DE1263875B - Secondary radar system with a device for suppressing interference signals and response pulse groups that are not synchronized with the interrogation signals - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. Cl .:

GOIs

German class: 21 a4 - 48/63

Number: 1263 875

File number: S 95752IX d / 21 a4

Filing date: March 3, 1965

Open date: March 21, 1968

The invention relates to a secondary radar system in which an interrogation station sends out interrogation signals characterizing different interrogation modes in periodic sequences and the response stations, after receiving each interrogation signal, emits a response pulse group corresponding to the respective interrogation mode, which contains at least two frame pulses following each other at a predetermined distance, between which each According to the information to be transmitted, code pulses can be present at predetermined pulse locations, with a device installed in the receiving station for the response pulse groups for the suppression of interference signals and response pulse groups which are not synchronized with the transmitted query signals, with a memory for the temporary storage of the received and demodulated ones Response pulse groups and with a comparison arrangement, which each received video-frequency response pulse group at the same time with that in _{ao of} the previous query period received and stored, the same interrogation corresponding response pulse group is supplied after a delay by an interrogation period, and only those pulses of the received response pulse group passes through that are also present at the corresponding pulse locations of the response pulse group received in the previous interrogation period.

It is known that in the case of secondary radar systems, aircraft are queried from the ground, which aircraft are on board Have facilities that enable it, either by an operator or automatically to answer the query. The response signals sent by the aircraft can be encrypted Contain data which cannot easily be obtained with conventional radar systems can, in particular the data for the identification of an aircraft and the indication of the altitude.

The interrogations of an aircraft are usually carried out in that pulse pairs are transmitted from the ground to the aircraft, which are precisely defined both in terms of their duration and their distance and have a precisely defined repetition frequency. The aircraft's response to the interrogation pulses is to return a coded response pulse group containing the desired response. This response pulse group is received on the ground and converted into video pulses, which are then evaluated. There are several possible query methods, for example by the size of the distance between the secondary radar system with a device
to suppress interference signals and from
not synchronized with the query signals
Response pulse groups

Applicant:

Societe Nouvelle d'Electronique et de la
Radio industry, Paris

Representative:

Dipl.-Ing. E. Prince, Dr. rer. nat. G. Hauser
and Dipl.-Ing. G. Leiser, patent attorneys,
8000 Munich-Pasing, Ernsbergerstr. 19th

Named as inventor:

Serge Mikailoff, Paris;

Georges Peronneau,

Celle Saint Cloud, Seine et Oise (France)

Claimed priority:

France of March 3, 1964 (965 879)

see the two transmitted query pulses are. Different response information can be obtained through the different query methods be called. The response pulse groups contain two frame pulses (also as start pulses and stop pulse), the distance of which is fixed independently of the query is. A predetermined number of code pulse positions is provided between these frame pulses, and the information is provided by the presence or absence of pulses at these code pulse locations coded.

A major problem with these secondary radar systems is the occurrence of undesired ones Response signals (referred to as "fruits" in English usage). These unwanted response signals can arise from various causes. The answering stations arranged on the floor namely usually all work with the same polling frequency, and since the intervals between the the impulses representing the response are standardized,

809 519/22 *

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these answering stations can have multiple response pulses - their front frame pulses with one pulse of the groups received from different reply senders, current polling period coincides. if which work on the same frequency and is met by this criterion, then there is a large external answering stations are triggered. This Ant- probability that it is just Words are then received in a repetition rate 5 received pulse around the front frame, which is most likely about the impulse of a response impulse group, and only The repetition frequency of the query concerned is in this case the subsequent pulses is divorced. These unwanted response impulses are evaluated at all.

groups are therefore "unsynchronized" and it is completely obvious that in this case the evaluation of the io-ended response pulse group in the previous desired response pulse group is severely disturbed. The interrogation period takes place in that it is determined that the conditions are particularly unfavorable if, behind the pulse received from the previous interrogation period, the pulse received at an interrogation point overlaps with the pulse groups that have just been received. Such a pulse in coincidence can also occur in a given overlap if only one interrogation point corresponding to the spacing of the frame pulses is operating, but a number of them are followed by a further pulse. If this remote transponder transmission is met, it is very likely to be triggered at the same time. As a result of the probability of a complete response pulse differing distances, the response signals meet
then staggered in time. . 20 In the secondary radar system according to the invention

