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Like a Superhet Receiver but the I.F. is Audible
- This receiver is based on the SA602 balanced mixer.
- The local oscillator is set between 37 and 43 kHz.
- The transmission on or very close to 40kHz is mixed down to an audible Intermediate Frequency between 300 and 3000 Hz.
- The transmitter and receiver can be set so both the wanted and the unwanted image frequencies are received.
- By careful choice of frequencies, the unwanted image can be greatly reduced by the narrow bandwidth of the ultrasound transducers.
- The Intermediate Frequency between 300 and 3000 Hz is processed by the computer sound card.
- Most superhet (supersonic heterodyne) receivers produce an I.F. well above the range of human hearing.
- 455kHz or 10.7 MHz are commonly used in AM and FM radio receivers.
- This receiver produces and audible I.F. so it's a "Sonic Heterodyne" receiver.
- The computer software used to create and decode the data signals is Digital Master 780 which is part of the Ham Radio Deluxe software suite.
The Mark 1 Tested Circuit
- The square wave local oscillator is unusual.
- A sine wave without harmonics would normally be preferred and should not be too difficult to design.
- As it's unlikely that there are any signals (apart from bats) near the harmonic frequencies, this design simplification should not matter too much.
- This two chip design seems to have enough sensitivity to work well across a room.
- There is scope for adding an ultrasound preamplifier before the mixer and an audio amplifier after it.
- The frequency stability is not perfect but the DM780 software locks on very well.
- If either the transmitter or receiver is moved, there is very obvious Doppler shift disrupting some transmission modes.
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