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Power-User

Join Date: Jul 2011
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How To Use Sweep Generator And Scope For Filter Test?

11/22/2016 7:57 PM

I want to use my sweep generator (Tenma 72-5015) with my scope (Hameg HM204) so that the generator sweeps across a range of frequencies as the scope timebase sweeps across screen. My intent is to look at things like LC filters. I understand most of what I am doing (I think) but the question is ..... the scope needs to be synchronised with the sweep generator somehow and the only BNC sockets on the sweep generator are ...OUTPUT ... TTL/CMOS ... EXT COUNTER .... VCF. Do I need to use the trigger on the scope somehow to accomplish what I want or should I be using the timebase output BNC on the back of my scope somehow?? Thanks for any help.

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#1

Re: How to use sweep generator and scope for filter test?

11/22/2016 9:27 PM

The VCF allows a voltage to control the frequency sweep on the signal generator.

The oscilloscope should be in XY mode.

Apply a sawtooth signal to the VCF input and also to the horizontal channel of the oscilloscope. The vertical channel of the oscilloscope should be connected to the rectified output of the filter.

See:

http://user.physics.unc.edu/~sean/Phys351/techresource/docs/Tenma72-5015.pdf

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#2
In reply to #1

Re: How to use sweep generator and scope for filter test?

11/22/2016 10:08 PM

This will work to some extent but not as well as one might think. The problem is that the amplitude will still be displayed as a linear scale on the scope. When working with analog filters (particularly multipole LC filters) a logarithmic scale for at least the amplitude is very helpful.

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Guru

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#5
In reply to #2

Re: How to use sweep generator and scope for filter test?

11/23/2016 9:37 AM

If you want log response, it's not difficult to build a log amplifier...

https://en.wikipedia.org/wiki/Log_amplifier

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#6
In reply to #5

Re: How to use sweep generator and scope for filter test?

11/23/2016 10:37 AM

I try to give somebody a helpful hint for them to explore for the well known solution. But then somebody else spoon feeds them a circuit that will sit on a positive rail half of the time.

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Power-User

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#3
In reply to #1

Re: How to use sweep generator and scope for filter test?

11/22/2016 10:25 PM

Rixter thanks for replying. Also thanks for pointing me at that PDF. My Hameg scope has the sawtooth timebase wave available at a BNC at the rear of the scope . I wonder if I could feed that into VCF providing voltage is 0-10v as mentioned on PDF.

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#4
In reply to #3

Re: How to use sweep generator and scope for filter test?

11/23/2016 9:31 AM

I don't see why not. Connect it to a scope channel and see what it looks like.

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Power-User

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#7
In reply to #4

Re: How to use sweep generator and scope for filter test?

11/24/2016 8:14 AM

Thanks for your help Rixter.

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