In the known secondary radar systems of the type that are received, only those received pulses of the type specified at the beginning are suppressed in the evaluation circuits which, with such an undesired response signal and sporadic probability, lead to a complete Disch interference signals because they belong to two response pulse groups ,
other interrogation periods in the same interrogation. at the second input the delayed by the periods in the same relative temporal position of the two frame pulses are present. This interference signal suppression is 30 captured response pulse group and on the third one, however, obviously imperfect, because every pulse by the distance of two frame knock-off pulses is regarded as valid, which e.g. When the three pulses coincide, an unsynchronized response pulse input signals emits an output signal that the group of the other interrogation period releases together in time.
falls. This development results in an additional security

The aim of the invention is to create a remedy against the exploitation of unwanted response

Kundär radar system of the type specified above, signals. Because it results in an additional test

with which an even better differentiation of the demand is whether also in the current query period

wanted response pulses from unwanted response 40 behind the pulse, which is used as the front frame pulse

word impulses and interference signals is possible. a response pulse group was viewed

According to the invention, this is achieved by a further pulse at a time interval that follows

Coincidence circuit that corresponds to the prescribed spacing of the frame pulses at two inputs.

Comparative arrangement supplied response impulse speaks. If this second impulse is absent, the

groups and at a third input the in the pre-45 evaluation circuits enabled and thereby the

incoming query period received response - previously received pulse group as invalid

impulse group with one of the distance the frame draws.

impulses corresponding advance receives and preferably contains the inventive seconds

Coincidence of the three input signals a control signal där radar system also an arrangement which the

emits that the transmission of the output signals of the 50 time control circuit back to their initial state.

Comparison arrangement to the evaluation circuits, if after a control signal another

triggers, and by a timing circuit, which follows the control signal at a time interval that is smaller

is restarted by each control signal than is the spacing of the frame pulses.

and after one of the duration of a response has expired- This additional measure enables the transfer

impulse group corresponding time since the delivery of all valid impulses from two opposing

of the last control signal the transmission of the overlapping valid reply pulse groups

output signals from the comparison arrangement to the output to the evaluation circuits,

value switching blocks. The invention is based on the drawing

In a secondary radar according to the invention described for the sake of play. In it shows

system is not only received the coincidence of 60 F i g. 1 is a block diagram of the invention

Genetic impulses in successive interrogation arrangements,

Periods are examined, but it is also fixed - Fig. 2 diagrams to illustrate the put through, whether a synchronized response signal passed through at all exists that the order in which the query is running is

period received pulses by a regular 65 F i g. 3 diagrams to illustrate the elimination

Response pulse group acts. The criterion for this is an unsynchronized response signal,

is the presence of a complete response - Fig. 4 Diagrams to illustrate the elimination

impulse group in the previous interrogation period, generation of an isolated interference impulse,

F i g. 5 a and 5 b diagrams to explain the measures for closing the gate circuits in the arrangement of FIG. 1 in the case of overlapping response pulse groups,

6 shows diagrams to illustrate the suppression of unsynchronized response signals in the case of the displacement of the response pulse groups, the response pulse group of the following period η leading the response pulse group of the following period (n-1) , and

7 shows diagrams to illustrate the suppression of unsynchronized response signals in the event of the response pulse groups being offset, the response pulse group of the following period n lagging behind the response pulse group of the following period (n- 1).

