LINEAR VCO AND CMOS CLOCK DRIVER             Scott R. Gravenhorst
                                          chordman@concentric.net
                                                         06-26-96

The schematic VCNGVCO.GIF depicts a linear VCO with two outputs: 
Audio and CMOS clock driver.  It was designed to allow building 
electronic music applications which require a VCO clocking.  The 
clock output circuitry is implemented in CMOS, so it is best to 
design appliances for the VCO to drive using CMOS as well.  The 
VCO itself is a standard design having 2 stages: an integrator 
and a schmitt trigger.  The output of the schmitt trigger is 
amplified to CMOS logic levels by one transistor, buffered and 
sent to two divider chains to provide a switchable 5 (or more) 
octave range for both the audio and CMOS clock outputs.  This 
allows a wide variety of harmonic relationships between the VCO 
itself and the device clocked by it.  The VCO has been tested on 
a PAiA FatMan and can be tuned to track it's 5 octave (c-c) 
keyboard range.  There are portions of the VCO's operating range 
that are not particularly linear, but could be useful for 
clocking non-tonal circuits like a noise generator.  The VCO has 
two outputs: one for clocking CMOS systems and the other is an 
opamp buffered audio output, 8 volts p-p.  My implementation 
clocks a noise generator and a complex waveform generator, both 
based on CMOS XOR feedback shift register systems.  These 
circuits will be placed in the archive as soon as their designs 
are finalized.  

The CV input circuit was designed to conform to the CV output of 
a PAiA FatMan, but with little change would work on any linear 
(volts / hertz) synthesizer.  A volts/octave synth would require 
an exponential converter in front of the CV input.

The CMOS output circuitry provides a logic level that swings from 
0 to 15 volts.  Other levels may easily be derived by diode 
limiting and buffering.  Do NOT reduced the supply voltage 
values.  My original intention was to power this from +/- 12 
volts, as it was already available in my system, but the 
oscillator refused to oscillate.

TUNING ADJUSTMENT:
There is an adjustment to remove any offset voltage present in 
the CV.  Offset voltage will effect the learity of the VCO.  
Start the adjustment by setting the offset pot to it's minimum 
resistance (zero offset null) and switching to the lowest (f/16) 
octave setting.  Set the frequency somewhere in the upper quarter 
of it's range.  The best linearity curve is in the high end.  
Play the highest available keyboard key and tune the VCO to it 
using the front panel tuning pots (the fine control will probably 
be necessary as the adjustment at the high end can be touchy).  I 
started at the high end as high as I could still detect a beat 
note.  I found the top octave to be not linear enough, but the 
next two down were.  Now play the lowest note and adjust the 
offset pot to match the pitches of the new and existing VCOs.  
Repeat the high and low end adjustments until they match.  If the 
VCO is in it's linear range, the keys in the middle of the 
keyboard should track very closely.  It is important to note here 
that nonlinearities in the CV supplied will make this VCO appear 
nonlinear.  Make sure the driving synth is in tune with another 
standard instrument (like a good digital synth) all across it's 
range.  I had to completely and precisely adjust my FatMan's CV 
scaling pots before I could adjust this VCO.  

FATMAN SPECIFIC NOTES:
If you want to be able to use the FatMan's pitch control as
master pitch, connect the CV input to the center lug of R34 
instead of the CV output RCA jack.

See FATMIXER.GIF and FATMIXER.TXT for details on how to connect 
this and other devices to a FatMan's VCF input.