Constant DC at RF input for one signal, otherwise works fine

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djsw
Posts: 8
Joined: Thu Jan 07, 2016 6:07 am

Constant DC at RF input for one signal, otherwise works fine

Post by djsw » Thu Jan 21, 2016 5:00 am

I'm trying to sample an ultrasonic transducer using the RF input.

I've hooked an oscilloscope up to the output of the cable I want to use and I'm seeing a noisy DC signal at about 1.15V, except when I trigger the transducer, when I see the signal I want to be able to sample. When I connect that same cable to the RF input of the Red Pitaya, the Red Pitaya only sees a constant DC voltage for the entire buffer (literally buff == buff[i+1] for the entire buffer).

When nothing is connected to the RF input, the Red Pitaya sees a noisy DC signal at about -0.013V.

I swapped to Python and reconnected the transducer and I still saw the perfectly DC signal. I generated a waveform on the RF Output and sampled it on the RF Input using Python, and matplotlib was able to perfectly reconstruct it.

I'm at a loss for what the problem could possibly be. The Red Pitaya is apparently sampling correctly and the output of the cable I want to sample is apparently the correct signal, but for some reason when I plug the cable into the RF input, the Red Pitaya only sees a DC voltage that stays constant down to the smallest decimal place. Has anybody got any ideas?

hans.dorn@gmail.com
Posts: 15
Joined: Wed Dec 16, 2015 10:33 pm

Re: Constant DC at RF input for one signal, otherwise works

Post by hans.dorn@gmail.com » Thu Jan 21, 2016 11:36 am

The RF inputs of the redpitaya are jumpered to +/- 1V range by default. The DC offset of 1.15V is just outside of that that range and causes the RF input to max out.
You could try jumpering the input to +/-20V, if the sensitivity is high enough with this setting, or better get rid of the DC voltage.

Cheers
Hans

djsw
Posts: 8
Joined: Thu Jan 07, 2016 6:07 am

Re: Constant DC at RF input for one signal, otherwise works

Post by djsw » Fri Jan 22, 2016 4:39 am

That was the issue, thank you very much.

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