DIY parts

[terry michaels]

-------------Forwarded Message-----------------

From:   terry michaels, 
To:     INTERNET:ijfritz at earthlink.net, INTERNET:ijfritz at earthlink.net
        
Date:   8/19/98  8:48 PM

RE:     Re: DIY parts

Ian - -

The gate capacitance issue is really a minor one, I think in most cases it
can be ignored.  The only time it might require consideration is if you
substitute a FET with a lot of capacitance (50 pf or more), or you want a
really perfect sawtooth.

The design of most FETs creates gate capacitance to the source and drain in
equal amounts.  The KE4859 has about 6 pf from the gate to the source, and
6 pf to the drain.  The fast rising gate pulse couples charge into the
input end of the integrator with the ratio 6pf / 2400pf, which is less than
1% error.  This can be ignored.

A bigger problem is the output impedance of the CA3130, which is at least a
few hundred Ohms, and possibly much more, when dealing with a fast pulse. 
An op amp in a closed loop has an output impedance of a fraction of an Ohm
at low frequencies, but rises to hundreds of Ohms at high frequencies,
because the internal gain of the op amp rolls off as you go up in
frequency, eventually reaching zero.  The result with my circuit is the 12
pf gate capacitance raises both the input and the output of the CA3130
together, creating a 2 volt spike on the top of the sawtooth, lasting 0.2
microseconds.  Again, this probably won't affect any practical use for this
VCO, unless you use a different FET with much greater gate capacitance.

The 2N4392 has about 1/3 the leakage current, and about twice the on
resistance of the KE4859.  I think it should work well enough that you
probably wouldn't see any difference if you substituted it.  At worst, you
might have to increase the value of the 18 pf cap across the LM311 a
little, to allow for more discharge time.  I can copy a data sheet for both
FETs and send it to you by snail mail, if you like. 

BTW, I recently found out my VCO has a low level, high frequency
oscillation in the exponential converter section at certain current levels.
 It can be fixed as follows:
* On the LM308, delete the 68 K resistor from pin 2 to +V.
* Change the 1K resistor from pin 2 to ground, to 1.5M (you can make this
smaller if you want, but no smaller than 100K).
* Add a 10pf cap from pin 2 to pin 6.  Make it 50 - 100 pf if you go with a
100K resistor above.
* Change the 6.8K resistor to 2.7K.

Please pass this on to anyone you know that is using this circuit.  My
apologies for not catching this in the beginning.

Terry