generate with odd behavior

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edgo
Posts: 34
Joined: Wed Jun 25, 2014 11:38 pm

generate with odd behavior

Post by edgo » Tue Nov 18, 2014 11:40 pm

We have a LabVIEW program driving a Red Pitaya, generating waveforms over a frequency range, then acquiring results from a circuit. Some recent testing shows an odd behavior around 235kHz, where there is a small increase in the generated signal amplitude leading up to this frequency, then it tapers back down to normal:

Frequency (kHz) and Amplitude (Vpp) [generating 2.0Vpp sine wave]
100 2.072
150 2.073
200 2.084
225 2.112
235 2.116
240 2.115
250 2.107
275 2.090
300 2.080
350 2.072
400 2.070

We have measured this generated signal with a "real" oscilloscope, too. Same behavior.

Example: Signal cable running from output channel 1 to input channel 1 (with 50 ohm termination).

This can be duplicated using Red Pitaya's "Generator & Oscilloscope" application.

1) Use "Signal generator" tab to create signal on appropriate channel

2) Use "Measure" tab to view Amp: Vpp

Sweep through the frequency range and observe.

We have two RP's and they both have this behavior, on both channels.

Does anyone else also observe this behavior, and does anyone have any clues?

john k2ox
Posts: 39
Joined: Sun Oct 05, 2014 6:47 pm
Location: New York
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Re: generate with odd behavior

Post by john k2ox » Wed Nov 19, 2014 3:52 am

2.116/2.0 => +5.8%
20*log(2.116/2.0) = +0.49db

That's not bad.

All devices have a frequency response. I know that the input (scope) has frequency response correction factors. I do not know if the DAC, sig gen, does.

The spec says: Gain error: <5%(G) and foot note (G) says: further corrections can be applied to make it better.

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