Electronic – What would cause this kind of amplitude interference on the ADC recording


I am using a 12-bit 6.4Mhz ADC to record a sensor reading.

It's an ultrasound doppler system. A DAC is used to generate a TX frequency, that frequency is bounced off a moving target and received via an ultrasonic sensor.

In my testing environment, the amplitude of this reading is fairly consistent over time:

However, when the sensor is installed on site, I see this weird amplitude-modulation type interference:

site rig

I'm familiar with interference where additional frequencies are received/interfere with the recording, however I don't know what causes this, where the amplitude seems to drop out at points.

I'm not even sure what this is called, does it have a name?
and, more importantly, what would cause this behaviour?

More info:

More results with FFTs:ffts

This isn't a display artefact. Zoomed in on one of the "zero crossings":


I even reproduced this in Matlab by mixing the adc data with a sinusoid of 200 Hz.reproduced

The top of each of the two FFT peaks has two separate frequency maxes instead of one.

Best Answer

This is not a very detailed answer.

It looks as if you are getting multiple echoes back from your target, where the phases between the echos are varying slowly. Sometimes they cancel each other out, sometimes they don't.

It's difficult to tell why this happen in a real setup but not in your test setup. Maybe your test setup does not accurately reflect the real situation.

I think you need to add even more information about your target (what you're measuring, what kind of speeds it's moving at) if you want more ideas.