Why Palestinians and Palestinian Statehood May Cease to Exist if Perry Becomes President
WorldNews.com
U.S. Should Not Have Lied Then...or Now
WorldNews.com
Fighting rages near Kadhafi's Tripoli compound
Jakarta Globe
Gaddafi claims attack in Tripoli crushed
The Independent
Egypt says Israel's regret is not enough
Al Jazeera
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 vacuum tube or transistor with its output connected to its input through a feedback loop, providing positive feedback. This circuit was widely used in radio receivers, called regenerative receivers, between 1920 and World War 2. Regenerative receiver circuits are still used in low-cost electronic equipment such as garage door openers.
In a regenerative receiver the output of the tube or transistor 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; for this reason the circuit is also called a regenerative detector.
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, 103 - 105, increasing the receiver's sensitivity to weak signals. The high gain also has the effect of sharpening the circuit's bandwidth (Q factor) by an equal factor, increasing the selectivity of the receiver, it's ability to reject interfering signals at frequencies near the desired station's frequency.
For the reception of CW radiotelegraphy (Morse code) signals, the feedback is increased above the level of oacillation (a loop gain of one), so that the amplifier functions as an oscillator (BFO) 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. This frequency is in the audio range, so it is heard as a steady tone in the receiver's speaker whenever the station's carrier is present. The Morse code is transmitted by keying the transmitter on and off, producing different length pulses of carrier ("dots" and "dashes") which are heard as "beeps" in the speaker.
For the reception of single-sideband 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.
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 1920's and early 1930's. The type 236 triode (US vacuum tube, obsolete since the mid-1930's) had a non-regenerative voltage gain of only 9.2 at 7.2MHz, 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 [or device] characteristics and [stability of] supply voltages which determine the maximum value of regeneration obtainable without self-oscilation." Intrinsically, there is little or no difference in the gain and stability available from vacuum tubes, JFET's MOSFET's or bipolar junction transistors (BJT'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 regerative detector has less gain with stronger signals, and because the stronger signals cause the tube or transistor to operate on a different section of its amplification curve (i.e. grid V vs. plate V for tubes; gate V vs drain V for FET's, and base current vs. collector current for BJT's).
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".
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, although the experts agree that the incorrect judgement had been issued. Armstrong received a standing ovation at the
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 or AC transformer and rectifier. 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 WW2 the regenerative circuit was used in some military equipment. The standard German army receiver wasa the Torn.E.b, manufactured from 1936 to as late as 1945. 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 WW2 in Friend or Foe identification equipment and in the top-secret proximity fuse.
In 1923, the superheterodyne design began to gradually supplant the regenerative receiver, as tubes became far less expensive. After WWII, the regenerative design was almost completely phased out of mass production, remaining only in hobby kits.
Positive feedback compensates the energy loss caused by R, so we may express it as bringing in some negative R. Quality with feedback is . Regeneration rate is .
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 . 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.
Types of Signals for Regenerative vs. Super-Regenerative (super-regen or spuerregen) Detectors. Super-Regenerative Detectors work well for wide-band signals such as FM, where it performs "slope detection". Regenerative Detectors work well for narrow-band signals, especially for CW and SSB which need a heterodyne oscillator or BFO. A super-regenerative detector does not have a usable heterodyne oscillator - even though the super-regen always self-oscillates, so CW (Morse Code)and SSB (Single side band) signals can't be received properly.
Super-regeneration is most valuable above 27 MHz, and for signals where broad tuning is desirable. The super-regen uses far fewer components for nearly the same sensitivity as more complex designs. It is easily possible to build super-regen receivers which operate at microwatt power levels, in the 30 to 6,000 MHz range. These are ideal for remote-sensing applications or where long battery life is important. For many years, super regenerative circuits have been used for commercial products such as garage-door openers, radar detectors, microwatt RF data links, and very low cost walkie-talkies.
Because the super-regenerative detectors tend to receive the strongest signal and ignore other signals in the nearby spectrum, the super-regen works best with bands that are relatively free of interfering signals. Due to Nyquist's theorem its quenching frequency must be at least twice the signal bandwidth. But quenching with overtones acts further as a heterodyne receiver mixing additional unneeded signals from those bands into the working frequency. Thus the overall bandwidth of super-regenerator cannot be less than 4 times that of the quench frequency, assuming the quenching oscillator produces an ideal sinewave.
This text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
