Reflectional receiver

A reflex circuit is a circuit in which the flow of information mirrored on an imaginary plane of reflection ( " reflected ", hence the name) and is a most cost-intensive assembly a second time as a separable signal - in theory even several times - passes. The reflex principle has been used for better utilization of electron tubes and bipolar transistors, especially of radio or radio amateurs in receiving circuits for radio signals and in some cases also in commercially produced radios and walkie-talkies.

In the assembly, there are two separable signals with a matching information. These signals are combined prior to or in the assembly Additive ( linear) and separated again after the assembly. A clear separation of the two signals is mandatory only within the loop.

The two signals can be emitted from the assembly to a connection in common. It is in principle also possible that the module has separate outputs.

• 2.1 The reflex receiver
• 2.2 Reflex receiver example in 1924
• 2.3 Transistorisiertes reflex receiver sample
• 2.4 levels of savings in the superheterodyne
• 2.5 The reflex circuit is now virtually meaningless

• 3.1 Reading the circuit
• 3.2 Error reading circuit

For use in the analog signal processing

Operation

The reflex circuit is to be defined here as a multiple use of an amplifier circuit. To this end, the original signal and that which has already passed through the amplifier stage, the input side added. Multiplication may not take place for reliable operation, since the signals are then inextricably linked. The space used for the reflex circuit ( s) amplifier stage (s) or must consequently have to work largely linear.

The original signal is sent to another frequency region after initial amplification by mixing or demodulation. The different frequencies can be separated from each other by high-, low - or band-pass filters and resonant circuits, so that the amplified signal for renewed strengthening can be attributed to the input again.

Possibilities and limitations

Reflex circuit allows a saving of components and may optionally also reduce the space requirements. The principle was therefore used in the construction of simple receiver circuits with discrete components mainly.

Simultaneous demodulation in the reflection stage itself is not possible because of the associated inter-modulation of the two signals to be amplified. Thus, the reflex circuit can not be simultaneously used as overlay or mixing stage also, since this would lead to a variety of parasitic mixtures.

A stronger RF positive feedback for de-attenuation of the RF input resonant circuit according to the principle of the feedback recipient must be avoided in a clean working reflex level: The associated, significant increase of the RF input level leads to a shift in operating point and can reduce the NF- side gain so drastically so that the original gain profit is canceled. In addition, then the RF signal is demodulated by shifting to the characteristic break in an undesirable manner already in the reflex level, so these as such can no longer act.

On the contrary, for stable operation and good linearity in the reflex level you just wanted a negative feedback signals. In order not to make unnecessarily complicated a reflex circuit, this is often done by suitable in this sense anschlossene coil tabs and / or transformer windings.

With a step in reflex circuit, a much larger total gain than a single stage may be achieved. However, they will always be less than that of two individual stages, as naturally always circuitry compromises must be made.

Separation from other circuits

Not every circuit, wherein an output signal is fed back, is a reflection circuit. The best known counter-examples are the following:

• Feedback ( positive feedback, negative feedback )
• Shrinkage control in the superheterodyne receiver

In a series of circuits that have been referred by their authors as a reflex circuit or as a reflex audion, it is not about circuits according to the reflex principle!

Feedback versus reflex circuit

• The input signal and the returned signal are not separable in the feedback
• At reflex circuit input signal and the returned signal are in different frequency ranges.
• A feedback occurs in phase, while in the reflection circuit, a fixed phase relationship between the two signals is not absolutely
• The number of loop iterations is unlimited in the feedback.
• The path of the feedback signal is not in the immediate path of the signal processing. The total circuit works so basically even without the returned signal.
• The feedback can be connected to the rectification
• Within a multi-use according to the reflection principle, the feedback module may be used in principle limited.

Special and general definitions

The definition of " tube circuit with multiple use " is both too general because it includes neither the common processing of independent signals or multiple use, for example, the " Audion tube " with the grid rectification from. On the other hand, it is too specific, because so yes the use of transistors or integrated circuits is excluded.

