[sdiy] HF VCOs and tracking problems

[Magnus Danielson]

On 11/11/2012 09:54 PM, Karl Ekdahl wrote:
> 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!

Quick brain-dump:

The reset of the capacitor takes a fixed time, while the time to charge 
the capacitor expects to half on the doubling of the current from the 
expo-converter (for a typical sawtooth oscillator).

The trick being used is to insert a serial resistor to offset the 
voltage level of the capacitor, such that it would have to charge a 
smaller voltage for the same current, and hence a shorter time. It turns 
out that for this simple model, you can perfectly cancel the reset-time 
by letting the RC match the reset-time.

It should be noted that the actual property of the reseting transistor 
is important, and that it vary from transistor to transistor.

For reference, I had my ASM-1 VCO clocked in up to 160 kHz, but the last 
octave wasn't tracking very good, since the expo-transistor went into 
saturation. So, for my expo-pair (MAT-01) I had a current-limit to 
fight, so scaling the capacitor differently will be needed. A different 
expo-pair might give you more current to work with.

More on ASM-1 here:
http://rubidium.dyndns.org/~magnus/synths/friends/stopp/

I've made some notes on the ASM-1 VCO reset-compensation, so you will 
find a little more details in there.

Cheers,
Magnus