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Setup to test a filter

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atferrari

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I have never done this before.

Today I started to test the state variable filter in the attached circuit.

My setup is just a (brand new) function generator sweeping from 30 Hz up to 30 KHz, 1 V pk-pk - 0 offset. Output impedance set to 50 ohms and signal applied to R7 which not yet connected to the preamp.

Soon I realized that I needed to trigger the scope (generator's 2nd channel, delivering pulse with the same period of the sweeping).

Basically I get a response that barely looks what I could expect for HP, BP and LP (albeit irregular and asymetric) but I cannot find anything that I could call a notch.

Op amps polarization checks as +4,4V, in case you ask.

Eventually I could take some pictures tomorrow.

My questions: am I doing right?

How to measure fc? I suspect there is an obvious way but I cannot think of any right now. I have no markers available.

Comments and suggestions will be aprreciated.
 

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  • Directional microphone.pdf
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My setup is just a (brand new) function generator sweeping from 30 Hz up to 30 KHz, 1 V pk-pk - 0 offset. Output impedance set to 50 ohms and signal applied to R7 which not yet connected to the preamp.
R7 ?
Do you mean R8 ?

1V should be OK, as long as you are not overdriving the filter.
30hz to 30khz, is the sweep "continuous" or does it go in steps of (say) 10hz, like you may get from a digital synthesiser?

Soon I realized that I needed to trigger the scope (generator's 2nd channel, delivering pulse with the same period of the sweeping).
That sounds correct.

Basically I get a response that barely looks what I could expect for HP, BP and LP (albeit irregular and asymetric)
All the pictures you have ever seen of frequency response have logarithmic scales (decibels) on the vertical axis, your scope will be showing linear scales.
Similarly with the horizontal axis, it is common to plot wide range frequency responses on a log scale, yours is probably linear.

but I cannot find anything that I could call a notch.
Depends on the sharpness of the notch.
Also on the sweep speed, if the sweep speed is too fast, the filter will "ring" and you will miss the notch.

My questions: am I doing right?
Sounds as though you are on the right track.

How to measure fc? I suspect there is an obvious way but I cannot think of any right now. I have no markers available.
Do you have a second audio oscillator?
There is a technique which works with RF when you are using a diode detector to measure the output of the filter/amplifier, but I have never seen it done at AF.

JimB
 
R7 ?
Do you mean R8 ?

1V should be OK, as long as you are not overdriving the filter.
30hz to 30khz, is the sweep "continuous" or does it go in steps of (say) 10hz, like you may get from a digital synthesiser?


That sounds correct.


All the pictures you have ever seen of frequency response have logarithmic scales (decibels) on the vertical axis, your scope will be showing linear scales.
Similarly with the horizontal axis, it is common to plot wide range frequency responses on a log scale, yours is probably linear.


Depends on the sharpness of the notch.
Also on the sweep speed, if the sweep speed is too fast, the filter will "ring" and you will miss the notch.


Sounds as though you are on the right track.


Do you have a second audio oscillator?
There is a technique which works with RF when you are using a diode detector to measure the output of the filter/amplifier, but I have never seen it done at AF.

JimB

Yes R8. My bad; used an old version for reference.

Continuous sweep.

Have to check tomorrow what could be in dB and log.

I am using 350 ms for the sweep period. Is it too fast?
 
Hi,

To see a notch you'd also have to make sure that the high pass and low pass do not overlap too much. I would think it would work if they both have the same -3db frequency point, but i did not analyze this circuit to see what that requires. You should do that or at least check it in a simulator, and that will also show you what kind of waveforms you should be seeing on the output.
Also try different sweep rates.
 
Hi,

To see a notch you'd also have to make sure that the high pass and low pass do not overlap too much. I would think it would work if they both have the same -3db frequency point, but i did not analyze this circuit to see what that requires. You should do that or at least check it in a simulator, and that will also show you what kind of waveforms you should be seeing on the output.
Also try different sweep rates.

Gracias for replying MrAl

The universal filter has the three outputs available so if everything goes well the notch can be built by adding HP+LP as I did with the 4th op amp.

All outputs enjoy a common fc.
 
Probably far too fast - adjust it manually and take manual readings at spot frequencies.

Sweep frequencies need to be FAR slower than the range you're sweeping, so 350mS would be OK for an RF sweep.

I came down to a 3 seconds period.

After a long time trying, I learnt that my DSO has a strange way of changing settings, even if I do not use the autoset mode.

The circuit is assembled in a very cramped perfboard. Maybe there is something wrong that I am not seeing.

This morning I assembled the same circuit in a protoboard **broken link removed**, brought back in service my old and friendly analog scope and enabled the generator's sync output.

The notch is there, as we use to say, "big as a house" together with other expected outputs.

Being my first time with this, I feel happy. **broken link removed**
 

Attachments

  • Fotos universal filter.pdf
    87 KB · Views: 131
Ahh, digital oscilloscopes!

They can do some very funny things to waveforms with widely varing frequencies.
Try displaying an AM modulated RF signal sometime. :rolleyes:

JimB
 
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