Reflex circuit
A reflex circuit is a circuit in which the flow of information is mirrored ("reflected", hence the name) on an imaginary plane of reflection and passes through a mostly cost-intensive assembly as a separable signal one more time - theoretically even several times. The reflex principle was used to make better use of electron tubes and bipolar transistors , especially by radio amateurs in receiving circuits for radio signals and occasionally in commercially manufactured radios and two-way radio .
There are two separable signals with matching information in the module. These signals are brought together additively (linearly) in front of or in the module and separated again after the module. A clear separation of the two signals is only mandatory within the loop.
The module can output both signals together at one connection. In principle, it is also possible for the module to have separate outputs.
Use in analog signal processing
Mode of action
The reflex circuit is to be defined here as the multiple use of an amplifier circuit . For this purpose, the original signal and that which has already passed through the amplifier stage must be added on the input side. A multiplication must not take place for reliable operation, as the signals are then inseparably linked. The amplifier stage (s) used for the reflex circuit must consequently work largely linearly.
After being amplified for the first time, the original signal is brought into a different frequency range by mixing or demodulation. The different frequencies can be separated from one another by high, low or band pass filters or resonance circuits so that the amplified signal can be fed back to the input for further amplification.
Possibilities and limits
The reflex circuit enables components to be saved and, if necessary, can also reduce the space requirement. The principle was therefore mainly used in the construction of simple receiver circuits with discrete components.
Simultaneous demodulation in the reflex stage itself is not possible due to the associated intermodulation of the two signals to be amplified. Thus, the reflex circuit cannot be used at the same time as a superimposition or mixing stage, especially since this would lead to a large number of parasitic mixtures.
A stronger HF positive feedback for undamping the HF input resonant circuit according to the principle of the feedback receiver must be avoided in a properly working reflex stage: The associated, significant increase in the HF input level leads to a shift in the operating point and can drastically reduce the LF gain so that the original gain in gain is canceled again. In addition, the RF signal is then already demodulated in an undesirable manner in the reflex stage due to the shift to the curve kink, so that it can no longer act as such at all.
On the contrary, for stable operation and good linearity in the reflex stage, negative feedback of the signals is desired. In order not to make a reflex circuit unnecessarily complicated, this is often done by means of coil taps and / or transformer windings connected in a suitable manner.
With a stage in reflex circuit, a significantly greater overall gain can be achieved than with a single stage. Nevertheless, it always remains smaller than that of two individual stages, since, of course, compromises in terms of circuit technology always have to be made.
Differentiation from other circuits
Not every circuit in which an output signal is fed back is a reflex circuit. The best known counterexamples are the following:
- Feedback (positive feedback, negative feedback)
- Shrinkage control in the superimposition receiver
A number of circuits, which their authors called reflex circuit or reflex audion, are not circuits based on the reflex principle!
Feedback versus reflex circuit
- The input signal and the returned signal cannot be separated during the feedback.
- With reflex switching, the input signal and the returned signal are in different frequency ranges.
- Feedback takes place in phase (positive feedback) or in phase opposition (negative feedback), while a fixed phase relationship between the two signals is not mandatory for reflex switching.
- The number of loop iterations is unlimited with the feedback.
- The path of the feedback signal is not in the direct path of the signal processing. The overall circuit therefore basically works without the returned signal.
- The feedback can be connected to the rectification.
- The feedback principle can only be used to a limited extent within an assembly that is used multiple times according to the reflex principle.
Special and general definitions
On the one hand, the definition of “tube circuit with multiple utilization” is too general, because it does not exclude the common processing of signals that are independent of one another, nor multiple utilization, for example in the case of “tube audion” with grid rectification . On the other hand, it is too special because it excludes the use of transistors or integrated circuits.
Normally the voltage and / or the current or the power of two signals of different frequencies are amplified at the same time. In principle, however, they could also be other sizes. With this simultaneous amplification of two different signals in one signal flow, the combination of HF and LF stages in the straight-ahead receiver was most common . However, it is also possible to amplify HF and IF, HF and NF or IF and NF in one superhet receiver at the same time .
The requirement for a reflex stage, which can sometimes be found in the literature, that the two frequencies or signals must be separated by switches on the input and output sides, is too special. Many circuit examples show that a simple addition can be sufficient on the input side. There are also circuits with separate inputs and / or outputs. In principle, a transistor could operate in a base circuit for the HF and in an emitter circuit for the LF, so that the addition takes place within the amplifier component. When the signals are brought together, the only thing that matters is that the flow of energy is controlled jointly by both components. In principle, one signal can also be picked up at the emitter and the other at the collector. On the output side, a separation is required within the loop, but a switch means that this separation is also carried out for the other output. This second separation is unnecessary if the original signal (for example the high frequency) is in any case ineffective in the further transmission channel.
It is pointed out at various points that the possible applications are limited, since mutual influencing of the signals, the mixing due to the curvature of the characteristic curve, the adoption of hum modulation, etc. represent problems that can often only be solved by an increased expenditure of switching means. This explains why reflex circuits have hardly achieved any greater practical importance.
