From: "Tom Donaly" on Wed, Jan 31 2007 6:05 pm
Ian White GM3SEK wrote:
Someone has asked my magazine column where the name "hybrid coupler"
came from; and why do some people reserve the name "hybrid" for only
certain types of couplers?
That isn't as easy a question as it first seems. Everybody seems to talk
about "hybrids" but few authors are prepared to define exactly what it
means - much less explain why.
The following is a draft outline for my magazine article. It looks much
more authoritative than I actually feel about the subject - especially
about the origins of the term "hybrid", so I would welcome any comments...
...
Hi Ian,
A hybrid coil in telephony was (is?) used to couple a 2 wire
line to a 4 wire line allowing bidirectional amplification. If you
have access to Reg's old bible _Communication Engineering_ by William
Everitt you can read an explanation of how this works starting on
page 317 (second edition).I suppose the term "hybrid" refers to
the fact that two different types of line (2 wire and 4 wire) are
involved.
Hello Ian and Tom,
The mention of "Communication Engineering" by William Littell
Everitt sparked my interest since it has prime occupancy on my
Basics bookshelf space (the book and I were "born" in the same
year...:-)
Going into the etymology of *ALL* "hybrids" in communications
will probably focus not on "radio" but the common telephone
handset and the long-distance, wired telephone system courtesy
of remarkable innovation by the American Telephone and Telegraph
laboratories. Note: I'm not waving a flag for "Americanism"
only that the AT&T system grew to be huge early-on and also
engineered for rather incredible longevity and good performance.
The common telephone set made here by the MILLIONs between
the 1930s and 1970s period all had a "hybrid transformer" as its
basic "coupler" to the telephone line, here a 2-wire balanced-
pair to the central office. Mouthpiece and earpiece were
actually isolated by this "hybrid" with a controlled leak-
through to the earpiece for "sidetone" (a term that came from
telephony as well). In addition, the "hybrid" coupled or
isolated the 40 VDC (give or take) always present on our
telephone lines and the 20 Hz Ring signal (with/without the
hook switch cut-out). Added to that were a (somewhat) primitive
"ALC" using thermistors-varistors to keep loud talkers from
blasting ears of other users. Rather complex arrangement but
very simple in terms of parts, rugged, long-lasting (better
than 30 years life), and cost effective.
It would seem that the POTS hybrid scheme grew out of
long-distance lines' repeater stations where two amplifiers
were needed to boost signal strength both ways...AND provide
a DC path for lots of different telco service needs. The
concept of linearity through negative feedback was a direct
result of "long-lines" repeater needs. A long-lines repeater
"hybrid" seems to have evolved from the already-existing
bridge circuit used for measurement of passive components.
I say "seems" because both repeater hybrid and measurement
bridges are configured for balance and "separation" of the
components' characteristics. All of that took place at
"audio" frequencies (under 15 KHz), including frequency-
multiplexed SSB voice channels. Yes, "SSB" was pioneered
by the various telephone companies in the world, on wire
lines first, then on "radio." :-)
Segue to World War II and some massive head-scratching
on both sides. "Radio" folks of older days may have
encountered a "Magic-T" in waveguide, particularly at X-Band
(3 cm or 8 to 12 GHz). A Magic-T is really a hybrid
transformer equivalent, RF energy coming into a common port
(inbound) is relatively isolated from RF energy into a 3rd
port intended to go outbound. The fourth port is generally
terminated in a resistive load for broadband balance but
can also be used as a bidirectional port. "Isolation" is
a result of good directional balance, a term that became
common in the microwave field after WW2 and Isolators and
Circulators came into being. Note: A "circulator"
performs the same function as the old Magic-T but has a
greater production yield, generally more compact.
The "rat-race hybrid" of all-coaxial lines probably
evolved out of the Magic-T as a semi-broadband hybrid
transformer but at RF lower than microwaves. It had
limited use until Stripline and Microstrip was innovated.
Most Rat-Races are still rather narrowband and incapable
of working to octave bandwidths common to microwaves.
With Stripline a whole vast array of "flat" configurations
were innovated but all based on basic transmission line
properties...including all the filter structures built
entirely on PCBs.
Broad-banding at RF also had to wait for better
powdered-iron and ferrite materials' better characteristics
that came about after WW2. One offshoot of that was the
broadband RF transformer that could have double decade
bandwidth. Such RF transformers could be configured very
much like the old POTS hybrid transformers. :-) Variants
of those appeared in HF through UHF solid-state power
amplifiers and "hot plug swapping" RF power amplifiers
composed of many modules, each relatively isolated from
one another. Does anyone working at RF not know about
the "balun?" [broadband impedance-changing/isolating
transformer]
It is my opinion that many, many terms became a sort
of jargon or "tech-speak" for the simple reason of the
names being short yet familiar to all working with a
particular field. For example, it is much easier to say
and pronounce "flip-flop" than the formal name of "Eccles-
Jordan Bistable Multivibrator." :-) Folks in radio say
"mike" for microphone or for microfarads of capacitance;
what they are talking about does a subtle unconscious
separation and ordering of the incoming information.
There isn't much confusion about such jargon even if
discussing 'condenser microphones.' :-)
Sometimes the jargon becomes over-used. I use the
term "Wilkinson Power Divider" [or "Power Splitter"]
for a familiar Stripline structure, yet I've heard and
seen it described as a "Wilkinson Hybrid." While it is
on the grey area of veddy proper useage, most RF folks
know what either is. Editors writing for a large,
unknown-technical-smarts-set readership do, understand-
ably, get a bit under stress trying to Explain It All.
[been there, done that, got a few T-shirts...:-) ]
I would trace "hybrid" in electronics all the way
back to the guy that invented the "Wheatstone Bridge."
Wheatstone spoke and wrote about that clever thing so
much that His name got stuck on the configuration, the
real inventor's name got rather lost. Balance and
isolation were essential there...and balance and
isolation was necessary in the telephone "hybrid."
Millions of telephone "hybrids" later, the jargon
happened. The "hybrid" essential principles were
applied to other fields within electronics and the
rest is slightly confused history...:-)
Cheers,