controlling oscillation frequency with a voltage controller current source

[JonRock]

Im trying to get a VCO to work at very low frequencies and I stumbled upon this short paragraph:

"For low-frequency VCOs, other methods of varying the frequency (such as altering the charging rate of a capacitor by means of a voltage controlled current source) are used"

unfortunately theres was no further elaboration. Does anybody have any expierience with this technique? What is probably the lowest frequency achievable and what might be the problems?

any advice is appreciated

 

[MikeMl]

I posted a circuit in this thread.

 

[JonRock]

Is there any way to do something similiar creating a sine wave?

 

[MikeMl]

Is there any way to do something similiar creating a sine wave?

Not easily. How wide a frequency range do you want to tune?

 

[JonRock]

best possible result would be something like 1Hz to 50Hz
When you say "not easily" do you acutally mean "no" or "not unless you find a 5MH coil" or do you have anything in mind how it might work?

 

[MikeMl]

Create a voltage-tuned astable, and then put a low-pass filter behind it. Tough to do over a 50:1 frequency range, though...

Better to use a ucontroller and a DAC, and have it synthesize the sine wave programmatically.

 

[BrownOut]

Is there any way to do something similiar creating a sine wave?

Yes there is. You just need a wave shaper. You can make a pretty good one with an emitter coupled pair. I have one, but it uses alot of transistors. And it doesn't work down to your frequency.

 

[JonRock]

yeah I thought about that (µc) but I was curious if there was an analog solution
thanks anyway

 

[JonRock]

Yes there is. You just need a wave shaper. You can make a pretty good one with an emitter coupled pair. I have one, but it uses alot of transistors. And it doesn't work down to your frequency.

could you elaborate that a little, I dont think I know what you mean yet? thanks

 

[BrownOut]

It's a triangle wave to sine wave shaper. You can generate a triangle wave by integrating a square wave. Alternatively, if you're using a timer to generate your signal, you can get a pretty good triangle wave from the timing capacitor. I'll try to cobble up a schematic when I get home....

 

[JonRock]

okay I guess I can figure out how to get a triangle output. How does a waveshaper work? Is it some sort of filter? Do you maybe have a link to a data sheet or something?

 

[crutschow]

You can generate low frequency sine-waves with a function generator IC, such as the XR-2206, which use wave shaping techniques. It has a small amount of distortion, so may not be usable if you need a very pure sine-wave.

 

[Diver300]

Use a DDS IC such as https://www.analog.com/static/imported-files/data_sheets/AD9833.pdf

 

[BrownOut]

okay I guess I can figure out how to get a triangle output. How does a waveshaper work? Is it some sort of filter? Do you maybe have a link to a data sheet or something?

Look at THIS LINK. Go to page 8 and look at figure 10a. See the logarithmic waveshaper made from the LM394 at the bottom of the diagram? The input to the left most device is a triangle wave, and the output from the right device is a shaped sine wave. The author claims the distortion can be kept low at .35%.

Now say thank-you. It took me for freeking ever to find this app note again.

 

[RCinFLA]

You can add a voltage controlled current source to a 555 timer circuit.

Most 555 timer circuits you see use R-C for frequency. The resistance can be replaced with a current source.

 

[JonRock]

Look at THIS LINK. Go to page 8 and look at figure 10a. See the logarithmic waveshaper made from the LM394 at the bottom of the diagram? The input to the left most device is a triangle wave, and the output from the right device is a shaped sine wave. The author claims the distortion can be kept low at .35%.

Now say thank-you. It took me for freeking ever to find this app note again.

That looks really helpful. Thanks alot!

 

[audioguru]

Instead of re-inventing the wheel, use an XR2206 function generator IC. It is still being manufacturered.
It makes a sine-wave from 0.01Hz to 1MHz.

 

[crutschow]

Instead of re-inventing the wheel, use an XR2206 function generator IC. It is still being manufacturered.
It makes a sine-wave from 0.01Hz to 1MHz.

Deja vu all over again.