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LW1DSE > TUBES 14.01.18 17:41l 279 Lines 15353 Bytes #999 (0) @ WW
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Subj: Regenerative Receivers (1/2)
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Regenerative circuits (1/2)
ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ
For CP437 !
ÄÄÄÄÄÄÄÄÄÄÄ
The regenerative circuit (or self-regenerative circuit) or "autodyne"
allows an electronic signal to be amplified many times by the same vacuum
tube or other active component such as a field effect transistor. It consists
of an amplifying tube with its output connected to its input through a feed-
back loop, providing positive feedback. This circuit was widely used in radio
receivers, called regenerative receivers (Regens), between 1920 and World War
II. Regenerative receiver circuits are still used in low-cost electronic
equipment such as garage door openers.
1.- How it works:
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
TICKLER THROTTLER
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄ¿
Û ³ ³
Û ³/ ³
Û ÄÁÄ ³ Figure 1.
Û ÄÂÄ ³
(AERIAL) ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ /³ ³ Single Tube (TRIODE)
ANTENNA ³ ÄÁÄ ³ Regenerative Receiver
ßßß /// ³| Schematic.
\|/ CG ÚÄ----- Û|
³ ³ ÕÍÍ͸ Û|
³ ³³/ ³³ ³ ³ CHRF Û| CBPR: By pass condenser
ÀÄ´ÃÄÄÄÄÄÄÄÄÂÄÄÄÄÄÂÄÄ´ÃÄÄ´ ³ Û| at RF frequencies.
/³³ ³ ³ ³³ ³ ³ Û|
ÚÄÄ´ ³ ³ ³ ³| CBPA: By pass condenser
COUPLING ³ ³ ³ ³ ³ ³³ ³ at AF frequencies.
Û ³ Àı±±±ÄÙ ÃÄÄÄÄ´ÃÄÄ´
Û ³ ³ ³³ ³ CBPR: By pass condenser
Û ³/ RG ÄÁÄ CBPR ³ at both RF and AF
Û ÄÁÄ /// O frequencies.
Û ÄÂÄ )
Û /³ O HEADPHONES
Û ³ MAIN CBPB ³
³ ³ TUNNING ³
ÀÄÄ´ ³³ ³
³ ÚÄÄ´ÃÄ´
ÄÁÄ ÄÁÄ ³³ ³
/// /// ³
+B o
The feedback was applied to the input (grid) of the tube with a
"tickler coil" winding on the tuning inductor. Any radio frequency feedback
oscillator topology can be operated as a regenerative receiver if modified to
provide a controllable reduction in feedback loop coupling, a method of
coupling the loop to an incoming signal source, and a method of coupling
audio frequencies out of the loop to a subsequent audio amplification stage
(or high efficiency headphones). It functions as a combination of an
oscillator and mixer which converts the modulation directly to the audio
baseband.
Because of the large amplification possible with regeneration,
regens often use only a single amplifying element. In a regenerative receiver
the output of the tube is connected to its input through a feedback loop with
a tuned circuit (LC circuit) as a filter in it. The tuned circuit allows
positive feedback only at its resonant frequency. The tuned circuit is also
connected to the antenna and serves to select the radio frequency to be
received, and is adjustable to tune in different stations. The feedback loop
also has a means of adjusting the amount of feedback (the loop gain). For AM
signals the tube also functions as a detector, rectifying the RF signal to
recover the audio modulation, by means of RG and CG; for this reason the
circuit is also called a regenerative detector.
2.- AM reception:
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
For AM reception, the gain of the loop is adjusted so it is just below
the level required for oscillation (a loop gain of just less than one). The
result of this is to increase the gain of the amplifier by a large factor at
the bandpass frequency (resonant frequency), while not increasing it at other
frequencies. So the incoming radio signal is amplified by a large amount,
increasing the receiver's sensitivity to weak signals. The high gain also has
the effect of sharpening the circuit's bandwidth (increasing the Q factor) by
an equal factor, increasing the selectivity of the receiver, its ability to
reject interfering signals at frequencies near the desired station's
frequency.
