# Help in tunable circuits

1. ## Help in tunable circuits

I need help in designing tunable circuits. I want to tune my circuit from 100 HZ to 500 Hz while designing filters. I have seen research articles based on eletronically tunable filters but they havent shown the tuning circuitry. How can I tune the circuit. Can I just tune by varying the biasing voltage?

2. ## Re: Help in tunable circuits

You don't say what you are trying to achieve here, it would help.
For this frequency range look at active filters, or digital filters. The topolgy to choose will depend on your application.
If you are just learning about filters then there are plenty of onlline tutorials to hellp you.

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3. ## Re: Help in tunable circuits

The 555 timer IC can be arranged to produce a pulse train of square waves. A potentiometer tunes the frequency. With proper values you should obtain a 5:1 range of frequencies.

Also there are simple square wave oscillators based on a capacitor and resistor creating an RC time constant. Relaxation oscillator is a general name for these. With experimentation you should be able to discover which resistor can be replaced by a potentiometer, allowing you to vary frequency.

Square waves can be filtered so you get reasonably sine-shaped waveforms.

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4. ## Re: Help in tunable circuits

Originally Posted by G4BCH
You don't say what you are trying to achieve here, it would help.
For this frequency range look at active filters, or digital filters. The topolgy to choose will depend on your application.
If you are just learning about filters then there are plenty of onlline tutorials to hellp you.
Thanks for your reply. I want to design a filter for biomedical applications. For the filter design I am using operational transconductance amplifier block. I want to tune it from 100-500 Hz.

I have read in one research article that In the transconductance gain gm of the OTA, it can be continuously controlled by an external voltage or current source over several decades, which lends electronic tunability to circuit parameters. Can we directly vary the current or voltage or is there some mechanism used to electronically controll this I and V.

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5. ## Re: Help in tunable circuits

Sorry I guessed wrong under the impression you wanted a tunable oscillator so you could test filter performance.

If you want a tunable bandpass filter then there are simple types which combine a few components (often capacitors and resistors), with a potentiometer adjust.
By adding an op amp you might obtain greater selectivity.
Possibly a Baxandall type reworked from the usual emphasize-attenuate function so it tunes frequency instead.

An LC series or LC tank is easy to suggest because it is a simple selective circuit. However your desired frequency range is low, so an inductor would probably need to be a large Henry value which tends to be large and expensive.

Or do you need a notch (bandstop) filter?

6. ## Re: Help in tunable circuits

I moved the thread to Analog Design because It's not related to RF and microwave.

OTA based filters, e.g. in gm-C topology are a popular implementation method of electronically tunable filters. There are also other ways, e.g. utilizing FETs as variable resistor. They have been discussed at Edaboard before and can be found in many internet articles.

OTA gm is tuned by varying the bias current of the differential pair. A second order filter needs usually two variable elements to tune the frequency without affecting other parameters.

7. ## Re: Help in tunable circuits

A gyrator circuit can emulate an inductor to allow tunable notch or band-pass filters.

8. ## Re: Help in tunable circuits

Thanks for replying FvM. You have said "A second order filter needs usually two variable elements to tune the frequency without affecting other parameters".

What two variables elements are used? Do I have to do the derivation and find out what two elements can be varied to tune the frequency or is there some other way?

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9. ## Re: Help in tunable circuits

Simulation of simple tunable bandpass filter. Two capacitors. One op amp.
The potentiometer changes the center frequency, through a range from 100 to 500 Hz. It also changes the amplitude. Of course this is not the only topology that could suit your purposes.

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