Undersampling with the Red Pitaya

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Dazza101
Posts: 6
Joined: Fri Feb 05, 2016 9:57 am
Location: Cape Town, South Africa

Undersampling with the Red Pitaya

Post by Dazza101 » Wed Aug 02, 2017 8:53 am

Hi All,

I'm working on an FMCW radar that uses the Red Pitaya for control and as the ADC. I'm using the provided C API and a decimation factor of 8, to perform undersampling in the second Nyquist band.
As my signal increases past Fs/2 my signal amplitude begins to drop drastically. For example: a 10 MHz 2 Vpp signal appears as 0.8 Vpp once sampled. All my development work takes place in C and I have yet to fiddle with the FPGA. I'm wondering if the Red Pitaya applies some sort of digital frequency compensation based on the set decimation factor? And If I could disable or correct this?

Kind Regards
Darryn

jeanminet
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Joined: Tue Aug 25, 2015 12:17 pm
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Re: Undersampling with the Red Pitaya

Post by jeanminet » Wed Aug 02, 2017 5:39 pm

Hi,

Maybe the decimation includes the averaging of the 8 consecutive samples in the FPGA.
The frequency response of the averaging is sin(x)/x where x = pi * 8 * f / 125 MHz.
That gives 0.9 Vpp for a 10 MHz, 2 Vpp signal.

Looking at the corresponding Verilog module (https://github.com/RedPitaya/RedPitaya/ ... dec_avg.sv), it looks like you can disable averaging.

Jean

Dazza101
Posts: 6
Joined: Fri Feb 05, 2016 9:57 am
Location: Cape Town, South Africa

Re: Undersampling with the Red Pitaya

Post by Dazza101 » Wed Aug 02, 2017 9:34 pm

Hi Jean!

Thank you for your reply! I think you are exactly right - I hadn't considered the effect of averaging.
I've attached the theoretical frequency response of an 8-point moving average, which appears to correspond to my recordings.
I'll test with other frequencies when I have access to my RP tomorrow.

Kind Regards
Darryn
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