Detector receiver

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Symbol Diode.svg Detector receiver
( English crystal radio )
Historical detector receiver from the Heliogen company (Germany 1935).  Attached are: (top left) an oscillating circuit coil and a crystal detector as well as a trap circuit at the bottom left.

Historical detector receiver from the Heliogen company (Germany 1935). Attached are: (top left) an oscillating circuit coil and a crystal detector as well as a trap circuit at the bottom left .

Device / component
Class: Straight-ahead receiver
Invention: 1st half of the 20th century
technology
Power supply: none necessary
Active components: Minimum 1 crystal detector or diode
Playback via: headphone
Frequency band: Depending on the attached antenna coil, long / medium and short wave (VHF possible to a limited extent)
Circuit example of a single-circuit detector receiver

Circuit example of a single-circuit detector receiver

In the early days of broadcasting , detector receivers (from the Latin detegere , `` discover '' ; also detector radio , diode receiver ) were the simplest devices for receiving radio broadcasts that were broadcast , for example, with amplitude modulation over short, medium or long waves . Even shortly after the Second World War , such simple devices were more common again in view of the poor economic situation. The Siemens company also built permanently tuned detector receivers, such as the so-called “Berlin connector” for receiving a strong local radio station .

Detector receivers consist of only a few components and can work without their own power source because all of the power that is fed to the connected headphones comes from the energy of the electromagnetic waves received by the transmitter .

The simple technology still makes it a popular object for hobbyists and for educational purposes today.

Structure and circuit

A self-made detector receiver with a diode (bottom right) as a rectifier, the three components coil (bottom left), (film) variable capacitor (center) and diode are clearly visible

The detector receiver is the simplest version of a straight-ahead receiver (strictly speaking, the resonant circuit-less fritter version mentioned below is the simplest). The basic structure consists of the inputs for antenna and earth , one or more resonant circuits for tuning to a specific transmitter, the envelope demodulator (detector, rectifier ) and the output, typically to headphones.

Although the circuit diagram has very few components, the structure used to be sometimes very voluminous because at that time more emphasis was placed on the mechanical design of some components and miniaturization was not yet an issue.

Detector radio from Sweden with banana socket connections

A receiver often consisted of a housing equipped with banana sockets in order to be able to connect the inputs (antenna, earth) and outputs (headphones). Often there were further banana sockets for externally changeable components, such as the detector itself (see below) and sometimes coils (see below) to change the wavelength range.

Detector receivers don't always have to be bulky, however, miniaturization already existed in the 1920s. A historical radio postcard can be found in the treasury of the Museum for Communication in Berlin and in the Thuringian Museum for Electrical Engineering in Erfurt . It is about four to five millimeters thick and the size of a normal postcard. There were other similar devices.

function

Above: low frequency
middle: unmodulated high frequency
Below: amplitude-modulated HF

Due to the multitude of electromagnetic waves ( radio waves ) in our environment, electrons in the antenna wire are set in motion, weak alternating currents flow. These are a mixture of currents of very different frequencies and amplitudes. If one were to demodulate this overall stream , one could hear a mixture of the programs of all nearby radio stations.

An oscillating circuit enables a desired frequency to be filtered out. In the arrangement shown here ( blocking circuit ) it represents a large resistance for the set frequency, with which this generates a corresponding voltage on the diode circuit. Other frequencies, however, are more or less short-circuited. A single oscillating circuit is still working imperfectly.

It makes no sense to connect headphones directly to the resonant circuit because the low frequency is symmetrically modulated onto the high frequency, see illustration. The listener would output the average value, i.e. the zero line, i.e. nothing. The diode eliminates this symmetry through rectification (demodulation).

Demodulation is achieved by cutting off either the positive or the negative half-wave of the AC voltage received on the resonant circuit. This is the job of the rectifier component, such as a crystal detector or diode . It only lets through a half-wave and turns the alternating voltage into a pulsating direct voltage . Its envelope (shown in blue in the picture below) is the desired low frequency. The headphones are unable to follow the individual power surges and move according to the average value, i.e. roughly the said envelope curve, which however corresponds to the desired low frequency .