3.- CW reception:
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
For the reception of CW radiotelegraphy (Morse code) signals, the
feedback is increased above the level of oscillation (a loop gain of one), so
that the amplifier functions as an oscillator as well as an amplifier,
generating a steady sine wave signal at the resonant frequency, as well as
amplifying the incoming signal. The tuned circuit is adjusted so the
oscillator frequency is a little to one side of the signal frequency. The two
frequencies mix in the amplifier, generating a beat frequency signal at the
difference between the two frequencies. If the frequency is in the audio
range, it will be heard as a steady tone in the receiver's speaker whenever
the station's carrier is present. Morse code is transmitted by keying the
transmitter on and off, producing different length pulses of carrier
("dots" and "dashes"). The audio tone makes the carrier pulses audible, and
they are heard as "beeps" in the speaker.
4.- SSB reception:
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
For the reception of single-sideband (SSB) signals, the circuit is also
set to oscillate. The BFO signal is adjusted to one side of the incoming
signal, and functions as the replacement carrier needed to demodulate the
signal.
5.- Advantages and disadvantages:
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
Regenerative receivers require fewer components than other types of
receiver circuit. The circuit's original attraction was that it got more
amplification (gain) out of the expensive vacuum tubes of early receivers,
thus requiring fewer stages of amplification. Early vacuum tubes had low gain
at radio frequencies (RF). Therefore the TRF receivers used before regens
often required 5 or 6 tubes, each stage requiring tuned circuits that had to
be tuned in tandem to bring in stations, making the receiver cumbersome, power
hungry, and hard to adjust. Regens, by contrast, could often get adequate gain
with one tube. In the 1930s regens were replaced by the superheterodyne
circuit in commercial receivers due to its superior performance and the
falling cost of tubes. JFETs are also used in regens today. In recent years
the regenerative circuit has seen a modest comeback in receivers for low cost
digital radio applications such as garage door openers, keyless locks, RFID
readers, some cell phone receivers. Regeneration can increase the gain of an
amplifier by a factor of 15,000 or more. This is quite an improvement,
especially for the low-gain vacuum tubes of the 1920s and early 1930s. The
type 236 triode (US vacuum tube, obsolete since the mid-1930s) had a non-
regenerative voltage gain of only 9.2 at 7.2 MHz, but in a regenerative
detector, had voltage gain as high as 7900. In general, "... regenerative
amplification as found to be nearly directly proportional to the
non-regenerative detection gain." "... the regenerative amplification is
limited by the stability of the circuit elements, tube characteristics and
[stability of] supply voltages which determine the maximum value of
regeneration obtainable without selfoscilation." Intrinsically, there is
little or no difference in the gain and stability available from vacuum tubes,
JFET's or MOSFET's.
A disadvantage of this receiver is that the regeneration (feedback)
level must be adjusted when it is tuned to a new station. This is because the
regenerative detector has less gain with stronger signals, and because the
stronger signals cause the tube to operate on a different section of its
amplification curve (i.e. grid V vs. plate V for tubes). Also the coupling
between the tickler and the grid tuned circuit becomes affected by the main
tunning setting.
A drawback of early vacuum tube designs was that, when the circuit was
adjusted to oscillate, it could operate as a transmitter, radiating an RF
signal from its antenna at power levels as high as one watt. So it often
caused interference to nearby receivers. Modern circuits using semiconductors,
or high-gain vacuum tubes with plate voltage as low as 12V, typically operate
at milliwatt levels-one thousand times lower. So interference is far less of
a problem today. In any case, adding a preamp stage (RF stage) between the
antenna and the regenerative detector is often used to further lower the
interference.
Other shortcomings of regenerative receivers are the presence of a
characteristic noise ("mush") in their audio output, and sensitive and
unstable tuning. These problems have the same cause: a regenerative
receiver's gain is greatest when it operates on the verge of oscillation, and
in that condition, the circuit behaves chaotically. Simple regenerative
receivers lack an RF amplifier between the antenna and the regenerative
detectors, so any change with the antenna swaying in the wind, etc. can
change the frequency of the detector.
A major improvement in stability and a small improvement in available
gain is the use of a separate oscillator, which separates the oscillator and
its frequency from the rest of the receiver, and also allows the regenerative
detector to be set for maximum gain and selectivity - which is always in the
non-oscillating condition. A separate oscillator, sometimes called a BFO
(Beat Frequency Oscillator) was known from the early days of radio, but was
rarely used to improve the regenerative detector. When the regenerative
detector is used in the self-oscillating mode, i.e. without a separate
oscillator, it is known as an "autodyne".
