Frequency Response Analyzer with Bode Plots

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Re: Frequency Response Analyzer with Bode Plots

Post by hexamer » Sat Nov 16, 2019 3:22 pm wrote:
Fri Nov 15, 2019 1:52 am
Hi hexamer.
Thanks for your reply.
I mean. for example:
- Input 1Vpp 1khz -> Vacuum tube amplifier -> 8Vpp output with perfect Sine Wave -> Capture
- Input 1Vpp 15khz -> Vacuum tube amplifier -> 8Vpp output with Sine Wave was cut off the head (not perfect Sine wave output) -> Capture

in the both case, 8Vpp Output capture but the second one is Distortion.
Thank you for the clear explanation. I see what you're talking about. Clipping distortion would result in energy being moved to higher order harmonics in the frequency spectrum. So, a frequency response would only show you is that the gain is limited. As long as the FRA sampling rate is high enough to avoid aliasing-in these higher order harmonics, then they won't show up in the frequency response. This sampling rate can be increased in the settings, but the default is 64x the stimulus frequency.

The only way that the Bode plot would indicate that some distortion may be occuring would be for you to compare the FRA plotted gain to what you think amplifier is set to (within the bandwidth of the tube). For instance, if your amplifier is set to amplify by 10x, and the gain response is lower than 20 dB, then there may be clipping distortion. There are diagnostic tools built into the FRA application that can measure purity of the input and output signals if you wanted an objective number of clipping distortion. But for a first pass to see if there is any distortion, you'd probably be better off using the PicoScope oscilloscope application.
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Re: Frequency Response Analyzer with Bode Plots

Post by » Mon Nov 18, 2019 1:49 am

Hi Hexamer.
Have a nice day.'Thank you very much for your clear description
I got it.That's is very mean to me.
I would to buy one for my purpose.
Thank you.

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Re: Frequency Response Analyzer with Bode Plots

Post by igorfier » Wed Dec 18, 2019 7:28 pm

Dear Aaron,

I just stumbled upon your very nice application while looking for ways to implement FRA functionality to my 2204A. It installed just fine in W10 x64 and initialized my scope upon startup.
I am now trying to measure simple circuits such as a resistor and capacitor in parallel, but I can't get any meaningful data. I suspect I didn't quite get the way the leads should be connected.
Here's what I'm doing, based on the posts I found about the same subject:

- At one end of the RC: inner bayonet of the CH-A BNC and inner bayonet of the AWG;
- At the other end of the RC: inner bayonet of the CH-B BNC and outer shield of the AWG.
- The outer shields of the CH-A and CH-B BNCs are left floating.

What am I doing wrong?
Thanks in advance for your answer and your time.

Best regards,
Igor Fier

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Re: Frequency Response Analyzer with Bode Plots

Post by kaimex » Wed Dec 25, 2019 2:15 pm

Hi Igor,

with Channel B input connected to the shield of the AWG you are shorting that input to ground.
The shields are all connected.
With channel A connected to the AWG output and your RC-circuit connected between the A and B inputs you get a voltage on input A that is Z_RC/(Z_RC+RoutAWG)*UoutAWG.
I have forgotten the exact Rout, guess it was somewhere around 600...700 Ohm.
When your RC-circuit has a very high impedance compared to that, you will get almost the full AWG output voltage. If Z_RC is very small compared to RoutAWG, you will get almost nothing.
You will get something meaningful, if Z_RC is not to far away from Z_out.
If Z_RC is very high, you have also to consider the input impedance of channels A & B of ~ 1 MOhm.

Regards Kai

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