Sonarbugg mottagare 

SYSTEM THEORY
The transducers are co-resonant near 40 kHz. If the carrier is tuned slightly away from resonance, frequency modulation of the carrier by audio generates amplitude modulation at the sending transducer and the receiving transducer. The pair acts as a slope detector. The receiver recovers audio by rectifying and lowpass filtering the carrier.

CIRCUIT FUNCTION
Ultrasonic transducer couples to the base of Q1, configured as a common-emitter amplifier by R1. S2 selects collector bias through R2 or tank Li -C2. Ql output couples through C3 to base of Q2 configured as a common-emitter amp by R4, R5, and R7. C23 rolls off response above 200 kHz. Q2 output couples through C5 and R8 to op amp U3, configured as a precision halfwave rectifier by D1, D2, and R9.U3 output (taken at resistor/diode juncture) couples through C6 to U 1-a, configured as a quasi -18 dB / octave lowpass filter by R10-13 and C7-9. Cutoff frequency is just above 3 kHz. U1-a output couples to U1-b, configured as a quasi -18 dB/octave highpass filter by C 10-12 and R14-16. Cutoff is around 700 Hz.U 1-b output couples through C 18 to U 1-c, configured by R20 and R21 as an inverting buffer with gain of 40 dB. Cl 9 limits high-frequency response; diodes D3 and D4 clip the output of the buffer at about 1.2 Vp-p. U1-c output couples R22 and C20 to tape output jack. U 1-b output also couples through C 13 volume control pot R17, whose wiper couples to the noninverting input of U2. C14 shunts RF at U2 input. R19 and C17 form the standard snubber. Audio couples through C16 to headphone output jack.U1-d is configured as a DC voltage follower whose noninverting input is biased at V+ by divider R23-R24. U1-d output serves as a stable bias reference for U3 and U1-a/b/c. 
C4, C15, C21, C22, R3, R6, and R18 decouple the supply.