PLAN B analog blog

....actually, any VCO with switchable integation timing:

In order to shift the frequency center of a VCO, the preferred method is to change the 
basic charge time in the integrator. The integrator is the thing that forms the core 
waveform.  Depending on how it recevies it's drive current and where the charge cap is 
located it will oscillate into either a saw or triangle wave.

By increasing the cap size you increase the charge time, and that increases the period of 
the waveform.  translate that into hertz (hz=1/period),  the frequency slows down.  The 
cheap and dirty way to do this is to simply have a switch-selectable cap value so that 
depending on how the switch is thrown, either one or two caps are in parallel with one 
another, sending it into either  high or low frequency.

The problem with the cheap and dirty way is the cap is essentially routed to the faceplate -  
the wires, traces, or combination of the two that introduce the second cap has to be 
physically routed through the switch, which is mounted on the faceplate.  It's not the 
faceplate part that's the problem....it's the length of conductor required to do that.  

The general rule of thumb is to make the traces in the core as short as possbile.  In the 
new rev2 PCB I paid close attention to that detail.  Where the older art he core components 
were sprinkled over one half of the board, in the new on the entire core is in a 1 1/2 x 1 
3/4 inch square on the board...and I still hang one filter cap onto the thing for safe 
measure.

The 'proper' way of doing this would be to utilize an analog switch which would be located 
'close to the action' which would pop the second cap in place based upon the faceplate 
control, which would be driving the enable end of that transimission gate - electroncally 
isolated from the signal path.

Why am I telling you guys this?

Not sure.

Probabay because I'm concerned how that would effect the fidelity and stability of the 
circuit, which in my way of looking at things kills the benfiit  of the lower range.  

(Notice I haven't said I'll look into incorporating this)

Possibly I may, but it'll be some tie  before we get to that.  A LOT on the plate right now.

- P