Can a MOSFET be used to vary frequency in a function generator?

[lcr2139]

I am trying to build a function generator and I am unsure of how to build a circuit to vary frequency. Would I use a MOSFET? Can someone help me?

 

[Bob S]

Use a NE555 astable timer. See http://www.doctronics.co.uk/555.htm#astable . If you need to keep the same duty cycle as you vary the frequency, then varying the capacitor is best. If you want a 50% duty cycle, then divide the NE555 output by 2 with a 7474 D-type flip flop, which would then permit varying a resistor to vary frequency.

 

[vk6kro]

There is an old function generator chip, called an Intersil ICL 8038. They are rare in Electronics stores, but Ebay sites have a few of them.

Here is a source of them:
http://www.ebay.com.au/itm/2PCS-IC-ICL8038CCPD-INTERSIL-DIP-14-/260968264013?pt=LH_DefaultDomain_0&hash=item3cc2ebb14d

These can provide sine, triangle and square wave output over the audio range and above.

Google for circuits of these chips.
Here is one:
http://www.eleccircuit.com/wp-content/uploads/2008/11/waveform-generator-with-icl8038.jpg

They are relatively simple to use but they need a power source with positive and negative voltages.

And you would need some way of calibrating it so that you would know what frequency was being generated. Look for a circuit with a digital display.

 

[DragonPetter]

They also make direct digitial synthesization chips. I know maxim makes one. I think this method is filling the space where analog waveform chips have begun to be obsolete or at least to not be manufactured anymore.

http://en.wikipedia.org/wiki/Direct_digital_synthesizer

You would most likely want to use it with a microcontroller, since they can be configured over serial links like a uart, i2c, or spi.

 

[Windadct]

If you are in the audio range ( 10 to 20kHz ) Another possibility is to use the Audio output of your computer, I believe there are some freeware programs that will give you what you need - I did find a $10 on on C-net ( http://download.cnet.com/Spectro-Signal-Generator/3000-2056_4-10858133.html )