Author Topic: OpAmp for high-Q inductor in ultra-narrow band notch filter in controller applic  (Read 1523 times)

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Offline ricko_ukTopic starter

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Hi all,
I'm trying to filter out a "single" frequency (200 KHz). i.e. a ultra narrow band pass filter. It works quite well using a simple high-Q LC filter but with the inductor and capacitor tolerances it is hard to tune it to that exact frequency and I need it to be very precise. I have a microcontroller on the board so ideally I want to be able to tune it for the highest amplitude.
I know that op-amps can replace inductors. So I think I should be able to adjust (through a digital pot) the resistor in the op-amp circuit that simulates/replaces the inductor.
Would such solution work?
Can I get a very high-Q response by replacing the inductor with a op-amp circuit?
If yes, how?

Many thanks :) :)
« Last Edit: December 21, 2019, 04:15:01 pm by ricko_uk »
 

Offline awallin

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the notch center frequency might help here... kHz, MHz, GHz ??
 
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Offline dzseki

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Also if you exactly know what you are looking for consider lock-in amplification instead of high-Q filtering.
HP 1720A scope with HP 1120A probe, EMG 12563 pulse generator, EMG 1257 function generator, EMG 1172B signal generator, MEV TR-1660C bench multimeter
 

Offline ricko_ukTopic starter

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Hi Awalling,
200 KHz is the Frequency.
Thank you
 

Offline ricko_ukTopic starter

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Thank you Dzseki,
I googled for hours to find a lock-in amplifier IC or schematic but couldn't. Do you have any pointers for a simple solution?
As I mentioned in the reply above the carrier freq is 200KHz and I do have the original clock available on the board as sinewave (I can also make it square wave).
Many thanks :)
 

Online Kleinstein

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The OP based filters usually need an OP GBW well higher than frequency times Q value. So a high Q at 200 kHz is not very practical with an active filter. Here an LC filter is probably better. If needed one could tune it with vari-cap or so.

There could be an option to use a kind of synchronous notch. Some AZ OPs use this method internally, so the corresponding literature could help here.

Is a really narrow notch really needed ? If it is only about getting a deep notch one could use 2 in series.
 

Offline thm_w

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Are you talking about a bandpass filter or a notch filter?

https://www.ti.com/lit/an/slyt235/slyt235.pdf
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Online nctnico

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What does the circuit do? Why do you need to filter the 200kHz signal?
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline T3sl4co1l

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Also, a PLL, which can be analyzed in terms of a bandpass filter.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
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Offline ricko_ukTopic starter

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Thank you T3sl4co1l,
that sounds interesting. How? Where can I find infos on how to do that?

Thanks
 


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