A frequency division bat detector circuit with amplitude restoration

J. L. Errington and Sylvain Frenehard

Examination of a frequency division detector showed that the output from this system was useful in detecting the presence of ultrasound, but gave no indication of its amplitude.  The design was therefore extended to incorporate an active detector and low pass filter to measure the amplitude, and by mixing this with the divided signal using an Operational Transconductance Amplifier (OTA) to reinstate the amplitude from that of the incoming sound.

After considerable testing and redesign it was decided to reject the use of the popular LM381 or 387 audio preamplifiers in favour of a circuit using LM353 dual operational amplifiers. The LM353 has a gain-bandwidth product of 10MHz allowing a stage gain of 50 with a bandwidth of 200kHz.  For full details see the report.

Sylvain tested several different approaches, and we feel that the design choices are becoming much more clear as some alternatives are eliminated. The problem of a suitable transducer remains. Tests on piezo-detectors show that they have a very narrow bandwidth, but they are relatively robust, sensitive, readily available, and cheap.  By contrast electret microphones have a much wider frequency response, and in particular hearing aid microphones due to their small size can have frequency responses that are substantially linear up to 40 kHz, and usable to 120kHz.

The new design runs from a 9V battery and provides a signal into headphones or a small loudspeaker. The PCB uses a ground plane as for radio designs to minimize oscillation, pick-up of radio signals and interference.

COPY OF SYLVAIN'S REPORT IN POWERPOINT FORMAT CLICK HERE (373k)