Normally, the voltage and / or the current or the power of two signals of different frequencies is simultaneously amplified. There could also be other sizes but in principle. The most common was at this simultaneous amplification of two different signals in a signal flow combining HF and LF stage in the straight receiver. But can also be the simultaneous amplification of RF and IF. HF and LF or IF and NF in a superheterodyne receiver.

The need sometimes to be found in the literature, exposure to a reflex level, the input and output side, the two frequencies or signals are separated by points to specifically. Many examples show circuit that the input side, a simple addition may be sufficient. But there are also circuits with separate inputs and / or outputs. Generally, a transistor for the HF could operate in common base configuration and in common-emitter circuit NF, so that the addition within the amplifier device takes place. When merging the signals, it is only important that the energy flux of the two components is controlled together. Thus, the signal can be taken from the collector to the emitter and, in principle, also the other. On the output side Although a separation is required within the loop, under a soft but understood that this separation is also made for the other output. This second separation is unnecessary but then, if the original signal ( ie, for example the high-frequency ) over the transmission channel is ineffective anyway.

It is in different places pointed out that the potential applications are limited because the mutual interference between signals, the mixture through characteristic curvature, the acquisition of Brummodulation etc. represent problems that can often only be solved by an increased effort at switching means. This explains why reflex circuits barely had a greater practical significance.

The addition of the input signal and the fed back signal to be understood in a general sense. When the analog signal processing, the addition of voltages or currents of different frequencies is possible in principle. In a still more general consideration of the term reflex principle can be applied in a figurative sense, to the multiplex experience. There is little, however, also be understood that encountered in modern computing technology using the ALU in a loop with a given number of passes as a form of reflex principle. The principle is not limited to the analog signal processing in an expanded in this manner definition.

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 amplified and then demodulated in a separate mainly realized with a diode detector in a linearly operating amplifier stage first. The resulting low frequency there is now filtered with a low pass back to the input of the same amplifier stage, and then amplified in the low-frequency level one more time, so that the signal is amplified twice in total. So reflex receivers use a tube or a transistor or possibly even a so realized, multi-stage amplifier arrangement in duplicate. This is possible because the frequencies to be amplified, therefore, are in different independent frequency bands by suitable switching means capable of being isolated.

In the circuit of the Patent 293300 of 1913 shown on the right, the high-frequency signal at the output of the amplifier through the transformer to the diode is given k l. The low frequency signal is fed to the amplifier again via transformer o1. P1 via the transformer, the amplified low-frequency signal is transmitted to the headphones m.

Reflex receiver example in 1924

The circuit diagram of 1924 from today's perspective a bit confusing at first, so a brief description of the operation. Demodulating the diode between the anode of the first tube and the transformer is used. The charging capacitor is located on the secondary side of the transformer. The parallel-connected to the transformer windings capacitors suppress the high-frequency components in the LF signal and bridge the LF signal for the path of the high frequency signal.

The well-known by some people's receiver types ago transformer coupling simplifies the separation of the signaling pathways and allows desirably an additional voltage gain. An upward transformation is possible due to the high impedance tube inputs, it is based on the voltage at about 1:4. Even with transistorized reflex circuits the transformer coupling has advantages. In bipolar transistors, however, a step-down is required, as these are current controlled.

Transistorisiertes reflex receiver sample

Better to see through the circuit is a Japanese pocket receiver from the 1960s. Caused by the received signal in the resonant circuit VC1/L1 high frequency voltage generated in the coupling winding L1b a small voltage which is used for power control of the base of the transistor T1. This works as an ordinary linear amplifier in the emitter circuit. The amplified RF signal is picked up at the collector of the transistor which operates in the RF- choke dr.2 on C6 is supplied to the RF choke Dr.1 and in the normal manner together with the RC combination with diode D1 output demodulated. The ohmic resistance is a potentiometer, which simultaneously adjusts the volume setting. The tapped at the wiper low-frequency signal is then fed back through the red signal to the switching capacitor to the input and about the same level. There occurring high frequency residues are shorted with C3, the coupling coil L1b acts for the AF signal practically as a short circuit. In which T1 is then increased again, but is now located in the low- field signal is supplied via the throttle dr.2, which also is effective for the NF almost as a short circuit, the low-frequency transformer TR.1. This adjusts the signal to the input of the built-up T2 LF amplifier. The capacitor C7 bridged the primary winding of TR.1 terms of high frequency and contributes to stable employment relationships with.