If the transmitter is not too far away, an oscilloscope between the top and bottom of the tank circuit will typically display a picture like the bottom, with a peak voltage of at least 200 mV, 600 mV is even better. If this value is not exceeded, current can never flow through the germanium diode because its threshold voltage is not reached. Then you can't hear anything, although a signal is present. Silicon diodes are unsuitable because their threshold voltage is 600 mV. An electronic component with a threshold voltage of zero would be desirable, but only exists in the form of a peak voltage rectifier ( see envelope demodulator ). However, this requires electrical energy to operate the comparator , which works faster than an operational amplifier up to a maximum frequency of around 10 MHz.

With amplitude modulation, the envelope of the pulsating DC voltage is the information transmitted - music or speech. The remaining high-frequency components cannot be heard, but they can be smoothed out (in the style of a charging capacitor ) by a small capacitor of about 1  nF that is placed parallel to the headphones . This can be necessary in order not to disturb a downstream amplifier with excessive high-frequency components.

Antenna and ground as inputs

Since the entire energy is only taken from the received high frequency radiation, a good antenna is essential for the function of the detector receiver . For medium waves, the optimum is a multi-meter long wire antenna. In the open air, however, the risk of lightning strikes must be taken into account, which means additional work, hence the friendly admonition of every radio operator at the time when the broadcast ends : "And please don't forget to ground your antenna ", i.e. H. The antenna and earth line were connected directly by means of a switch, which diverted the lightning energy past the electronics. If an outside antenna is not possible, a few feet of wire in the attic or anywhere near the outside wall inside a house is worth a try.

As grounding , for. B. the pipes of a central heating system or a gutter drain. Originally, a special electrode was anchored in the ground and connected to the detector.

In practice, surprisingly, the heater is sometimes also a good antenna. The experience of decades of experiments shows that, depending on the local conditions, earthing may not be necessary and everything works perfectly. In the course of the didactic occupation with the matter, own experiments are instructive especially here.

Incidentally, the antenna wire does not have to be bare. Iron wire is less suitable, copper is the most commonly used. The high-frequency waves do not interfere with the insulation that is customary in switching or bell wires. Only metallic shields are unsuitable because they block radio waves - and they are used precisely for this purpose to prevent disruptive radiation .

The selectivity can often be improved if - especially with very long antenna wires - a small capacitor of around 30 pF is placed between the antenna and the resonant circuit ( series connection ).

Alternatively, loop antennas or ferrite antennas can also be used as antennas . They also form the coil of the resonant circuit at the same time, see Sect. u. The smaller ferrite antenna , like the equivalent loop antenna, mostly provides too little reception energy for the direct operation of headphones, so that it is only useful in detector-receivers for local reception and a downstream LF amplifier may be necessary to to hear anything at all.

Oscillating circuit for tuning

The resonant circuit must always be of particularly high quality (low loss) in order to ensure sufficient selectivity. A resonant circuit always consists of an inductance ( coil ) and a capacitance ( capacitor ). With the coil in particular, you could achieve a lot by choosing a special material. So one used high-frequency stranded wire and wound in such a way that the self-capacitance of the coil was as low as possible (honeycomb coils, basket bottom coils).

In order to tune an oscillating circuit to the desired transmitter , the resonance frequency of the oscillating circuit must be set to its frequency. Then the resonant circuit is in parallel resonance (high resistance) with the transmission frequency, while all other frequencies are short-circuited to earth.

Either the capacitance or the inductance in the resonant circuit had to be changed for coordination. For this purpose, either a variable capacitor was used (air variable capacitors were common or, if the quality was lower, cheaper, foil-insulated types) or the coil was made variable. The latter was easy to make yourself with a little manual skill via a sliding contact; Another possibility for changing the inductance is a powder core (bound iron powder) in the coil that can be moved or adjusted with a thread.

Air-core coils were mostly used on coil designs . Extremely complex honeycomb or cross-wound or basket base coils (see picture at the top) or pairs of coils that can be rotated into one another for coordination were also used (see examples under the web links below). Such coil geometries primarily reduce the parasitic capacitances between the windings and thus increase the quality. The coil wire used is high-frequency stranded wire, which consists of a particularly large number of very thin individual strands in order to limit the skin effect at high frequencies as much as possible.

Variable capacitors are less suitable for self-construction. Fixed values ​​can, however, be made of high quality aluminum foil and thin foil (adhesive tape, cling film).

A simple coil, as it is used in detector receivers, consists of a cardboard tube on which about 100 turns of an insulated (switching or bell) wire are wound. The inductance can also be made variable by fixing the turns with a few strips of adhesive parallel to the roll and then exposing the bare wire between two such strips by grinding or scratching, on which a grinder can then be moved. But exotic approaches, such as a basket base coil based on a radially slotted beer mat, have already been implemented.

