrite rod antenna, are surface-mount devices (SMDs). The signals from X1 are very small (microvolts, rather than millivolts). A low-noise FET input stage, TR1, has minimal loading effect on X1 and provides a low-impedance source for the passive high-pass filter. This simple filter consists of capacitors C1 and C2 and resistor R3. It attenuates signals below about 15 kHz, but the rolloff is very gradual. Mid-range and lower-frequency audio is strongly reduced. The falling sensitivity of transducer X1 further limits the audio frequencies reaching the mixer IC1. Despite this level of attenuation, a little audio still gets through the system, but it has no effect on the ultrasonic operation. When the receiver is tuned close to the 1-MHz local oscillator, a little audio feedback will be heard. This is a real help with tuning in because it readily identifies the low-frequency end of the bat band. The transistor TR2 is a low-noise amplifier, and the SMD-type BSX70H works well in this circuit. IC1 is a double-balanced mixer and Colpitts oscillator in one package. The signal inputs at pin 1 and pin 2 are used in unbalanced mode, with pin 2 decoupled by capacitor C4.



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