Super regenerative receiver

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The super regenerative receiver , also known as pendulum audion , is a comparatively simple radio receiver circuit principle that is suitable for receiving amplitude-modulated and sometimes even frequency-modulated transmissions (AM or FM). Simple audio receivers with feedback had a comparatively good reception performance in terms of effort, but because of the feedback that had to be activated in addition to the transmitter tuning, operation was not very convenient. Especially when receiving weaker radio stations or radio transmitters , it had to be set more or less sensitively with a separate rotary knob. With feedback receivers, the maximum sensitivity is reached shortly before the start of the vibration. If this point is exceeded, the circuit works as an oscillator and the natural oscillation is superimposed on the transmitter oscillation . If the tuning is only slightly inaccurate, an interference tone corresponding to the frequency difference arises, which makes a reasonable reception of AM or FM broadcasts impossible. Even in the early days of radio technology, methods were therefore sought that would make the operation of a feedback redundant or, if necessary, automate it. A solution to this problem had been found with the super regenerative receiver, which is often referred to as a pendulum receiver or commuter for short. Initially, the circuit was often referred to as pendulum audion, which is technically incorrect, since the audion principle is not used in the demodulation itself in the broader sense.

history

Armstrong pendulum audition

Edwin Howard Armstrong filed a patent application for the pendulum receiver in 1921 as a super-regenerative receiver . The circuit diagram on the right shows a variant from the patent specification. The left tube works as an oscillator on the receiving frequency in a Meissner circuit . The right tube works as an oscillator for the pendulum frequency. Both tubes are connected in such a way that the operating point of the audio tube changes between the operating points of amplification and oscillation due to the pendulum frequency. The oscillation frequency is usually 30 kHz so high that the headphones can no longer reproduce it. The patent also contains a pendulum receiver variant with only one electron tube. In order for a grid audion to become a pendulum receiver, often only a higher time constant is necessary in the grid combination. The capacitor C and possibly the resistor R of the grid combination are increased for this. The undesired oscillation of a grid audion to an audible oscillation frequency is called motorboating , because the resulting noise is reminiscent of a ship's diesel.

For experiments with ultra-short waves, pendulum receivers were used by amateurs in the 1920s and 1930s.

In Germany gained pendulum recipients in the early days of FM - Radio - so in 1949 to about 1,953 - and more important. There were u. a. Assemblies were produced with which radio receivers set up by the manufacturer could be expanded with the VHF range. At this time, attachments working according to the super-regenerative principle, which could be connected to the record player or pickup socket of the existing radio, were of great importance. Despite the poorer reception performance compared to the superhet , super-regenerative receivers received relatively good acceptance as an inexpensive alternative for VHF reception due to the generally low purchasing power after the Second World War .

In the 1950s, receiver assemblies based on this principle, initially equipped with a tube in the high-frequency section and later instead using a transistor, were increasingly used for radio remote control of models. Since around the mid-1960s, super-regenerative receivers have also been found as receiving parts in handheld radios ( walkie-talkies ), and in very simple such devices even today. They are also used for remote control or telecontrol purposes and for the transmission of measured values ​​and data. B. in wireless thermometers, wireless doorbells and wireless alarms still used today.

technology

Generally, a regenerative circuit is a RF - oscillator , the strong due to feedback after reaching the state of vibration and displacement caused by rectification of the HF of the DC operating point as a blocking oscillator operates. The amplification of the oscillation after the end of the blocking time and the reaching of the linear working range takes place through the noise filtered by the oscillating circuit and through coupled in external signals. The oscillation is increased exponentially until it stops. The time required for this decreases with stronger signals and the blocking time remains relatively constant, which means that the oscillation frequency increases slightly as the signal increases. The operating current or the duty cycle contain logarithmically distorted the envelope of the received signal.

