"gbowne1" ) writes:
Well now, I'm onto bigger and better things.

I was doing a bit of reading lately and came across some interesting
topic one being VCO. The VCO I saw used a Optical Shaft Encoder
(OSE). The setup used was rather odd looking to me and didn't look
that mechanically stable.

Could someone explain VCO to me? And, also what type of VCO would be
the best in a transceiver considering the technology of the past 10
years? Are there digital VCO's?

Greg

A VCO is a Voltage Controlled Oscillator. It's not a different type of
oscillator, it's merely some existing type of oscillator with some element
changed so a variable voltage will vary its frequency.

What is controlled by the varying voltage depends on the design, but for
pretty much all radio type VCOs, a voltage variable capacitor replaces the
mechanical variable capacitor.

At it's simplest that means you have an oscillator just like you had before,
but you tune it with a varying voltage. So you'd have a potentiometer on
the front panel that would supply a varying voltage to that VCO. This
oscillator would be no different from the same oscillator with a variable
capacitor, give or take how well you regulate the control voltage.

That in itself doesn't do much, other than maybe making the oscillator
smaller (the voltage variable capacitor is usually a small semiconductor
which obviously takes far less space than a variable capacitor), and in
some cases it can be easier to get the parts than find a variable capacitor.

Where they really shine is as part of a PLL, Phase Locked Loop, that locks
the VCO to some standard frequency. There, a phase comparator compares
the VCO with the standard frequency, and the output of the phase detector
is a varying DC voltage that controls the VCO. If the frequencies of
the two oscillators don't match, then the control voltage out of the phase
detector adjusts the VCO to the exact frequency. By adding circuitry, one
can synthesize every single needed frequency, which is where a PLL really
becomes useful.

You aren't seeing VCO "controlled by an encoder". The optical encoder
is mechanical, and cannot directly control the VCO. (The optical encoder
just sends out a stream of pulses, the more pulses the more you turn the
knob. And there is a secondary output, also a stream of pulses. The
difference is, one of those outputs goes high before the other; which
one is dependent on which way you turn the knob.) You have to turn
the pulses from the optical encoder into something else. And in this
case, it's being used to control something digital, what would depend
on the actual design in the book. One scheme would be it controls a
series of up/down counters, and the outputs of them control a variable
divider chain in the PLL, between the VCO and the phase detector. Other
schemes might have the optical encoder feeding a CPU of some sort, and
the CPU does some of the work before controlling something else (such
as a variable divider chain, or directly control some other type of
synthesizer).

It doesn't have to be mechanically stable, because the encoder is
not directly controlling the VCO. It merely supplies information, which
way you turned the knob and how far.

One thing to be considering is that the book is intended to be a construction
project. It takes the whole book to describe the transceiver, and it's not
really about teaching a more general theory. Once you start wanting
to change things, you'd be better off starting with some other book, that
is less specific, and even shows simpler circuits. Because then you get
the background information, and it's far easier to start with a basic
SSB transceiver (basic in the sense that it's single band and uses
a mechanically tuned variable oscillator), get that going, and then
add the "bells and whistles" (done right, you can then replace the
VFO with some synthesizer).

Michael VE2BVW