But even an oscillating circuit of the very best quality can be dampened so much by headphones that are too low-resistance that the selectivity and volume leave a lot to be desired. This is remedied by an LF transformer as an impedance converter . However, active impedance converters require energy from a battery.

Crystal detector for demodulation

For demodulation of the amplitude-modulated high-frequency a served rectifier from semiconducting material, the so-called crystal-detector .

output

Typical contemporary headphones

After rectification, a half-wave of the high-frequency voltage is cut off, but two-way rectifier circuits were also used. This signal can be reproduced directly with headphones because the mechanical inertia of the system does not follow the high-frequency component of the signal and only the time-averaged voltage curve (which is proportional to the low-frequency envelope), i.e. the desired audio signal, is reproduced.

1600 ohms.

In some circuits, a blocking capacitor (typically 2 nF) is additionally arranged parallel to the output as a charging capacitor for even better integration of the high-frequency signal or to reduce the detuning caused by the manual capacitance.

The electromagnetic headphones used were relatively high impedance (for example around 4 kΩ). If the signal level was high enough, the electromagnetic horn loudspeakers that were common at the time could be controlled directly - these were also high-impedance. Today's piezoelectric earphones are much more high-impedance, but somewhat less sensitive. In addition, a resistor of about 100 kΩ must then be connected in parallel in order to close the DC circuit through the diode, which is necessary for it to work properly. The very high internal resistance of the crystal receiver is not sufficient for this. After all, the internal headphone capacity - together with the capacity of the headphone connection cable - replaces the parallel capacitor that is common with magnetic headphones (closing the HF circuit).

Suitable headphones as well as loudspeakers must be high-impedance (several kΩ) and have a high degree of efficiency in order to generate sufficient volume with the limited signal level. Low-resistance electrodynamic headphones or earphones must be adapted using a transformer.

Circuit variants

If a strong local transmitter outshines the entire waveband, you can hardly receive other transmitters. Then a blocking circuit (for which practically the same applies as for the oscillating circuit above) is inserted into the antenna line , which fades out this local transmitter so that weaker transmitters can be heard.

However, if you are satisfied with this one local transmitter, you can simplify the construction even further and largely do without tuning elements. With a germanium diode , which has a lower threshold voltage than a silicon diode , connected in parallel to the headphones and then connected to larger metal parts as an antenna, as well as to a grounding (e.g. water pipe), that was enough for reception. The antenna works together with the earth as a relatively broadband resonant circuit. Radio reception is no longer possible since most medium-wave transmitters were switched off .

The most important disadvantage of this reception principle is the poor selectivity . Therefore, experiments were also carried out with multiple oscillating circuits, but even with two oscillating circuits it is hardly possible to bring them to the necessary synchronization, be it by hand using separate tuning means, or be it with double rotary capacitors.

In practice, the antenna coupling takes place via networks of inductances or capacitances , the rectifier diode is usually connected to a tap within the coil winding. Both serve the purpose of reducing the damping of the oscillating circuit and thereby increasing its quality and thus, in turn, the selectivity.

In order to increase the output voltage, later in the age of the semiconductor diodes experiments were also carried out with full-wave rectification, which brought a certain improvement.

There were also devices that connected a detector receiver with a downstream amplifier stage (from a tube, later transistor). However, since a power supply was necessary for this, active circuits could also be used in the receiver part (see section on further development ), so that this variant was not widely used.

The detector as a component

The detector receivers in the early days of radio reception were mostly equipped with crystal detectors as rectification components and were accordingly also referred to as crystal detector receivers. Early series-produced components were, for example, a carborundum detector or Greenleaf Whittier Pickards Perikon .

Crystal detector

IV characteristics of various diodes
Typical crystal detector Schottky diode around 1923
Encapsulated crystal detector around 1960 WISI
Detector crystals in the commercial packaging
Old designs of germanium diodes

In crystal detector mainly came galena and pyrite used as ores in the nature occur. In times of crisis, related materials ( sulfur compounds) were also produced artificially. These were coveted trade goods ( black market ).

