I have been using an RP with an aluminum case and I am now at the stage to daisy chain with another RP. I was amazed to find that I can't do that with the expensive aluminum case but I can with the plastic one. So I will have to revert to the plastic one or use a drill and hacksaw to make an opening in the metal one.

Thanks.

Statistics: Posted by JohnnyMalaria — Wed May 24, 2017 8:09 pm

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It will fit but you need to have correct thickens off the thermal pad.

recommended thickens is 2mm.

Best, Zumy

Statistics: Posted by redpitaya — Fri May 19, 2017 10:04 am

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Fast analog inputs are not differential i.e they have common ground.

But not sure if I understand this question: "I have to use them both at the same time."

You can use them at the same time, just inputs are not differential.

Kind Regards, Zumy

Statistics: Posted by redpitaya — Fri May 19, 2017 10:01 am

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Will a 3M 14mmx14mm thermal pad fit for the aluminium case?

I am not really sure about the dimensions of the Xilinx Zynq 7010 SoC and i do not have my pitaya with me to measure it at the moment.

Thanks in advance for your help!

Best regards,

Dim

Statistics: Posted by mitsoyba — Tue May 16, 2017 11:17 am

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I want to use Red Pitaya's (v 1.1.1) analog inputs as a DAQ device for measurement of a few things on a DC/DC step up converter. I wanted to use LabVIEW, and will have to use them in HV setting. The main question is are the inputs differential? I have to use them both at the same time.

Best regards

Statistics: Posted by marin268 — Thu May 11, 2017 4:34 pm

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you must be sure to have well defined setup and on that setup you define the crosstalk levels.

So, I think that correct way of measuring crosstalk for example

between OUT1 and IN1 is:

1. OUT1, OUT2 , IN1, IN2 are all terminated with 50OHM. OUT1 and OUT2 are designed for 50Ohm.

2. OUT1, OUT2 amplitudes are set to 0V.

3. Acquire signals on IN1 and calculate FFT. When calculating FFT you need to use correct equations for calculating dB.

4. Save this FFT "trace" as "noise_fft"

5. OUT1 amplitude is set to 0.7V (don't use max amplitude in order to avoid non-linearities of DAC)

then you make freq. sweep for OUT1 frequency - from 100Hz to 50MHz.

6. For each f. step of OUT1 acquire signal on IN1, calculate FFT -> you get "cross_talk_FFT(f.step)"

7. for each f. step subtract "noise_fft" from "cross_talk_FFT(f_step)"

8. plot all "cross_talk_FFT(f_step)" one above another. (don't average them!!!)

8a. instead of 8. you can pick up max values for each f.step from "cross_talk_FFT(f_step)" matrix.

with that you will extract worst cases.

9. When you start STEMlab board start Oscilloscope application and then close it.

This will load fpga filter parameters.

then you can go with SCPI and remote control.

Now, when doing acquisition you should allays have same sampling rate in order to get

correct FFT calculations.

Preferably you should use max sampling rate.

But if you wish to look at lower freq. range you can use lower sampling rate

and repeat steps from 1-8 while max OUT1 freq is limited by Nyquist.

"I am still not sure how the coupling works ... I also tried to connect 47nF to the +3,3V to ground on the connector, but that did not have any visible effect on cross coupling. My idea was that a ripple on the supply might cause this."

- Yes, power supply ripples and ground plane can affect outputs performances.

but I think this will not affect crosstalk noticeably.

https://ln1985blog.wordpress.com/2016/0 ... rformance/

This is the reason for step 4. exclude environment noise form crosstalk measurements.

The code bellow is for calculating power noise using FFT.

