Fast VCOs/V->F converters

[Douglas R. Kraul]

This has been a while so bear with me...

Linear V-F converters can yield disappointing results when used as VCO
substitutes. Do the math on the linearity spec taking into account the fact
that the advertised linearity (.05%) in this case is on the *full scale*
range of the frequency output.  I think that you'll find the error expressed
in cents to be too high for practical use.

Almost all VCO and similar devices use an integrate and dump circuit
technique.  These devices fail at higher frequencies primarily do to delays
in the control path - how long it takes to detect the time to reset and how
long it takes to respond.  Parasitic effects also become problems at higher
frequencies.  Semiconductor junctions have capacitance that varies with
junction conditions.  Smaller capacitors in the integrator will make these
effects more pronounced.

Finally, capacitors themselves do strange things under transients like when
they are discharged.  Polystyrene (sp?) are the best and were used almost
exclusively in the finest VCOs of days past.  The problem is sometimes
called "hook" and really amounts to the capacitors having  more capacitance
at higher discharge rates, hence the oscillators go flat.

These effects are problems even at the upper end of the normal VCO range.
At higher frequencies they become even more pronounced.  Newer technology
semiconductors could improve these problems but I doubt it.  Again, the true
pro VCO designs of the mid 70's used compensating tricks to keep them from
going flat.  Emu's stuff was by far the best.  Most of the tricks also
relied on oscillator matching in order to prevent the errors from being
obvious - the famous tracking adjustments.

As for using a frequency multiplier approach, waveform shaping is about the
best but not practical above 4x or so do to again circuit errors.  Stay away
from PLL approaches because loop stability can be a big error factor in
musical uses.

All that said, good luck.  Just go into it with your eyes open that it is a
serious technical challenge.

-----Original Message-----
From: Arthur Harrison <theremin1 at worldnet.att.net>
To: DIY <synth-diy at mailhost.bpa.nl>
Cc: Karl Helmer Torvmark <karlto at invalid.ed.ntnu.no>
Date: Saturday, November 21, 1998 7:14 PM
Subject: Re: Fast VCOs/V->F converters


>Karl H. wrote:
>>
>>I've been thinking about doing a wavetable oscillator project lately, and
>>like several others people have remarked, getting a fast enough VCO to
>>drive it is difficult.
>>
>>However, I've just looked through my favorite Linear Technology appnote
>>(AN14 - Designs for High Performance V-to-F converters), and the "Fast
>>Response 1Hz->2.5MHz V->F converter" design looks promising. Now, how do
>>we convert this from a V->F to a I->F converter (so we can use a regular
>>expo. converter)? Anybody with more analog design experience that me care
>>to comment?
>
>An I to V converter could be an operational amplifier (signal fed to
>the summing node).  Perhaps the exponential function may be
>implemented directly in this stage's feedback loop.
>
>There are some Ananlog Devices VCOs, too, that might have a large
>enough range.
>
>>If it can be made to work, it can provide 20Hz-20kHz bandwidth with a 64
>>byte wavetable with speed to spare, also fast response ensures you can use
>>FM on it as well. Linearity of the V-F itself is spec'ed to 0.05%.
>
>Sounds good to me!
>
>-Art
>
>