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# low frequency tuned oscillator

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#### michiel

##### New Member
Recently i started a project: trying to build my own function generator.
The most important reason for me doing this project is learning more about oscillators and their limits and possibilities.
So far i found a few designs such as the colpitts oscillator and the wien bridge oscillator.
The problem i have with both is their low tuning range due to the low values of availible variable capacitors(100pF max on farnell).
I'm aiming for a sine with a frequency from 10Hz to 500kHz though i could realise this with multiple oscillators.
I would appreciate your input in this.

Look up the historical Hewlett-Packard model 200. It used a Wein bridge with a dual-section variable capacitor out of an AM desk-top radio. AFAICR, that oscillator had several ranges, the lowest of which went down to 20Hz.

Maybe you can find one and "transistorize" it...

Of particular interest is how Drs H & P stabilized the oscillator so that it has low harmonic distortion.

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I'm aiming for a sine with a frequency from 10Hz to 500kHz though i could realise this with multiple oscillators.
Using "simple" oscillators, it is usual to limit the frequency ratio to no more than 3:1 for LC oscillators (often only 2:1 in more exacting applications), for Wein bridge types using a variable R or a variable C to tuning ratio is usually no more than 10:1.

You are trying for a 50,000:1 ratio. Using simple oscillators you are not going to achieve that in one range.

It could be done by mixing two oscillators, one fixed and one variable.
You could have a fixed 10Mhz oscillator, and a variable 10 to 10.5 Mhz oscillator (nice easy 1.05:1 tuning ratio), feed these two oscillators into mixer and lowpass filter the output to get 0 to 500khz.
But this is getting away from "simple".

Another way of obtaining a wide tuning range is to use a Direct Digital Synthesiser (DDS).
Modules for these types are cheaply available:
But then this is not simple, but all digital and complex.

JimB

It could be done by mixing two oscillators, one fixed and one variable.
You could have a fixed 10Mhz oscillator, and a variable 10 to 10.5 Mhz oscillator (nice easy 1.05:1 tuning ratio), feed these two oscillators into mixer and lowpass filter the output to get 0 to 500khz.
But this is getting away from "simple".
JimB
thanks for the response.
this seems feasible to me. i have an AD633(multiplier) ic laying around from a FM project.
i would use a crystal for a fixed frequency and a varicap diode for the variable lc/rc oscillator.
the AD633 has a bandwidth of 5MHz which might be due to the op amp buffer.
I'll test the multiplier with 10MHz after the weekend and post the results for anyone interested.

it doesn't "HAVE" to be 10Mhz. you said the multiplier has a bandwidth of 5Mhz, you could so it with a 5Mhz fixed source and a 4.5-5Mhz variable oscillator. you will get the same results, with the only difference being the direction of the tuning. using a mixer to get difference frequencies works whether your variable oscillator is above or below the fixed source.

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