The oscillation frequency without an input signal depends on the gain of the active element, the quality of the oscillating circuit, the degree of overload, the time constant of the operating point generation and any additional damping of the oscillating circuit during the blocking phase. The pendulum frequency is the sampling rate of the transmitter signal; it is normally aimed for above the audible range well above 20 kHz so that it does not interfere with the reception of speech and music. In order to avoid overload and intermodulation phenomena, it is attenuated by suitable low-pass filtering in front of the audio amplifier. For data transmission and for special telecontrol purposes, much higher but also lower frequencies are conceivable.

Due to the incorrectly interpreted functionality for a long time, artificial circuits were created, but mostly only have disadvantages.

Completely misleading was the “point of maximum sensitivity reached once per run” known from Audion , which Burkhard Kainka vividly refuted for the first time with his microprocessor-controlled quartz commuter; the strictly logarithmic function is also easy to understand there.

Super regenerative receiver with transistors (1970s)

A distinction is made between self-oscillating and externally controlled commuters. A distinction is also made between linear and logarithmic operation, with the latter being encountered much more frequently. A special form of the commuter is the quench circuit commuter, in which the HF oscillation and, by means of an additional resonant circuit (the quench circuit), the sinusoidal pendulum oscillation are generated in one and the same stage. The circuit of an externally controlled super-regenerative receiver working with transistors, like the one shown here, shows the basic mode of operation of the super-regenerative principle very well. Such a circuit is discussed at.

The greatest disadvantage of the super-regenerative receiver is the fact that it produces very strong interference radiation, which can be emitted via the antenna without a preamplifier ( isolating amplifier ) and impair the reception of neighboring devices. Another disadvantage is the very strong noise that occurs when no station is received. In connection with this, however, the volume of received signals is hardly dependent on the received field strength, so that special control circuits are not required. Very detailed descriptions of the functionality can be found in various editions of "Das Große Fernsteuerbuch" by Heinz Richter .

application

Super regenerative receivers were initially mainly used for short-wave reception . They are hardly suitable for receiving medium waves , and not at all for long waves . They are best suited for frequencies from approx. 20 MHz to well over 1 GHz. They are only suitable for applications in which low demands are made in terms of selectivity , because the reception bandwidth is usually in the order of about a hundredth of the frequency to be received. The sensitivity is astonishingly good with values ​​around 5 µV measured against the effort.

The superregenerative receiver is basically an AM receiver. It can only be used for FM reception if the frequency deviation is sufficiently large, namely through edge demodulation , i.e. by placing something next to the transmitter frequency when setting. With narrowband FM (NFM) the signals are drowned out in the noise. If modest requirements are to be met, it is therefore suitable for VHF radio reception, but not for hi-fi applications.

The main field of application is the amplitude-modulated transmission of audio signals or data of small size over short distances, in which interference from other radio applications can be accepted. In other words, wherever it can be accepted that the transmission may have to be repeated once or several times. In the post-war years up to around 1965, the commuter was installed in almost all receiver parts for model radio controls - it was only in the following years that the significantly more expensive superimposition receiver gradually replaced it . With today's components, the manufacturing price of super regenerative receivers is minimal. They are therefore almost only found in devices at the lower end of the price range.

Most of the super-regenerative receivers now operate in the ISM bands on 27 and 433 MHz and the SRD band on 868 MHz.

literature

Footnotes and individual references

  1. ^ EH Armstrong, US Patent 1424065 , Signaling System, filed June 27, 1921
  2. ^ IF Jackowski, How to prevent motorboating , Popular Mechanics, Nov. 1927
  3. Burkhard Kainka: The quartz commuter http://www.b-kainka.de/bastel118.htm
  4. Claus Schmidt: Accessories and additional devices built in-house. Verlag für Technik und Handwerk, 1999
  5. Heinz Richter: The great remote control book. Franckh'sche Verlagsbuchhandlung, 1966
  6. ^ Heinz Richter, reworked by Richard Zierl: The great remote control book. Franckh'sche Verlagsbuchhandlung, 1974