[sdiy] Temperature Compensated VCO attempt - help?

[Magnus Danielson]

From: Don Tillman <don at till.com>
Subject: Re: [sdiy] Temperature Compensated VCO attempt - help?
Date: Sun, 2 Feb 2003 22:53:16 -0800

Hi Don,

>    > Don:
>    > > I'm really curious how the thru-zero VCO works?  (Perhaps I may have
>    > > to break down and finally buy a copy of Electronotes.)
>    > 
>    > Basiclly, start of with a triangle oscillator core and when you
>    > go though zero you reverse direction by chaning load current
>    > direction. This can be done in several different ways, but for a
>    > triangle oscillator core you allready have an OTA to do the
>    > current direction anyway, so that part comes for free.
> 
> Hey Magnus,
> 
> Sure, but I'm always interested in innovative ways to actually
> implement this.  I have yet to see one that goes through DC real
> gracefully.

My bid is:

Put a Gilbert-cell on top of an expo-converter. Put the linear (sign-swapping)
modulation on one of the Gilbert-cell input pair, put the Schmitt-triggered
output waveform on the other input pair of the Gilbert-cell. The current output
of the Gilbert-cell is connected to an integrator and this is followed by a
Schmitt-trigger. The current in the Gilbert-cell is modulated from the
expo-pair. This should be both a compact and pretty well-behaving oscillator
and since there is non abrupt switching of current but rather linear change in
current balance I think there is a good chance of a nice behaviour.

So, you've got both V/Oct and V/Hz inputs.

Sounds reasnoble?

Must I draw a schematic for it to be clear (it should be pretty clear if you
look at for instance Jürgen's FS-1 schematics).

No separate switches and directional comparator is needed. All that is hidden
away in the Gilbert-cell.

The Gilbert-cell actually doesn't HAVE to be linearized, but if you like to
avoid non-linearitizes in modulation it would be a good thing to have it.

>    > > I would think that the linearity of the OTA would be a major issue.
>    > > OTA's aren't especially linear... in the sense that they make great
>    > > triangle to sine converters.
>    > 
>    > This is why Scott and Jim used two different forms of
>    > linarizations. Scott used the builtin linearization diodes where
>    > as Jim used the other half of a LM13600 to do the linearization.
> 
> Yes, but what's the linearity error for these in practice? 

Well, any uncompensated diff-pair would have the output proportional to
tanh(Vdiff/26mV). The distorsion is considerable above a 26 mV.

> While temperature compensation is a good thing, to have temperature
> compensation at the expense of any less VCO linearity would be a bad
> thing (assuming you're using the VCO in the normal way).  Temperature
> compensation is just a convenience and without it I can always tweak
> the scale pot and be fine, but a nonlinear VCO means I can't play in
> tune. 

This is exactly why Jim stressed the point of temperature compensation for the
sake of stable scale. He has actually ignored the pitch offset, but that's
pretty darn good too in his scheme.

> Is a linearized OTA accurate enough?  I don't know.

According to Jim's measurement, yes.

> I would think that this is an area where some alternate VCA topologies
> would win.  A multiplying DAC f'rinstnace, though the digital part is
> not really appropriate here.  Or that OVCE arraangement that the SSM
> VCAs use; the feedback might help in this case.  Or one of those hifi
> VCA topologies that are connected opposite from the typical, so the
> control voltage accurately runs a current source for a diff pair, and
> the gain is set by the base voltage on the diff pair.

There's many different ways to filé a fish and there's pros and cons with each
one of them.

Where cutting our fingers on the bleeding edge here, but soon we shall all be
drinking from the holy grail, so at least a little more pain we should be able
to handle ;O)

Cheers,
Magnus