For a crystal detector is a about 5 mm was large crystal into a metallic holder clamped, which formed the one pole of the diode. From the other pole, a metal tip was pressed onto a point on the crystal, so that a Schottky contact was created. To be precise, the crystal detector was a Schottky diode . The operation of the receiver with a detector crystal was very difficult and required a lot of skill and a steady hand, as a suitable place on the crystal had to be found with the help of a metal tip that had a rectifier effect. Commercial versions of a crystal detector had enclosed the arrangement in a small glass tube that was mounted transversely on two banana pins and was thus plugged into the corresponding banana sockets of the detector receiver. At one end a metal tube with a handle poked out, with which one could move the metal tip and use it to poke around at the crystal. Elegant devices carried out alternating lifting / lowering and rotating movements when the handle was rotated, so that new contact points were always reached by simply rotating the handle.

In the case of self-assembly or demonstration experiments, such a Schottky contact can be established with very primitive means. B. a rusty sheet metal serves as a pole and a graphite pencil as the other pole, which presses on this sheet metal with a wound wire spiral as a spring.

Semiconductor diode

Later, instead of the more difficult-to-use crystal detector, diodes made of industrial semiconductor single crystals (e.g. from the 1940s peak diodes and from the 1950s germanium diodes ) were used, which were far superior to the old crystal detectors in terms of size, price, handling and operational reliability. However, due to their higher internal resistance, PN silicon signal diodes must be carefully adapted to the resonant circuit and listener. Particularly modern specimens with internal resistances of a few MΩ have to be flowed forward, this was also common with the crystal detectors, with their hardly predictable properties, from the beginning if it is a precursor to the transistor ) even an amplification can be achieved. Using were also evacuated tubes (z. B. a tube diode), but due to some drawbacks such. B. a relatively high heating power, four to seven times lower sensitivity and because of the shortly thereafter possible alternative with the tube triode , in the more complex but much more powerful Audion circuit, were only used for a short time.

history

Historically older detector modules included the coherer (also called: fritter see below) and the magnetic detector , which was used to detect high-frequency vibrations. The crystal detector, however, was a big step forward because with these older components no speech or music reception could be realized, only the presence of a high-frequency vibration could be indicated or only a Morse code received. Another predecessor of the crystal detector, but can already be used to demodulate sounding broadcasts, was the electrolytic Schloemilch cell . However, it was awkward to use and, like the fritter, needed a preload. Also Reginald Fessenden's recipient of his first radio broadcast of 1906 built on this electrolytic principle on.

Small time scale in the temporal environment of the crystal detector:

Modern detector

Improved detector with external power supply and impedance converter

A classic detector takes all the energy from the resonant circuit and reduces its quality factor because the connected headphones have a dampening effect like a low-resistance resistor connected in parallel. The consequences: The bandwidth becomes very large and the RF amplitude becomes smaller. Selectivity and volume can be significantly improved if the load on the resonant circuit is reduced by an impedance converter . The field effect transistor BF244 has an input resistance of around 1 MΩ for medium waves, the output resistance is a few 1000 Ω. A voltage doubler can be connected at this point in order to achieve a higher volume. Only germanium diodes or Schottky diodes are suitable as rectifiers , because they have sufficiently low threshold voltages of around 0.2 V. The resistance of the headphones should be between 500 Ω and 50 kΩ, if necessary a corresponding resistor must be installed.

A further improvement is possible through cathode rectification , the energy-intensive electron tube being replaced by the use of tube-like performance features of a junction field effect transistor (FET). Since this circuit has no threshold voltage, signals of a few millivolts can be demodulated.

As part of competitions, e.g. For example, the “Annual Crystal Set DX Contest” , hobbyists fight for the best receiver with self-made detector receivers. In the 2003 competition, the winner was able to listen to over 190 stations up to 4000 km away. Also in 2013 there was an elimination with detector disciplines, the "Homebrew Radiocontest" .

Further development

From the early 1920s, there was another straight-ahead receiver in addition to the detector radio - the Audion . Until now, the detector had the advantage that it was around a third cheaper to build it yourself than it was to build an audio receiver, and that it did not need its own battery or any other external energy source. In Germany, the level of equipment with detectors was still over 50% in 1924. But the audion was u. a. The inexpensive receiver of Loewe Audion GmbH local receiver OE333 , produced from 1926 onwards, made it more popular, the success of which caused some changes on the German market. For the first time, the sales of detector radios fell, and the prices of other audio receivers on offer also fell. In Germany, the inexpensive people's receivers replaced many detector radios. But as early as the 1930s, the technically superior heterodyne receiver pushed the detector and audio radios considerably on the market. After the Second World War , however, the popularity of detector and Audion receivers rose again in Europe in the post-war period. Publishers such as Radio RIM in Munich published circuits for reproduction.

