From: "gbowne1" on Wed, Mar 28 2007 11:45 pm

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?

"VCO" is an acronym for Voltage Controlled Oscillator.

The mechanism for tuning is that the frequency-determining
resonant circuit uses (usually) a voltage-controlled
variable capacitor to adjust the oscillatory frequency.
A voltage- or current-controlled variable inductor my be
used to do the same thing. "VCO" has become a rather
generic term applied to any variable frequency oscillator
controlled by a voltage or current. A few mix that up
with "VFO" or Variable Frequency Oscillator, an even more
generic term for a (usually) manually-controlled oscillator.

Typically, the voltage-controlled variable capacitor is
a reverse-biased diode in which the diode junction
capacitance varies dependent on the reverse-bias voltage.
The more rare variable inductor type of VCO changes its
inductance dependent on the amount of DC current changing
its partial magnetic field saturation characteristics.

"Mechanical stability" of a VCO has no direct parallel to
the old-time mechanically-variable manual-tuning variable
oscillator. The fixed components of a VCO require stable
construction, yes, but the frequency stability depends on
the CONTROL VOLTAGE (or current) REGULATION AND STABILITY.

A VCO as a subsystem on an IC may not require any external
capacitance or inductance; some, including component
assemblies in a small enclosure include all of that. An
older form of VCO is the free-running (astable) multi-
vibrator circuit where the square-wave frequency is varied
by bias voltage on the base-gate-grid of the multivibrator.
The demand for digital-IC-based VCO multivibrators has been
slight in the last two decades so most of those have been
withdrawn from production.

"OSE" is not a common acronym for a rotary incremental
digital shaft encoder. [Ocean State Electronics in Rhode
Island uses "OSE" as a logo] A shaft encoder REQUIRES
an interface circuit to convert the directional rotation
digital signals to something that can CONTROL a VCO or
some other circuit producing a controlled, variable-
frequency output. Call it "shaft encoder" for short.

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, in all fairness to most of the readers of Homebrew,
you are asking questions which are too general, too basic,
and require book-length replies which should have lots of
illustrations to fully explain details. I would suggest
you FIRST engage in some self-study on specific topics to
round-out your own knowledge on circuit basics. I doubt
that most of us want to "explain how to solder" for example
since that is something that can be self-taught on the bench.

Today's transceivers, receivers, transmitters use variations
of the "DDS" or Direct Digital Synthesizer for their main
tuning circuit sub-system. The DDS is a later advancement
on the PLL or Phase Locked Loop, an incremental-step frequency
synthesizer. A DDS (almost always a SOC or System On a Chip)
can produce a much smaller incremental step of frequency.
Both the PLL and DDS use a single quartz crystal oscillator
as the frequency reference; all output frequency steps of a
PLL or DDS are of quartz crystal stability. Most manually-
tuned PLLs or DDSs are set/adjusted by a form of rotary
shaft encoder supplying (through a rotation-sensing digital
interface sub-circuit) the "up" and "down" incremental
frequency step control signals. Frequency display in today's
radios is obtained indirectly from the DDS IC, converted from
the step control sub-circuit output to the DDS output
frequency plus/minus whatever the transceiver needs to make
the display equivalent to the "air frequency" at the antenna.

A DDS is about as close to a "digital VCO" as today's
technology gets...but it CANNOT be explained easily without
lots of text, illustrations, diagrams, knowing some basics
about "digital accumulators" or "analog-digital conversion."
A DDS is RELATED TO, but not quite the same as a PLL.

A PLL uses a voltage-controlled oscillator as its main output.
That output frequency is fed to a variable digital divider
that produces an output that is an integral-division of the
frequency of the VCO. That divider output is compared, in a
phase-frequency detector (digital) to a fixed, stable
reference frequency. That comparison output is filtered to
produce an up/down control voltage for the VCO. In this
frequency-control loop the VCO is maintained on-frequency
to the reference frequency times the number of divisions in
the digital divider. Changing the divider increment changes
the VCO output frequency. The digital divider increments
may be fixed by switches or it may come from another digital
sub-circuit that is directly controlled by a rotary shaft
encoder for manual adjustment.

The above is a rather basic description of a standard PLL.
DDSs have various forms but their control-comparison to a
single reference frequency is CONSIDERABLY different from a
PLL. A DDS is enormously useful in a modern transceiver,
especially in SSB tuning. My new Icom 746Pro, like many
modern transceivers, can tune in 1 Hertz steps making SSB
reception a snap for clarity...plus having more functions
to allow easy digital control of modes like RTTY plus a
direct internal interface to its digital signal processing.
It uses a DDS as the basic frequency control. My older Icom
R-70 receiver used a PLL, actually three PLL sub-systems, to
achieve 10 Hertz step tuning, quite good for SSB but not as
precise as modern versions. The even older Heath SB-300
family of transceivers used indiidual-band quartz crystal
oscillators and mixing with a manually-tuned analog VFO
(excellent stability) in a "Collins Radio" architecture.

Now, in all seriousness, if you want to experience learning
about transceiver frequency control systems, I'd suggest you
concentrate on the old, conventional L-C non-digital all-
analog methods. It doesn't have all the high-tech buzzwords
attached, but they DID work quite well when attention was
paid to certain critical factors in their design/assembly.
Basic theory (not just assembling someone else's design) of
those will apply to more sophisticated circuit methods. In
progression from L-C analog methods to PLLs, one gets to know
digital circuitry and how those behave. In progression from
PLL to DDS one can get into spectral output and theory of
modulation (in general) along with sampling theory and that
old devil, "aliasing."

If you start in on self-learning, there is an enormity of
material available to you in printed and electronic form.
I'd say most of us in RRAH have been there and done that
in various areas and ask/discuss more definitive details
and comparisons.

73, Len AF6AY