6.- History:
ÄÄÄÄÄÄÄÄÄÄÄ
The inventor of FM radio, Edwin Armstrong, invented and patented the
regenerative circuit while he was a junior in college, in 1914. He patented
the super-regenerative circuit in 1922, and the superheterodyne receiver in
1918. Lee De Forest filed a patent in 1916 that became the cause of a
contentious lawsuit with the prolific inventor Armstrong, whose patent for
the regenerative circuit had been issued in 1914. The lawsuit lasted twelve
years, winding its way through the appeals process and ending up at the
Supreme Court. Every court up to the Supreme Court had ruled in favor of
Armstrong. However, the Supreme Court ruled in favor of De Forest.
At the time the regenerative receiver was introduced, vacuum tubes were
expensive and consumed lots of power, with the added expense and encumbrance
of heavy batteries. So this design, getting most gain out of one tube, filled
the needs of the growing radio community and immediately thrived. Although
the superheterodyne receiver is the most common receiver in use today, the
regenerative radio made the most out of very few parts.
In World War II the regenerative circuit was used in some military
equipment. An example is the German field radio "Torn.E.b". Regenerative
receivers needed far fewer tubes and less power consumption for nearly
equivalent performance.
A related circuit, the super-regenerative detector, found several
highly-important military uses in World War II in Friend or Foe identification
equipment and in the top-secret proximity fuse. In the 1930s, the
superheterodyne design began to gradually supplant the regenerative receiver,
as tubes became far less expensive. In Germany the design was still used in
the millions of mass-produced German "peoples receivers" (Volksempfänger) and
"German small receivers" (DKE, Deutscher Kleinempfänger). Even after WWII,
the regenerative design was still present in early after-war German minimal
designs along the lines of the "peoples receivers" and "small receivers",
dictated by lack of materials. Frequently German military tubes like the
"RV12P2000" were employed in such designs. There were even superheterodyne
designs, which used the regenerative receiver as a combined IF and
demodulator with fixed regeneration. The superregenerative design was also
present in early FM broadcast receivers around 1950. Later it was almost
completely phased out of mass production, remaining only in hobby kits.
7.- Operating limits:
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
Quality of a receiver is defined by its sensitivity and selectivity.
For a single-tank TRF (tuned radio frequency) receiver without regenerative
feedback,
Bandwidth = frequency / Q
where Q is tank "quality" defined as
Q = Z / R
Z is reactive impedance, R is resistive loss. Signal voltage at tank is
antenna voltage multiplied by Q. Positive feedback compensates the energy
loss caused by R, so we may express it as bringing in some negative R.
Quality with feedback is:
Qreg = Z / (R - Rneg)
Regeneration rate is:
M = Qreg / Q = R / (R - Rneg)
M depends on stability of amplification and feedback coefficient, because if
R - Rneg is set less than Rneg fluctuation, it will easily overstep the
oscillation margin. This problem can be partly solved by "grid leak" or any
kind of automatic gain control, but the downside of this is surrendering
control over receiver to noises and fadings of input signal, which is
undesirable. Modern semiconductors may offer more stability than vacuum tubes
of the 1920s, depending on other circuit parameters as well. Actual numbers:
To have 3 kHz bandwidth at 12 MHz (short waves travelling all around Earth)
we need:
Q = F / f = 4000
A two-inch coil of thick silvered wire wound on a ceramic core may have
Q up to 400, but let's suppose Q = 100. We need M = 40, which is attainable
with good stable amplifier even without power stabilizing.
To be continued
ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
º Compilled from Wikipedia.com and "Regenerative and Reflex Receivers" by º
º Kim Smith and The Radio Electronique. Translatted and drawings in ASCII º
º by LW1DSE Osvaldo F. Zappacosta. Barrio Garay, Almirante Brown, º
º Buenos Aires, Argentina. º
º Made with MSDOS 7.10's Text Editor (edit.com) in my AMD's 80486. º
º April 15, 2012 º
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º Osvaldo F. Zappacosta. Barrio Garay (GF05tg) Alte. Brown, Bs As, Argentina.º
º Mother UMC æPC:AMD486@120MHz 32MbRAM HD SCSI 8.4Gb MSDOS 7.10 TSTHOST1.43C º
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