Regenerative circuit

For other uses, see Heat exchanger § Regenerative heat exchanger.

Homebuilt one-tube regenerative shortwave radio set with vintage headphones.

Rear view of the above radio, showing the simplicity of the regenerative design.

The regenerative circuit (or self-regenerative circuit) 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.

How it works

Regenerative receiver schematic. The feedback was applied to the input (grid) of the tube with a "tickler coil" winding on the tuning inductor.

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, 10^{3 - 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.

Description

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), so the TRF receivers used before regenerative receivers often required 5 or 6 tubes, each stage requiring tuned circuits that had to be tuned in tandem to bring in stations. Regenerative receivers could often get adequate gain with one tube. Transistors, either bipolar or JFETs are used in regenerative receivers today. The low cost of transistors caused the regenerative circuit to be replaced by the superheterodyne circuit, although it 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 10,000 or more. Typical gains obtainable from amplifying devices in regenerative circuits are: 100,000 for bipolar transistors; 20,000 for JFETs; and a few thousand for vacuum tubes. This is quite dramatic considering the fact that the non-regenerative gain of these devices at RF frequencies is very low, often 20 or less.

A disadvantage of this receiver is that the regeneration (feedback) level must be adjusted when it is tuned to a new station. 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 normally operate at milliwatt levels—one thousand times lower. So, interference is far less of a problem today. In any case, the addition of a preamp stage (RF stage) between the regenerative detector and the antenna 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. Both of 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.

History

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. The Court ruled in favor of De Forest, although the experts agree that the incorrect judgement had been issued.

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 early military equipment. A related circuit, the super-regenerative detector, found wide use in WW2 in military Friend or Foe identification equipment and in the top-secret proximity fuse.

In 1930 the superheterodyne design began to supplant the regenerative receiver, and after WWII the regenerative design was almost completely phased out of mass production, remaining only in hobby kits.

Operating limits

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. Note that modern semiconductors offer much more stability than vacuum tubes of the 1920s.

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.

Super-regenerative receiver

The super-regenerative receiver uses a second lower frequency oscillation (within the same stage or by using a second oscillator stage) to provide single-device circuit gains of around one million. This second oscillation periodically interrupts or "quenches" the main RF oscillation. Ultrasonic quench rates between 30 and 100 kHz are typical. After each quenching, RF oscillation grows exponentially, starting from the tiny energy picked-up by the antenna plus circuit noise. The amplitude reached at the end of the quench cycle (linear mode) or the time taken to reach limiting amplitude (log mode) depends on the strength of the received signal from which exponential growth started. A low-pass filter in the audio amplifier filters the quench and RF frequencies from the output, leaving the AM modulation.

A big difference between a straight regenerative ("regen") and a super regen receiver is that there is no heterodyne (no squealing interference) as the set is tuned and operated. The super regen uses far fewer components than more complex designs and it is easily possible to build VHF, UHF, and even microwave radio receivers that operate at microwatt power levels. These are ideal for remote sensing applications or where very long battery life is essential. For many years, super regenerative circuits have been used for commercial applications such as garage-door openers, radar detectors, microwatt RF data links, and very low cost walkie-talkies.

On the other hand, in the normal operating mode a super-regenerative system has an inherent contradiction limiting its use to relatively free and clear bands. 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.

However, recent experiments have shown that the self quenched super regenerative circuit may be operated at a level in between the regenerative and super-regenerative states. At these levels, something very interesting happens: A received narrow band FM (RF) signal can be set to heterodyne with the super regen detector's RF oscillations such that sum and difference frequencies are produced. The difference frequency, if close to the detector's quenching oscillation frequency, will lock onto and FM modulate the quench frequency. This produces a large variation in gain which exactly follows the FM modulation of the received signal. The gain variation creates a frequency to amplitude conversion (FM detection) of very high efficiency. To date, this effect has only been studied by electronics experimenters but could eventually lead to very high frequency, ultra low power FM receivers.

Patents

• Armstrong, E. H., U.S. patent 1,113,149, Wireless receiving system, 1914.
• Armstrong, E. H., U.S. patent 1,342,885, Method of receiving high frequency oscillation, 1922.
• Armstrong, E. H., U.S. patent 1,424,065, Signalling system, 1922.
• Braden, R. A., U.S. patent 2,211,091, Superregenerative magnetron receiver, 1940.

See also

• Tuned electrical circuit

External links

• Some Recent Developments in the Audion Receiver by EH Armstrong, Proceedings of the IRE (Institute of Radio Engineers), volume 3, 1915, pp. 215–247.
• a one transistor regenerative receiver