VCO help needed.

[Rob01]

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From: haig at cleo.murdoch.edu.au
To: synth-diy at horus.sara.nl
Subject: VCO help needed.
Date: Tuesday, May 27, 1997 12:52 PM

Greetings.  I'm new to this list and new to building so please bear with
me.

To date I have only completed the +/-15v dc regulated power supply.  I'm in
the process of constructing the VCO labelled "National Semiconductor
Appnote
AN-299: A temperature controlled exponential VCO" (I'm afraid I've
forgotten
the source).  It has gone very well:  I have the basic breadboarded circuit
working.  I added trimmers at the LM329dz (to calibrate the reference
voltage to the required 6.9v), inline with the offset resistor, and (along
with a smaller  resistor) in place of the 250 ohm resistor near the "cv in"
for calibration of the range.

My main concern is with the comment that "An actual application would
include output decoupling".  Being new to electronics I'm not clear on
this.
Does it refer to the fact that the ramp signal may be riding on a dc
voltage, and that it is this 'offset' which must be 'decoupled'.  If so
would a simple highpass filter (consisting of, say, a 0.47 uF capacitor and
a 1 MOhm resistor) at the 'ramp out' centre the signal at ground.

This particular configuration WOULD most certainly drop off the DC offset.
>From what i can tell, the main reason you do this is so you dont exceed the
input voltage of later stages such as filters, env, etc. IF this is what
they mean by "output decouple". Basically, you want to have a basically
purely resistive element to ground ie resistance to ground, and a cap to
the next stage. This combination should force the DC bias to drop off to
ground, leaving you with the waveform on the other end of the cap. DC will
see the cap as an open, and not be able to travel through it. So it will
dissipate through to ground. The AC waveform will see the cap as a short,
and continue on to your next stage. Makes sense, what youve said above! I
just think that perhaps your coupling cap may be a little high.

My other query is with the LM3046.  Apart from the limits imposed by the
required connections, does it matter which transistors in the package are
used.  I have left out the one with its emitter connected to the substrate,
as some directions I found indicated the substrate should be connected to
the lowest voltage in the external circuit.  When I connected the substrate
to ground there seemed to be no difference in the result.

I would say that to leave ANALOG circuits unconnected shouldnt pose a
problem. I have left ANALOG circuits unconnected without too much probs. I
personally only worry about digital stuff, like 7000 series stuff and esp
CMOS stuff when it comes to leaving stuff unconnected, cuz they will float
and the energy left in the substrate and semiconductive material will
sometimes trigger a false data state, but i dont think it changes analog
stuff too much. Im no real design expert, but from what i have messed with,
to leave these unconnected seems feasible.