[sdiy] High frequency VCO as BBD clock

[Pete Hartman]

John Blacet's Time Machine uses a VCO in some ways similar to this.  He has
a VCO constructed with an LM13700 and a single op amp from an LM324, and
then uses a transistor buffer into a CD4013 to make the two-phase clock
that the BBD uses (an MN3005).

For the MN3005 the clock out from the 4013 only has to go from 2KHz
(shortest delay) to 200Hz, so you don't get up into the tens or hundreds of
kilohertz for that.  In John's case he also put clock driven aliasing
filters around the BBD (LTC1063), and those need 80KHz - 800KHz -- those
are generated with a 4046 and a 4518 to make a clock multiplier from the
VCO range, so the filters adjust their cutoffs to accommodate the different
BBD rates.

Pete
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