The addition of the input signal and the returned signal must be understood in a more general sense. In the case of analog signal processing, voltages or currents of different frequencies can in principle be added. In an even more general view, the term reflex principle can also be applied in a figurative sense to multiplex use . There is little to be said against the use of the ALU in modern computing technology in a loop with a given number of runs as a form of the reflex principle. With a definition expanded in this way, the principle is not limited to analog signal processing.
Application of the reflex circuit
The reflex receiver
The best known application of the reflex circuit is the reflex receiver. Here, the high frequency is first amplified in a linear amplifier stage and then demodulated in an independent detector, mostly implemented with diodes. The resulting low frequency is then filtered with a low pass, fed back to the input of the same amplifier stage and then amplified again in the LF level, so that the signal is amplified twice. Reflex receivers therefore make double use of a tube or a transistor or, if necessary, a multi-stage amplifier arrangement implemented with them. This is possible because the frequencies to be amplified are in different frequency bands that are independent of one another and can therefore be separated from one another by suitable switching means.
In the circuit shown on the right from patent 293300 from 1913 ( Otto von Bronk and Wilhelm Schloemilch ), the high-frequency signal at the output of the amplifier is passed to diode 1 via transformer k. The low frequency signal is fed back into the amplifier via transformer o 1 . The amplified low-frequency signal is transmitted to the headphones m via the transformer p 1 .
Reflex receiver example from 1924
The circuit diagram from 1924 is initially a bit confusing from today's perspective, hence a brief description of the mode of operation. The diode between the anode of the first tube and the transformer is used for demodulation. The charging capacitor is located on the secondary side of the transformer. The capacitors connected in parallel to the transformer windings suppress the high-frequency components in the LF signal and bridge the LF signal for the path of the high-frequency signal.
The transformer coupling known from some popular receiver types simplifies the separation of the signal paths and, if desired, enables an additional voltage gain. An upward transformation is possible due to the high-resistance tube inputs; it is approximately 1: 4 in relation to the voltage. The transformer coupling also has advantages in transistorized reflex circuits. In the case of bipolar transistors, however, a step-down transformation is required because they are current-controlled.
Transistorized reflex receiver example
The circuit of a Japanese pocket receiver from the 1960s is easier to understand. The high-frequency voltage caused by the received signal in the resonant circuit VC1 / L1 generates a small voltage in the coupling winding L1b, which is used to control the current of the base of the transistor T1. This works as a normal linear amplifier in a common emitter circuit. The amplified HF signal is taken from the collector of the transistor, which works HF-wise on the choke Dr.2, via C6, fed to the HF choke Dr.1 and with the diode D1 usually together with an RC combination at the output demodulated. The ohmic resistance is a potentiometer, which is also used to adjust the volume. The low-frequency signal picked up at the grinder is now fed back to the coupling capacitor via the signal path marked in red and then to the input of the same stage. High-frequency residues occurring there are short-circuited with C3, the coupling winding L1b practically acts as a short-circuit for the LF signal. The signal, which is now amplified again in T1, but is now in the LF area, is fed to the LF transformer Tr.1 via the choke Dr.2, which also almost acts as a short circuit for the LF. This adapts the signal to the input of the audio output stage built with T2. The capacitor C7 bridges the primary winding of Tr.1 in terms of high frequency and contributes to stable working conditions.
Step savings in the superhet receiver
The reflex circuit was also used in more complex circuits working according to the superhet principle in the intermediate frequency amplifier, which could then also be used for audio pre-amplification. These were mainly small radio receivers and two-way radios of Japanese origin. Circuit examples can be found among others in and in a publication that appeared in the early 1970s. In receivers for VHF radio, the reflex principle was also used for the simultaneous amplification of high and intermediate frequencies in one and the same stage.
The reflex circuit is practically meaningless today
Although the demodulation in reflex circuits mostly took place with semiconductor diodes and did not work according to the audio principle, this type of receiver circuit was also incorrectly called "reflex audion" in some publications.
In the early days of broadcasting, i.e. in the 1920s, the reflex circuit acquired a certain importance, as it could be used to create very powerful circuits with little material expenditure and with only one or two tubes. Competitions by radio clubs and magazines made a significant contribution to this, rewarding their members 'or readers' suggestions for switching for the least amount of effort with maximum reception performance. The critical working conditions and the associated poor reproducibility made them uninteresting for industrially manufactured devices.
Furthermore, the reflex circuit was used so seldom in the tube era that it was treated, for example, in 1958 as a “special receiving principle” alongside similarly insignificant principles such as “synchrodyn receiver” and “receiver with low-current tube”. In many other radio and radio technology books from this time it is no longer mentioned at all. An extensive amateur manual from 1966 shows a tube circuit in which the demodulated LF signal is filtered by an RC element before it is merged with the HF signal . The subject index of another extensive manual for radio amateurs from 1978 does not contain a subject word with “reflex-”. In 1962, Heinz Richter wrote in: "While reflex receivers have survived for a long time in tube technology, they have regained importance in transistor technology." However, this only applied to handicraft circuits, building templates in electronic experiment kits and some of the cheapest devices briefly appeared in supermarkets and as promotional gifts, too.
