About NATIONAL APP NOTE #299 VCO

[Joachim Verghese]

On Wed, 24 Apr 1996 gstopp at fibermux.com wrote:

>      Yes - please try this VCO and tell us how it works. I'd like to do it 
>      all over again now that I think I know what happened, but I haven't 
>      found the time.

I built the AN-299 VCO on a breadboard last December, and here are some
notes I made:

-  -  -

First of all, I'd like to point out that most circuits that are presented
in application notes are "ideas" only, not complete designs. IMO, the
essence of AN-299 is the 3046 temperature regulating scheme. The VCO
itself is not what I'd use for serious work.

Anyhow, what's missing from the AN-299 circuit is what experienced
designers take for granted: decoupling capacitors at supply rails
and *DC references*. The first thing to do is to put 0.1uF capacitors
between the supply rails (+-15V) and ground, close to the VCO op-amp.
The most important modification, though, is to place a large
capacitance across the LM329 reference. The current drawn through
the 600 ohm resistor is relatively large, and also dependent upon the
output waveform. If you monitor the voltage across the LM329 reference
without decoupling, you'll see saw-shaped fluctuation. What we want
is a stable, ripple-free 6.9V reference, so decoupling here is essential.
I connected a 100uF/16V electrolytic in parallel with the LM329, and
this made the circuit behave properly.

Another thing, I found large positive spikes at the sawtooth retrace
phase. These might have been caused by bad circuit layout, didn't have
time to experiment more. The phenomenon might also be due to the fast
response of the retrace transistors. Retrace time was found to be approx.
1 microsecond. To get rid of the spikes, I connected a 1N4148 diode from
the inverting input of the op-amp to ground (cathode to ground). This
also improved high frequency stability a lot. The VCO now oscillated well
above 40kHz. The drawback to adding a diode is that it might affect
temperature stablilty.

Also, while it might not be obvious from the drawings, the op-amp(s) have
to be powered by a dual supply for the circuit to oscillate at all.

-  -  -

-joachim