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Simple Ultrasound AM Receiver

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40kHz Ultrasound AM Receiver

The receiver below is really too sensitive. The noise generated by the first op-amp is noticeable and limits the performance of the whole receiver. Also with the gain turned up it becomes unstable. Try leaving out one of the op-amp preamplifiers.

The circuit consists of ...

the ultrasound transducer which is resonant at 40 kHz and "tunes" the receiver.
This is the only filtering selecting the received frequency. The data sheet quotes 40 +/- 1 kHz for the transducer.
a Non Inverting Amplifier with a gain of 34.
Gains larger than this are not recommended for the low cost 741 chip because you will run into the Gain Bandwidth Product limitations of the chip.
a second Non Inverting Amplifier with a gain of 34.
an inverting amplifier with a gain of -6.8. The diode in the feedback reduces the gain to zero for the negative half of the incoming signal. This subsystem acts like an ideal diode and it demodulates the received AM signals.
The bandwidth of the ultrasound sensor is too narrow for good quality audio AM although muffled transmissions can be heard.
The receiver works exceptionally well with narrower bandwidth data signals including ...
- BPSK-31
- Slow Scan TV
- Olivia 2000-64. This uses Frequency Division Multiplexing with multiple carriers.
- MT63-2000. This uses Frequency Division Multiplexing with multiple carriers.

Increasing the Range

Find two disused satellite dishes. The mesh ones work as long as the holes are a lot smaller than 8.25mm. Mount the sender and receiver at the dish focal points and see how far you can get the signal.

Slow Scan TV Reception

This screen shot came from a student ELEC6 project to make an ultrasound AM transmitter.

The receiver above picked up this image, fairly free of noise and the colours are rendered well.

The audio spectrum of the received image can be seen on the right.

The transmit and receive software was Digital Master 780 which is part of the Ham Radio Deluxe suite.