[sdiy] VCO Tuning goals

[Ian Fritz]

At 05:02 PM 10/2/2009, David G. Dixon wrote:
> > With regards to the HF trim: isn't the diode a critical component?
> > What can happen when the diode is less than perfect?
>
>Diodes are less than perfect by definition.  However, to me, the key
>question is, how does a given diode's I-V response correlate to the needs of
>the circuit for a certain amount of correction current vs frequency (CV);
>i.e., the I-V requirement for HF correction?  Is the exponential response of
>a forward-biased diode the right curve?

If the diode characteristic is a worry then you can simply use a 
diode-connected transistor of the same type as the converter 
transistor.  BTW the purpose of the diode is to produce a resistance that 
follows the Rbe of the converter to produce the appropriate bucking voltage.


>  What about the square-law transconductance of a JFET?

Not sure what this refers to.


>I think (again, don't quote me here) that I'm referring to the finite
>on-resistance of the cap-shorting JFET, which imposes a finite time
>requirement to shunt the charge on the cap to ground.  This time is
>inconsequential at low frequencies, but substantial at high frequencies.  I
>don't believe that the comparator per se has anything to do with it,
>presuming that an actual comparator (ultra-fast) or a reasonably fast opamp
>(TL07x, with a slew rate of 13V/us) is used.  If I'm wrong about this, I'm
>sure I'll be corrected...

There are two common sources of tracking error.  The imperfect log 
conformity of the expo converter, and the dead time during which the core 
switches its state, eg the capacitor discharge time.  It is common to just 
put in one HF tracing circuit to take care of both of these.  For example, 
the popular EN-76 VCO uses the reset correction circuit to cover both. 
(Franco compensation.)  But some super fussy people sometimes actually put 
both in:
http://home.comcast.net/~ijfritz/sy_cir2.htm

   Ian