Errors and mistakes in the approach
In connection with (bipolar) transistors, “-audion” simply means “receiving circuit with one transistor”. It is therefore not an audion in the sense of a tube audion with audio rectification.
The circuit, which is convincing at first glance, shows at first glance in an exemplary manner the addition of the high-frequency and low-frequency signals typical for the reflex principle.
Reading the circuit
At the collector, the high frequency is picked up on the one hand for the feedback and on the other hand for the rectification according to the principle of voltage doubling. The time constant of the RC element after the rectification is, as expected, C = 10 nF, R = 1.8 kΩ in the order of magnitude of 10 −4 s.
The LF signal is taken from the collector via a choke which, on the one hand, suppresses the high frequency and, on the other hand, represents part of the operating resistance of the transistor for the high frequency.
Error reading the circuit
The transistor is not driven by the sum of the voltages shown in the picture, but by the current that is caused by these voltages. For the high-frequency signal, this was taken into account in such a way that it is not the higher voltage at the upper connection of the resonant circuit that is used for control, but rather the higher current that can be used at the tap.
The error lies in the actually elementary confusion of current and voltage of the LF signal: The resistance of the element with the time constant is bridged by the constantly open base-emitter path of the transistor, so the time constant only meets expectations when viewed superficially. The holding times of the charging capacitor are almost completely eliminated: the supposed LF signal becomes a series of high-frequency pulses.
The high-frequency impulses cannot be separated from the high-frequency input signal at the collector and lead to feedback. Therefore it is not a reflex circuit in the sense of the definition. However, the direct component of the high-frequency impulses is amplified and emitted as in a neatly developed reflex circuit (as long as the impulses are not limited by the modulation range).
Some authors use the partial word “reflex” in a misleading way for circuits that are not reflex circuits for various reasons: There is no feedback, the signals cannot be separated because they are largely in the same frequency range, or because the signals are mutually exclusive influence multiplicatively.
Web links
- Single-tube reflex receiver The circuit diagram is very problematic because the capacitor parallel to the headphones that is required to close the anode circuit for the HF is missing. Even with rectification, you should not rely on the capacity of the transformer winding.
- The transistor can also serve two masters. A problematic, but very interesting solution, because it is not the LF signal, but rather high-frequency semi-oscillations that are fed back. As shown in the article, wrong polarity of the diode can lead to feedback. The prerequisites for a proper implementation of the reflex principle are therefore not met. On the contrary, the circuit is reminiscent of the differential amplitude demodulation mentioned in connection with reflected audion.
Sources and Notes
- ↑ In 1924 it was mentioned in particular that the small HF signal makes poor use of the tube's dynamic range and still leaves enough space for the NF signal. - A very modern view that is reminiscent of the concept of information.
- ↑ In the case of a long transit time within the loop, echo effects can occur due to mutual, disruptive influencing of the signals.
- ↑ For example, the following part of the arrangement can be insensitive to the high-frequency signal. Then its clearly recognizable suppression can be omitted.
- ↑ See the grid rectification with linear gain and the anode rectification with nonlinear gain.
- ↑ Handbook for radio frequency and electrical technicians. V. Volume, specialist dictionary, publishing house for radio-photo-cinema technology, Berlin-Borsigwalde 1957/1970.
- ↑ a b Meinke, Friedrich-Wilhelm Gundlach : Pocket book of high frequency technology. Springer-Verlag, Berlin / Göttingen / Heidelberg 1956.
- ↑ Ing. HF Steinhauser: VHF hand-held speech radio construction book. Franz'sche Buchdruckerei G. Emil Meyer, Munich 1956.
- ^ A b Walter Conrad: Basic circuits of radio technology. Fachbuchverlag Leipzig 1958, p. 100.
- ↑ Der Radioamateur magazine, 2nd year, 1924, issue X, page 254
- ↑ Werner W. Diefenbach: Miniature receiver for travel and radio sports. Jakob Schneider Verlag, Berlin-Tempelhof 1963.
- ^ Bernd Jacobi: Japan radio communication devices. Verlag für technical literature Conrad, pp. 12-13.
- ^ Author collective, elektronicum, amateur handbook for communications technology and electronics, German military publisher , Berlin 1966/1967
- ↑ Amateur radio, A handbook for radio amateurs, edited by Karl-Heinz Schubert, Military Publishing House of the German Democratic Republic, Berlin 1956, 5th, completely revised edition, 1978
- ↑ Heinz Richter, Das Große Transistor Bastelbuch, 1962, Franckh'sche Verlagsbuchhandlung W. Keller & Co., Stuttgart
- ^ Karl-Heinz Schubert, Practical Radio Tinkering III, German Military Publishing House, Berlin 1969, page 54, picture 36
Circuit for 17 is a plagiarism from Siemens semiconductor circuit examples edition April 1959 p. 28/29