Levels of savings in the superheterodyne

Reflex circuit has also been used in more complex, operating on the Superhetprinzip circuits in the intermediate frequency amplifier, which could then be used for both low-frequency pre-amplification with. These were mainly small radio receivers and radios Japanese origin. Circuit examples can be found among others in and in a paper published in the early 1970s. In receivers for FM broadcasting the reflex principle has also been used for the simultaneous amplification of high and intermediate frequency in the same stage.

The reflex circuit is now virtually meaningless

Although the demodulation with reflex circuits usually performed with semiconductor diodes and not working on the audio principle, this type receiver circuit was mistakenly called in some publications "Reflex Audion ".

In the early days of broadcasting, so in the 1920s, the reflex circuit got some importance, as could be realized quite powerful circuits with it with little effort and great with only one or two tubes. This was due to a considerable extent in competitions of radio clubs and magazines, which rewarded the circuit proposals of its members and readers for the least effort and maximum receive power. However, the critical adjusted working conditions and the associated, poor reproducibility made ​​them unattractive for industrially manufactured devices.

Furthermore, the reflex circuit was in the tube era so rarely used that they are under a " special reception " principle in addition to the similar principles as insignificant " Synchrodynempfänger " and " receiver with low power tube " is treated as 1958. In many other radio and wireless technology books from this period, they will no longer mention. An extensive amateur Manual from 1966 shows a tube circuit in which the demodulated AF signal is filtered by an RC network with the RF signal before merging. The index of another comprehensive handbook for the radio amateur from 1978 does not contain a tag with " reflex ". 1962 Heinz Richter writes in: " While in the tube technique, the reflex recipients have survived for a long time, they could become important in the transistor technology again. " This was true, but only for hobby circuits, project documentation in electronic experiment kits and for some best-buy devices that appeared briefly in supermarkets and as giveaways to.

Mistakes and errors in the approach

In conjunction with ( bipolar ) transistors " Audion " simply means " receiving circuit with a transistor." So it is not is an Audion within the meaning of Röhrenaudions with audio rectification.

The convincing at first glance circuit shows at first glance by way of example typical of the reflex principle of addition of the high frequency and the low frequency signal.

Read the circuit

At the collector of the high frequency is taken on the one hand for the feedback on the other hand for the rectification based on the principle of voltage doubling. The time constant of the RC element behind the rectification is with C = 10 nF, R = 1.8 kOhm expected to be in the order of 10 - 4s.

The low-frequency signal is taken from the collector of a throttle on the one hand suppresses the high frequency and the other hand, for the high frequency is a part of the load resistor of the transistor.

Error reading circuit

The transistor is not controlled by the sum of the voltages shown in the figure, but on the current which is caused by these voltages. For the high-frequency signal, this was observed in the manner that not the higher voltage at the upper terminal of the resonant circuit is used for driving, but the higher current that can be used in the tap.

The fault lies in the fact elementary confusion between 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 corresponds only superficially expectations. The holding times of the charging capacitor almost completely eliminated: The putative NF signal becomes a series of high frequency pulses.

The high-frequency pulses can not be separated from the high-frequency input signal at the collector and lead to a feedback. Therefore, it is not a reflex circuit in the sense of the definition. The DC component of the high frequency pulses is, however, amplified and emitted (if the pulses are not limited by the dynamic range ) as in a neatly designed reflex circuit.

Some owners use the partial word " reflex " in a misleading way for circuits that are not reflex circuits for various reasons: it lacks the feedback, the signals can not be separated because they are essential parts in the same frequency range, or because the signals are mutually multiplicative influence.