Description
This article is from the Antique Radios
And Phonographs FAQ, by Hank van Cleef vancleef@netcom with
numerous contributions by others.
40 I keep hearing about "Neutrodyne," "Regenerative," "TRF," and"Superheterodyne." What do these terms mean?
The first home entertainment radios were crystal sets which used a
single tuned antenna circuit and a crystal detector. When tubes were
added for amplification, these were set up with tuned circuits that had
to be individually tuned to the station being received. These are "TRF"
sets, for "tuned radio frequency." Later on, manufacturers learned how
to build TRF stages using either mechanical coupling between the tuning
condensors or a single ganged condenser, and to provide adjustments to
get them to track (i.e., all tune to the same frequency across the range
of broadcast frequencies), so later TRF sets have one-knob tuning.
The Neutrodyne refers to a method of "neutralizing," or compensating
for, detuning effect of grid-plate capacitances by feeding back an
opposing signal. These sets are TRF sets with neutralizing circuits in
them---generally, another coil in the tuned circuit used to generate the
neutralizing signal.
The superheterodyne uses the physical principle that two oscillators
running at different frequencies will produce "beat" frequencies equal
to both the sum of and difference between the two frequencies. This can
be heard when tuning musical instruments; the principle is the same for
radio frequencies. The incoming RF signal is "mixed" with a local
oscillator signal and fed to a fixed tuned stage that is sensitive to
the difference frequency between the two signals. Use of one or more
fixed-frequency tuned stages gives the set relatively constant
sensitivity and selectivity, both of which are difficult to get in
variable tuned stages. To illustrate what these words mean, take a
common five-tube US table radio and a station at 1000 Khz ( 1
megacycle). An antenna coil and one section of the tuning condenser
(capacitor) are tuned to resonate at 1000 Khz, "selecting" that
frequency. A local oscillator is tuned by the other section of the
tuning condenser to 1455 Khz. In a set with a 12SA7 tube, the
12SA7 is wired as an oscillator, with the oscillator signal appearing on
the first grid (g1). The tuned RF signal is fed to the third grid (G3).
The plate circuit is connected to a transformer tuned to 455 Khz, to
respond to the difference between the frequencies being injected on G1
and G3. Signals at 455, 1000, 1455, and 1455 Khz all appear on the
12SA7 plate (the two fundamentals and the sum and difference), but the
tuned "intermediate frequency" (IF) transformer selects only the 455 khz
signal. This intermediate frequency is generally amplified by one or
more tuned (455 khz) stages---in our example, a 12SK7 with double-tuned
input and output IF transformers (i.e., both the plate and grid circuits
are tuned to resonate at 455 Khz) is used, and the output of that stage
is fed to the a diode detector.
This may sound a bit complicated, and I've left out all the fine points
of the design to focus on "what's supposed to happen."---a good
engineering text discusses design details beyond this description. One
point of terminology----the mixer stage (12SA7) was often called a
"first detector" in early designs; thus, the 12SQ7 diode detector in our
example is called the "second detector," a term that has persisted
through the decades.
One other common early design was the "regenerative" set. In these
sets, an RF amplifier was designed as an oscillator, but provided with a
control that could be adjusted so that the stage wouldn't go into
oscillation. The positive feedback in the stage provided substantially
more gain than a simple tuned circuit would provide. Misadjustment of
the feedback control would make the stage oscillate, producing squeals
in the output, and quite powerful RFI (radio frequency interference) as
well. The "superregenerative" circuit is a refinement that prevents
sustained oscillation, but was generally not used in home entertainment
sets.
(1/95) Roy Morgan forwarded me a description of the super-regen by Dan
Knierim for inclusion---here it is.
>P.S. What's the diff between a super-regen and a regen detector?
>I basically understand the regen circuit (gain stage near oscillation
>behaving as high Q filter) but I don't recall what the principle of
>the super-regen circuit is. And I'm definitely not an RF kinda
>guy these days.
A super-regenerative detector is a gain stage with positive feedback greater
than unity (so that it will oscillate), but with an RC circuit in the plate
or grid supply, so that the increased current during oscillation will lower
the gain over a period of time proportional to the RC time constant, and
finally kill the oscillation. Of course, once the oscillation quits, the
current draw goes down, the RC circuit recharges, the gain goes back up, and
the oscillation starts again. The frequency of this blocking oscillation is
set (by picking the RC time constant) to be well above audible frequencies,
but far below the RF oscillation frequency.
So how does it detect? Any RF input signal at the frequency of the main
oscillation (not the blocking oscillation) will help the main oscillation
restart when the stage is coming out of the blocking mode. If the RF input
increases, the main oscillation will restart faster, the stage will
spend a higher percentage of its time in the oscillating mode, and the
average plate current will be higher (where the average is taken over several
cycles of the blocking oscillation). Thus the detected audio output is just
the plate current run through a low-pass-filter.
The average plate current as a function of RF input amplitude is not very
linear; in fact it has a 1 / natural logarithm nature to it due to the
exponentially rising nature of an oscillator starting up. This makes the
audio quality from a super-regenerative detector low, but also acts somewhat
like AVC. The pk-pk audio output amplitude is more proportional to the
pk-pk RF input amplitude *ratio*. The steep slope of a logarithm near
zero also implies a high sensitivity with very small input signals, which
is one of the super-regens claims to fame.
Some of its many drawbacks are: it makes a racket when not tuned to an
input signal (in other words, it also has a high sensitivity to very small
amounts of noise, in the absence of an input signal above the noise floor);
it is tricky to keep running right; and it radiates like crazy if not
preceded with a separate RF input stage.
By the way, don't sneeze at regen sets just because they don't have a
lot of tubes. I recently read a posting in another group that talked
about a 1920's one-tube setup that blew smoke around some fancy radios.
Edwin Armstrong, who contributed the straight regen, the super-regen,
and FM, was a real genius.
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