expo converters

[macdonald at evenfall.com]

I have just been experimenting with a LM13600 based VCO (sawtooth/pulse
circuit in figure 16 of the National datasheet).  I used an expo
converter similar to the one in Tom G's VCO5a, (two 3906's with the
emitters tied to V+).  I can get decent V/oct performance at low
frequencies, but the oscillator goes flat as the frequency climbs.  I
understand this is common in VCOs and can be compensated for ("high
trim").  I guess I have a few questions:

Does anyone know of an LM13600 VCO circuit that has high trim circuitry
that I can look at?  I haven't been able to find one.  Or maybe it can
be done in the expo converter circuitry.

I substituted a 500pf cap (actually 2 1000pf in series) for the
specified 50pf item (smallest ones I have - time to order some more
parts).  Do larger cap values increase the high frequency "droop"?

Is there a general or common approach to implementing "high trim" or is
it a circuit-specific problem?  

Why do some VCOs not have high trim (for instance the ASM-1)?  Is it
because these VCOs have a very fast discharge time and thus there is
less error at high frequencies?

Thanks,

Chris MacDonald