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I have a 40 kHz ultrasonic transducer that transmits a pulse. The pulse is generated by switching a phase shift oscillator on and off with a NE555 timer. For some reason the sender is making a faint audible noise. This is the first time I am noticing it. I thought the sender might be damaged but the pulse at the receiver seems fine although there is more coupling at the receiver than before.

Any idea what might be wrong? enter image description here

Update: Using larger resistors for the oscillator solved the problem.

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    \$\begingroup\$ Subharmonic oscillation, the nightmare of a power supply designer. I would say it has something to do with passband of the oscillator feedback loop. Do you have the driver's schematic? \$\endgroup\$ – venny Oct 18 '15 at 13:15
  • \$\begingroup\$ @venny I added the schematic. \$\endgroup\$ – Ace Oct 18 '15 at 13:34
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    \$\begingroup\$ Have you tried FFTing the output? Spectrum analyzer etc. \$\endgroup\$ – Fizz Oct 18 '15 at 13:48
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    \$\begingroup\$ In general the design calculations for a phase-shift oscillator assume the filter stages don't load each other. Which might be far from reality here given that you use 100-ohm resistors. And what kind of caps are you using? \$\endgroup\$ – Fizz Oct 18 '15 at 13:56
  • \$\begingroup\$ What's the pulse repetition rate? \$\endgroup\$ – Andy aka Oct 18 '15 at 14:09
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It looks like the pulse repetition rate is about 30 ms. This is based roughly on the 348kohm and 100nF in the 555 circuit. The pulse width is in the region of 200 us. I assume the main oscillator is running at 40 kHz and 40kHz has a cycle time of 25 us.

In a perfect world exactly 8 cycles of 40kHz would be gated by each pulse but, it's not going to work that way. It is likely to be a fractional number of cycles that are gated and this is going to cause a low frequency audible blip that evolves with time as things move into and out of sync.

I don't think it's a problem but you might be better served deriving a pulse from the 40 kHz and gating the 40 kHz to dump an exact number of cycles onto the transducer. You could even arrange it to start at a zero cross. I'm sure this wouldn't produce as much audible noise but then again, on the other hand is it such a big deal. After all, if it performs its main task and the noise isn't annoying then let it be.

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