At the beginning of the transistor era in the 1950s, Audion-like transistor circuits or reflex receivers with similar technology were used. Today, however, only superimposed receivers are practically in use. For special purposes (e.g. training, experiments) the detector receiver is still a worthwhile project today.

Detector receivers can also be implemented for VHF reception with the same basic structure - the frequency modulation can be demodulated according to the principle of the edge demodulator. To do this, the resonant circuit must be somewhat out of tune with respect to the transmission frequency. As an antenna z. B. a half-wave loop dipole. Such devices were never made commercially, but building and using an FM transmitter in the near field is instructive.

Regarding the energy supply, Rectenna (from the English of rectifying antenna) can be seen as the successor. This is the name of a circuit arrangement that receives high-frequency energy and then converts it into direct voltage.

By switching off many powerful radio transmitters in the medium wave range , fewer radio stations from Europe can now be received with detector receivers than 25 years ago. However, with the decreasing station density, remote reception with a detector radio becomes easier.

Quote

In his novel It's a Battlefield (1934), the setting of which is the city of London, the writer Graham Greene mentions the detector receiver in a brief dialogue. In the German translation by Walter Puchwein, the dialogue reads:

“I heard Moscow,” said Crabbe, “I heard Rome, New York, but I can't get Geneva. What kind of device do you have? ”He snapped at Surrogate.
“Forgive me,” he replied, “but I think you have misunderstood me. I didn't talk about the radio station earlier ... "
"Just don't tell me about a crystal," Crabbe interrupted. “I'm an old man, but not stupid yet. What stories people tell me about stations that they can supposedly get with crystal detectors ... "

See also

Web links

Commons : Detector Receiver  - Collection of images, videos and audio files

Individual evidence

  1. ^ Friedrich Benz: Introduction to radio technology. Springer-Verlag, Vienna 1937.
  2. Image of a radio postcard .
  3. Datasheet LT1016 , (PDF; 1.3 MB), accessed on September 16, 2016.
  4. ^ Hans Heinrich Meinke , Friedrich-Wilhelm Gundlach : Pocket book of high frequency technology - Volume 2: Components . Springer-Verlag, Berlin 1992. ISBN 3-540-54714-2 . S. N16.
  5. Kristalloden Technik , R. Rost, Verlag Wilhelm Ernst and Son 1954, p. 157.
  6. ^ Radio handicrafts for boys , Heinz Richter , Franckh'sche Verlagsbuchhandlung 1974, p. 27.
  7. RIM craft book in 1964 ( "RIM book"), Radio RIM , Munich 1964, p 83rd
  8. Vladimir Gurevitch: Electric relays: Principles Applications . Taylor & Francis Group / CRC Press. Boca Ratin (Florida USA) 2006. pp. 211ff. ISBN 978-0-8493-4188-5 .
  9. Thomas H. Lee: Planar Microwave Engineering: A practical guide to theory, measurements and circuits . Cambridge University Press. Edinburgh 2004. pp. 297ff. ISBN 978-0-521-83526-8 .
  10. Receiving circuits , in: "Des Funkbastlers Ratgeber" . Brochure from the company Anschütz & Co. Kiel, 1926. pp. 14-22 and 32-43.
  11. Radio reception without tubes , Herbert G. Mende, Franzis Verlag, 1959, p. 16.
  12. ^ Siegfried Wirsum: "Radio tinkering with field-effect transistors" . Radio RIM . Munich 1973. p. 7ff.
  13. Berthold Bosch, Remote Reception Detector
  14. publications by Dave Schmarder theradioboard.com , accessed April 20, 2014, English.
  15. Eva Susanne Breßler: “From the experimental stage to the propaganda instrument: The history of the radio exhibition 1924 to 1939” . Böhlau Verlag GmbH and Cie. Cologne 2009. p. 73ff. ISBN 978-3-412-20241-5 .
  16. ^ Graham Greene: It's a Battlefield . William Heinemann, London 1934. German translation by Walter Puchwein. Battlefield of life . Rowohlt, Hamburg 1952 / April 1953, p. 69.
This version was added to the list of articles worth reading on April 21, 2007 .