[sdiy] HF VCOs and tracking problems

[Karl Ekdahl]

Hi list

I'm trying to design a high frequency VCO, something that can span a frequency range of ~6-200kHz. I'm basing it off of a standard sawtooth core and i'm experiencing tracking problems. I realized this might be an issue before i begun but i don't really understand what the underlying problem is, nor the remedy.

I've seen that a lot of VCOs have HF-adjustments, i figured however that this was so as to be able to have a greater frequency range, rather then just controlling high frequencies in general? Obviously i was wrong about my assessment that simply changing the main capacitor to a lower value would do the trick.

So what is the problem here, is it capacitor leakage? Something inherent to the classic transistor exponential converter? I'm absolutely horrific at maths so any explanation using heavy equations with strange looking symbols won't really help me :)

Thanks!

Karl