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Linear VCO's

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kieran.french

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Hey,
My name is Kieran and I am a final year Electronic Engineering Student in University College Cork, Ireland. I am currently designing a synthesiser for motor/encoder simulation. My design is based around the concept of a voltage to frequency converter being linear 'through zero'. By this I mean that I require a voltage to frequency converter to be directly linear right down to zero, and remain linear right up to frequencies up to 2 - 2.5MHZ. I am having trouble locating a commercially availble VCO to perform this function, the best I have seen is linear up to 1MHz.
Can anyone suggest where to find such a V/F converter, or even enlighten me as to their existence. Any help would be greatly appreciated.
My email is kieran.french@gmail.com.
 
hmm that is a very large range
you would need a 21 bit A/D converter to get that range , which is why you probably arent going to find one..
why does it have to be to 2.5 MHZ..?
 
I agree with 'willib', that's a massive range - I'm not suprised you can't find one. If you do, I would expect it to be pretty expensive as well.

As a possible solution, as you are talking radio frequencies and not audio ones - how about two RF oscillators, one fixed (preferably crystal controlled) and the other variable between the same frequency and 2.5MHz higher or lower. Apply both oscillators to a double balanced mixer and the output will be adjustable between 0-2.5MHz as you alter the variable oscillator, you could fit a low-pass filter on the output of the mixer to futher reduce any oscillator bleed-through.

The frequencies of the two oscillators are up to you, but bear in mind you want a linear sweep, so a higher frequency is probably better - and the low-pass filter will be more effective.
 
I'd be a bit concerned about the zero end of the scale too. It's going to take some time to test it at the low end of the range. I guess I'm too old to build stuff like that, I'd be dead long before I'd verified that 0Hz output. But wait - the OP said "through zero", so does that mean negative frequencies have to be generated?
 
JohnBrown said:
I'd be a bit concerned about the zero end of the scale too. It's going to take some time to test it at the low end of the range. I guess I'm too old to build stuff like that, I'd be dead long before I'd verified that 0Hz output. But wait - the OP said "through zero", so does that mean negative frequencies have to be generated?

My two oscillator scheme would do that as well, assuming the sum was positive, and the difference was negative :lol:
 
hi guys,
Just want to say thanks for all the replies. I will try to implement WilliB's idea, it sounds perfect for the project objective. I will keep in touch and let ye know ho I get on.
Thanks,
Kieran :D
 
kieran.french said:
Sorry Nigel, your idea was the one I will try to implement.
Thanks

Thank you!.

It's not an original idea by any means, it's a common method of making wideband communication receivers and spectrum analysers - a very old tried and tested technique.
 
kieran.french said:
Sorry Nigel, your idea was the one I will try to implement.
Thanks
I agree with Nigel that the heterodyne scheme should work well - if you can keep the two oscillators from "talking" to each other. I know from experience, and from reading the literature, that it is difficult to prevent one oscillator from injecting a small amount of energy into the other, causing "frequency pulling". When you get down near zero difference frequency, you may see the VCO suddenly lock to the crystal oscillator's frequency. You need to plan your ground plane very carefully, and Faraday cage(s) might be required.
If you only need the linearity, but don't actually need very low frequencies, then frequency pulling might not be a problem, but you might still see some phase jitter.
 
One other thing to keep in mind: If your "baseband" VCO (e.g., AD652) is at 10kHz, and it drifts 0.1%, it will move 10Hz. If your heterodyne output is at 10kHz, and its VCO is 10.01MHz (crystal oscillator is 10.00MHz), and the VCO drifts 0.1%, the output will move 10kHz - a 100% change! Obviously, it gets even worse at lower frequencies. Now, you could use a lower crystal frequency, but then your VCO deviation would have to be proportionally larger, which gets to be more difficult at RF frequencies. I'm not saying it's impossible (but it might be), and if you don't need to go that low in frequency then maybe it's not a problem, but it's something to think about. If the high-frequency VCO is in a feedback loop like a PLL, then these drift problems may go away, but I don't know how to do what you want with a PLL.
If you don't need instantaneous frequency agility, think about direct digital synthesis.
 
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