[sdiy] EMS VCS3 VCO

[JH.]

> You can not run a current source (like expo transistor collector) directly
> into a capacitor.

Nothing wrong with a current source into a cap.

> Start up transient is infinite and stalls simulation.

Not infinite. Just the current value of the current source.

> Put a resistor of some milliohms between transistor collector and cap.

I would be surprised if that would work. (But then again Spice is full of
surprises,
and I got Spice circuits running - which should have been runnng in the
first
place, but didn't, that is - by just storing them under a new name. Crazy.)

But back to the problem. The one thing undefined is the initial capacitor
voltage, and consequently the state of the hysteretic switch - Spice cannot
know this, and when it guesses, it may be right or more
often it will be wrong. So put a switch acros the cap that opens at
something
like t=1us, or simply give the cap some resonable initial condition - IC -
(i.e.
starting voltage). In oscillators, it often helps to set this IC slightly
outside
the normal oscillation range: Asuming the cap voltage normally goes between
+5V and 0V (not speaking about a particular VCO here), when you set the IC
to +4.5V, Spice can still not know if you want it in an upwards or downwards
(chrching or discharging) state. So it's bettet to set it to, say, +5.5V.
Then
the hysteretic switch of the VCO has a well-defined state, and the cap
*will*
be discharged when simulation starts. Normally the oscillation will then
be kick started just right, with a slightly wron first period, of course.
You may want to mask this first period when you trace your output, i.e.
start recording the output into file after one period.

This works great for me. (I'm using PSpice V8.) Hope this helps somehow.

JH.