[sdiy] STEAL this circuit!!!

[John Blacet]

Here is an interesting circuit that is designed to do precise up/down
octave switching on a VCO:

http://www.blacet.com/octSwitch.gif

In addition, it provides precision +10V and -10V references for the VCO
circuit and provides a handy "Tune" button for setting the scaling
trimmers.

Reference diode D1 is a handy device that is laser trimmed and very
precise without any adjustment nonsense; it does not even need a cap on
the output. It provides +5.00V which is inverted and scaled and inverted
again by two sections of U3, a TL054 op amp (a more precise TL074
version). This gets us about +10V and -10V. In this case we don't care
if it is *exactly* 10V, we just want it to be stable.

What we DO want to be precise is the +1.00V and -1.00V outputs because
we are going to insert them via a comparator controlled analog switch
into the frequency CV summing node of a VCO. (Remember: in a 1V/octave
VCO, adding a volt will cause the VCO frequency to go up one octave.)
Two more sections of U3 are used to do a divide by 10 on the +10 and
-10. Two trimmers allow us to dial in 1.000V pretty easily. Notice that
the +/-1.00V outputs are used as inputs to the analog switch located
just above on the schematic.

The analog switches, which are used in two parallel sections to lower
"on" resistance, are controlled by two comparator circuits built with
two sections of op amp U5. The 15K and 100K voltage dividers (R54, RN2
and R46, RN2) are used to set up comparator trip points at +2.5V and
-2.5V. An external input at J2 will turn the comparators on when a
voltage > 2.5V or > -2.5V is input. This in turn activates one or the
other of the analog switch groups to sent +1.00V or -1.00V to the
commoned switch outputs. This voltage when summed into the VCO freq
summer (not shown) will cause a one octave jump up or down.

Notice TP1, where a DMM is connected when adjusting RT3 and RT4.
Locating this point post analog switch avoids any scaling errors
introduced by the small amount of on resistance of the analog switches.
Also notice S2 "Tune". This is a PCB mounted push button switch that
will cause the VCO to jump up an octave. This can be used when trimming
the scaling of the VCO during calibration, avoiding having to procure
and connect a precision voltage source.

Regards,
___________________
John Blacet
Blacet Research
http://www.blacet.com