When dealing with amplitude ratios your FFT equations will look different

In equation 1 (look at code) you should use magnitude of complex value and not complex conjugate multiplication

Amplitude spectrum in volts RMS = sqrt(2)* mag(FFT(A))/N

in our case

Amplitude spectrum in volts RMS = sqrt(2)* mag(xdft_hann)/N

and equation 2 should look like

A= 20*log10( Amplitude spectrum in volts RMS/OUT1 amplitude);

Code:

`%% data - acquired signal in volts `

Fs = 125E6;

t = 0:1/Fs:1-1/Fs;

N = length(data);

freq = 0:Fs/N:Fs/2;

R_load=50;

%% Hann window;

w = hann(N)';

K = 64;

tmp = hann(K)/K;

tmp = fft(tmp);

tmp = tmp.*conj(tmp);

dcspan = sum(tmp(1:K/2)>1e-6)-1;

win_gain = sqrt(sum(tmp));

xdft_hann = fft(data.*w)/win_gain;

xdft_hann = xdft_hann(1:N/2+1);

%% Equation 1

Pmag_hann=(xdft_hann.*conj(xdft_hann))./(N^2); % Calculate power spectrum to Fs/2

Pmag_hann = Pmag_hann./R_load; % Calculate for 50 Ohm

%% Equation 2

Pdb_hann = 10*log10(Pmag_hann); % Power in dB 1W as reference

PdBm_hann = 10*log10(Pmag_hann./1E-3); % Power in dBm 1mW as reference

Best, Zumy

PS 1: Double check the new equations for amplitude - I didn't test it

PS 2: A lot of people use same equation for FFT spectrum calculation and then getting wrong results.

mag(a+jb) is not equal to (a+jb)x(a-jb) (complex conjugate)

Statistics: Posted by redpitaya — Thu Apr 20, 2017 11:58 am

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Probably in this cases (amplitude difference) board is not calibrated.

With calibrated board if you set OUT1 to 900mV

connected to IN1 (using 50OHM termination) you should get on IN1 = 900mV +- 0.5mV

Kind Regards, Zumy

Statistics: Posted by redpitaya — Thu Apr 20, 2017 10:57 am

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For using LA pro application on the STEMlab 125-14 the LA fronted is required.

With the LA fronted maximum sampling rate (125MS/s) is guaranteed.

http://redpitaya.readthedocs.io/en/late ... onnections

Kind Regards, Zumy

Statistics: Posted by redpitaya — Thu Apr 20, 2017 10:45 am

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In the meantime I got new interesting results. I compared the crosstalk out2 to in1 and a second time out2 to in1 but with out1 also switched on at same amplitude and frequency but 180 degrees phase. Both outputs are left open. It does help quite a lot in the range 30 kHz to 300 kHz (around 10 dB better!).

I am still not sure how the coupling works ... I also tried to connect 47nF to the +3,3V to ground on the connector, but that did not have any visible effect on cross coupling. My idea was that a ripple on the supply might cause this.

Statistics: Posted by gmb — Thu Apr 13, 2017 7:50 pm

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Can you please describe in more details how you performed measurements

(sweeping of out1 and out2).

I.e have you used SCPI commands etc.

Regarding the second part:

If you have inductive coupling then in case of larger current going from outputs (load connected to the output) you will get more crosstalk.

and if the output current is small (no load) the crosstalk will be also smaller.

Also, which version of STEMlab you are using

STEM 125 -14 or STEM 125-10?

Best, RP team

Statistics: Posted by redpitaya — Wed Apr 12, 2017 9:26 am

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Does not seem to be capacitive coupling ... should get worse not better as the voltage is higher without load.

Statistics: Posted by gmb — Wed Apr 05, 2017 10:51 pm

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Where can we find the "moderately simple C program [that] can update the generator frequency" that you are talking about ?

I have a similar problem because I want to generate a signal swept in frequency.

I tried to generate one arbitrary waveform with the entire signal but with the generating sampling rate at 125Msps and a time-signal length of 34ms I have a ~130mo file...

Statistics: Posted by APP_IE_2018 — Wed Apr 05, 2017 4:24 pm

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I put the Red Pitaya into the metal casing (the original one) and made a second run of measuring. See both files - one before, the other one after. There's not too much change.

Any idea where the maximum at exactly 200 kHz comes from? Is there any inductors somewhere that are resonant at 200 kHz and cause magnetic coupling?

Statistics: Posted by gmb — Mon Apr 03, 2017 7:55 pm

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If this is not the case, we will investigate what could be the problem and let you know.

kind regards, Red Pitaya team

Statistics: Posted by redpitaya — Wed Mar 29, 2017 9:17 am

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