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frequency vs voltage graph

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confounded

New Member
hi, i'm building an active filter and would like to produce a graph of frequency vs voltage out, is it possible to trick my oscilloscope into producing this?
If not how would one go about achieving this?
 

audioguru

Well-Known Member
Most Helpful Member
Usually the frequency and the amplitude on graphs are logarithmic. You would need to make two logarithmic converters for your oscilloscope.
 

Nigel Goodwin

Super Moderator
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You don't need to be that complicated or clever - just do it on paper.

Inject different frequencies from your signal generator, read the output level on your scope, write the frequency and result down, then plot a graph from them.
 

confounded

New Member
i have my heart set on getting a graph on my oscilloscope. Im working on the logarithmic convertors now, how do i connect them up to display frequency vs voltage out?
 

confounded

New Member
yes i have a picoscope pc oscilloscope i can store with and an old tektronix analagoue bench scope (no storage)
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
Then you need an audio generator you can sweep the frequency of, do this at a very low speed (as the filter won't respond that fast), rectify the output of the filter, apply to a log amplifier, and display the output on the storage scope.

The Picoscope might even have a log option?, have a look.
 

audioguru

Well-Known Member
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The signal from the output of the active filter has a linear amplitude. Feed it to a logarithmic converter and the output feeds the vertcal of the 'scope and it will be in decibels.

If your frequency sweep (that feeds the input of the active filter) is a linear ramp then add a logarithmic converter in between.
 

confounded

New Member
thanks for your help nigel, i'm still constructing the circuit, just thought i'd find out how i'm going to see the results.
When i've made it i'll see if i can get my plot.
A quick look and it doesn't seem i have a log function.
 

Hero999

Banned
Wow, this sounds complicated, just to test one circuit.

Why don't you build it permanently and use it to test other projects?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Wow, this sounds complicated, just to test one circuit.
And if he's only doing it on a Picoscope, why not just do it manually as I suggested in the first place, but instead of paper, plot it on the PC in a nice graph - no problems with hardware log conversions then. Spreadsheets generally have nice graphing functions.

By doing it in that way, it could have been done by now :D
 

Hero999

Banned
I've done it that way before, in college we had to build an amplifier with a specific frequency response curve. This was nothing complicated, just an inverting op-amp ciruit with RC networks in place of Rf and Rin.

I used a signal generator and scope to measure the gain and phase shift at different frequencies, then plotted a graph using a spreadsheet.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I've done it that way before, in college we had to build an amplifier with a specific frequency response curve. This was nothing complicated, just an inverting op-amp ciruit with RC networks in place of Rf and Rin.

I used a signal generator and scope to measure the gain and phase shift at different frequencies, then plotted a graph using a spreadsheet.
And how long did it take you?, considerably less than the three hours this thread has been running I'll bet.

I probably did the same in college, except using graph paper, no computers back then :D
 

confounded

New Member
It may take a long time to set up, but once done i can switch in any filter and actually save time producing a graph.
Still havn't built the full circuit yet, the logarithmic convertor design im using needs a matched pair of transistors.

I've attached my circuit design, this is just my 2nd attempt to fudge different circuit parts together to form a working complete circuit so i expect i've made some mistakes. Hopefully someone could check it over for me please.

I'm afraid i don't understand the connections required from the oscilloscope to get frequency vs voltage displayed, does it require x-y mode?

My circuit is a 555 astable producing 100Khz square wave, this is required by the MF10 switched capacitor IC.
The 12.6V is required to give 5V + and - 5V square wave output from 555.
I havn't bothered including MF10's components values as this is specific to the filters qualities.
The logarithmic convertor is taken from a textbook.

All critiscm welcome, thanks for all your help guys

--circuit attached-- plz excuse paint i havnt worked out ltspice yet
 

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audioguru

Well-Known Member
Most Helpful Member
The last circuit you posted is for the vertical channel of the 'scope for it to display the amplitude.
Now you need a logarithmic ramp for the horizontal channel of the 'scope and to drive a voltage controlled oscillator that feeds the other circuit.
 

Mikebits

Well-Known Member
I have done something like this before using the X ramp output on the back of scopes. I doubt the pico scope has that feature but your analog scope might.

But I think I agree with NIgel. Good old fashion paper and take data points.
 
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confounded

New Member
i've downloaded the latest software from picoscope (1st time for a year) and i think it now has the x-y operation :)
 
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