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Frequency Response with Pico 4262

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Frequency Response with Pico 4262

Postby Glubbein » Wed Sep 18, 2013 5:26 pm

Hello,

I want to mesure frequency response of amplifiers from 5Hz to 20kHz. Thus I saw the video about it in Youtube with the 4262.
I also have a 4262.
So I've tried with the following settings (quite the same as in the video) :
1°) Generator :
- sin wave
- start frequency : 5Hz
- amplitude : 400mV
- up
- stop frequency : 20kHz
- Increment : 100Hz
- Time : 60µs
2°) Scope :
- 200µs/div
- 500S
3°) Spectrum :
- 50kHz
- 512
- Flat-top
- Pick hold
Result :
Frequency response 1.jpg


Why do I have this strange begining (cercled in red)???
If I use loudness fonction of the amplifier, the curve is OK except the begining : same as in the picture above.

I've done many things :
- changing settings (aquisition time, frequency increment in the generator, time also in the generator).
- other amplifier in test (I've allready try 3 differents types)
- changing probes (pico and tektronix)
- changing cable beetween generator and amplifier
- update the pico software...
- using other computers (all are under windows XP SP3)
- Even trying to put a capacitor beetween probe and amplifier output...
Nothing changes!

Thanks a lot for help.

Glubb
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Re: Frequency Response with Pico 4262

Postby docali » Thu Sep 26, 2013 4:58 pm

Hello,

I think the reason is that the sweep begins with a psoitive number like 20 Hz and the x-axis starts at 0 kHz. It:s the same as you see after 20kHz

Best regards,
docali
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Re: Frequency Response with Pico 4262

Postby Glubbein » Fri Sep 27, 2013 2:45 pm

Hello,

thanks for the answer, but I would say : "so what???"

Another test (done with other amplifier) :
Settings are :
1° Generator :
sin
20kHz
up
200Hz
200Hz (increment)
60µs
2°) Scope :
+/- 1V
200µs/div
500S
3°) Spectrum :
256
Flat top
Peak hold
200kHz
All those settings are similar as setting in this video :
http://www.picoscope.tv/general/frequency-response-measurement.html

See the result :
Frequency response 2.jpg

Still the same!!! :x

My picoscope is ten months old... I bought it at Farnell, it's still under warranty. If I have no other solution, I will send back and ask for a new one...
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Re: Frequency Response with Pico 4262

Postby PeterF » Mon Sep 30, 2013 11:20 am

Hi,
Please ensure that trigger is set to auto and I think you will find the peak at LF will be resolved. If the signal is not triggered, the start point for each Fourier transform starts at a different DC value each time and the displayed value at 0Hz will be the peak value of the waveform, giving the visible peak. There is nothing wrong with your scope.
Please contact us again if you have any other questions.
Regards,
PeterF.
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Re: Frequency Response with Pico 4262

Postby Glubbein » Mon Sep 30, 2013 9:09 pm

Hello,

:D :D THANKS A LOT PETER : it works perfectly!!! :D :D

Just few comments about the generator of the 4262 :
- I think it would be better to setup before activation, and not after (especialy for amplitude).
- the highest frequency with the sweeping mode (20kHz) is too low : 50kHz would be very fine. Is it a software limitation or an hardware limitation?

It would be fine to have the possibilty saving measurements with curve, and also part of a curve (zoom in for exemple).

Except those three points, pico 4262 is a very good device!

Regards.

Glubbein
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Re: Frequency Response with Pico 4262

Postby PeterF » Tue Oct 01, 2013 9:07 am

Sinex4.csv
AWG .csv file to give x4 the set frequency
(31.34 KiB) Downloaded 96 times

Hi,
The AWG will only meet it's full flatness and distortion specification up to 20kHz. You can trick it into producing a higher frequency but the response falls off rapidly above 40kHz. If you program the AWG waveform with, for instance, a waveshape that is four sine-wave cycles then the output waveform will be four times higher frequency than the nominal frequency set. I attach a suitable .csv file to import into the AWG of the 4262. (Signal Generator, select "Arbitrary [AWG]" and set click the "Arbitrary" button.)
You can also, of course, use the "Zoom" button to zoom in on any part of the waveform and also have a "Measurement" at the bottom of the screen. These are not saved when the whole file is saved as a .psdata file but you can save a "Screen capture" by selecting "Edit", "Copy entire Window as image". You can then "Paste" this into any graphics program of your choice.
I hope that helps!
Please contact me again if you have any other questions.
Regards,
PeterF.
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Re: Frequency Response with Pico 4262

Postby docali » Sun Oct 20, 2013 9:36 am

Hello

could you please also provide such a file that doubles the frequency only?
So, if you configure 5-20khz it would be 10-40khz. That would give a reasonable range for any amplifier testing.

And I would fully agree that the generater should be configurable before activating it.

Best regards!
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Re: Frequency Response with Pico 4262

Postby Martyn » Wed Oct 23, 2013 8:14 am

This is straight forward to create in the software.

Open the Signal Generator dialog
Enable the "Signal On"
Select the "Arbitrary" button to open the AWG dialog
Set Sample to the maximum
Select the Sine Shape button which will create a two cycle sine wave (which is what you need)
Finally "Normalize" the signal using the button on the top line (between Clear and Undo)
Then click OK to apply
Martyn
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Re: Frequency Response with Pico 4262

Postby Glubbein » Tue Nov 05, 2013 11:35 am

Hello Peter, hello Martyn,

Thanks a lot for your answers.

- I think it's better to work only from 1 to 20kHz because, as Peter said, the frequency response of the AWG is not very good above 40kHz (even above 30kHz...).

- About activation of AWG after or before configurating it : (that's for Martyn) :
you write : Enable the "Signal On"... yes but then you activate the AWG, and then you have a signal at the output!This could be a problem if the device connected at the output doesn't support hight input level.
It would be better to have the possibility of configuring the AWG (with NO SIGNAL on the output) AND THEN activate it.
(I think that docali wanted to say the same).

Best regards.

Glubbein
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Re: Frequency Response with Pico 4262

Postby Martyn » Fri Nov 15, 2013 8:20 am

I have put in a request to the development team suggesting that the setup of the SigGen/AWG should be possible before enabling the output. I will post back when I have some news.
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