VCO
 

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

VCO
 

"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

VCO
 

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

VCO
 

Quote: "VCO" is an acronym for Voltage Controlled Oscillator.

Well, yeah I gathered that, after more reading. I see that people
have used them in music synthesizer modules.

I'm sorry for being a bit elementary and maybe long winded here, I'm
still trying to learn as much as I can while I am doing and reading,
etc.

The VCO in the transceiver project in the book used some sort of motor
control and shaft encoding.

I did a google type search for HF DDS Generator using WebFerret. Came
up with over 175 results. Seems people are using 16F chips and AD98xx
chips.

Well, anyhow thanks for all your replies.

Greg
Seattle, WA

VCO
 

On Mar 30, 4:47�pm, "gbowne1" wrote:
Quote: "VCO" is an acronym for Voltage Controlled Oscillator.

Well, yeah I gathered that, after more reading. *I see that people
have used them in music synthesizer modules.

I'm sorry for being a bit elementary and maybe long winded here, I'm
still trying to learn as much as I can while I am doing and reading,
etc.

Keep that up, by all means. Amateur radio is about radio-
electronics technology applied to communications.

The VCO in the transceiver project in the book used some sort of motor
control and shaft encoding.

That's the common method of manual control now...but in-
between the rotary shaft encoder (attached to the manual
knob) and the VCO itself is a large collection of digital
circuits. Be prepared to learn more about digital circuits,
gates, flip-flops, etc. Not difficult, just strange if your
background is only analog circuits.

I did a google type search for HF DDS Generator using WebFerret. *Came
up with over 175 results. *Seems people are using 16F chips and AD98xx
chips.

Analog Devices is the most-often used supplier of the
basic DDS IC. Microcircuits' PIC series (the '16F'
family) of microcontrollers is used to do the "translation"
between manual frequency data input and the data
needed to set the DDS to the correct frequency. Now
that translation IS more difficult for many, experienced
in electronics or not.

If you are intending to go into frequency synthesizers for
your own projects, I'd suggest you concentrate on PLLs
first. Getting to know those and WHY they work will
lead into the various DDS methods of frequency
generation. The general digital circuits of both PLLs and
DDSs are related.

Well, anyhow thanks for all your replies.

Greg
Seattle, WA

I try to give "Sound" advice...my wife and I have a house
in Kitsap County west of Tacoma, SE of Bremerton. :-)

[as in "Puget Sound" folks...bad pun but the Sound is
very very long...and lovely territory]

73, Len AF6AY

VCO
 

I'm understanding this more and more as I go along. Thanks to the
first two replies on this thread. I'm still learning more about
electronics in general. But learning about RF and HF electronics
really has been interesting.. and may prove hefpful.

I'd at least like to start learning more about the project in that
book though. I know that it's primarily a building block learning
process.

To Len, I know exactly where that is.. :-)

Greg