The block diagram of FIG. 1 shows the main components of the arrangement for eliminating undesirable Response signals in a secondary radar system according to the invention. The organs for Duration and amplitude normalization (quantization) of the received pulses and for storage and for reading the response pulse groups received in successive interrogation periods are not shown. These organs usually include a minimum value limiter stage for the incoming video signal, a pulse duration discriminator circuit, which eliminates the pulses, the duration of which is less than a predetermined minimum duration of approximately 0.35 μβ, an amplitude normalization arrangement for the incoming video signal, a memory device and the associated circuitry. The ones belonging to the memory Circuits contain in particular a clock generator, which by one of the synchronizing pulses the radar system is triggered. This clock generator provides pulses which the specified Have the impulses of the response impulse groups spaced apart. Each query period is thus in divided into a certain number of precisely defined time levels. A circuit analyzes every polling period the received video signal, which in a secondary radar system by a sequence of position-coded Pulses is formed and it determines the presence or absence of valid pulses in every time step.

The pulses contained inside the various time levels are assigned to the Recorded storage elements of the storage device. If with several different periodically alternating query modes are used, each query mode is assigned a memory, the selected by a switching device operating synchronously with the switching of the query modes will. If, for example, the radar system runs in three different directions one after the other with each antenna revolution In query mode, three storage levels can be provided, each of which is at each antenna revolution sequentially in three of the three interrogation modes.

In the next query period, the content of the relevant query mode is then assigned Read memory level. This query takes place with an advance of the duration of a response pulse group; this means that the stored response pulse group is earlier to this advance Is available as a response pulse group received from the same distance in the current interrogation period. The memory is cleared and the newly received response pulse group is recorded in it.

The in F i g. The organs shown in FIG. 1 follow on from the arrangements just described. They contain a shift register R 1, the input of which is supplied with the pulses emitted by the memory at the moment of reading. These pulses, which correspond to the one in the previous sampling period (n— 1)

ίο correspond to the received video signal and are labeled Vq (n-1) appear at terminal 1. The video signals for the current sampling period η appear at terminal 2. They are labeled V (ri).

The shift register R 1 has the capacity of a response pulse group. As a result of the lead corresponding to the duration of a response pulse group when reading the memory, the pulses Vq (n-T) appear at the input of the shift register R 1 with an advance of the duration of a response pulse group, i.e. 20.3 μ8, with respect to the one at terminal 2 incoming pulses V (ri). The video signal Vq (n-1) is advanced in the shift register R 1 at the rate of the pulse repetition frequency of the response pulse groups. Thus, an entire response pulse group of the sampling period (n-1) at the point in time at which the first video pulse of the corresponding response pulse group of the current sampling period arrives is recorded in the shift register R1.

The output of the last stage of the shift register Rl is connected to one input of a comparison arrangement PO, which is designed as a coincidence circuit. At the other input, the comparison arrangement FO receives the video signal V (n) from the terminal 2. The comparison arrangement PO thus only emits an output signal V '(n) when a pulse I (n) of the video signal V (n) receives a pulse I (n-T) of the video signal V (n-T ) corresponds to the previous sampling period (n-1).

The outputs of the first and the last stage of the shift register R 1 are connected to two inputs of a coincidence circuit Tl , the third input of which receives the video signal V (n). The output of this coincidence circuit Tl is connected to the set input of a bistable multivibrator B 1 whose output opens a gate Pl or blocks. The signal input of this gate circuit P1 is connected to the output of the comparison arrangement PO. A delay arrangement L1 is inserted into this connection, which compensates for the delay in opening and closing the gate circuit P1.

The coincidence circuit T1 brings the flip-flop circuit J51 into its working state when there is coincidence between the three signals which are fed to its three inputs. As a result, the gate circuit P1 is opened, so that it transmits the output signals of the comparison arrangement PO to the output terminal 4. If the recorded signals Vq (n-1) contain the two frame pulses Flq (n-1), F2q (n-1) of a standardized response pulse group, these frame pulses are at the moment at which the leading frame pulse of the current sampling period arrives corresponding response pulse group arrives, in the last or in the first stage of the shift register R1. This means that the coincidence

7 · 8

conditions met. The gate circuit P1 is thus possible at the end of a response pulse group. This opened when the recorded signals assembly includes a bistable trigger circuit B 2, Vq (n-1) two με at a distance from each other whose set input 20.3 to the output of Koinziliegende pulses included that show the framework denzschaltang is connected Γ1, and whose Ausmenimpulse are, and if at the same time a pulse 5 passage through a capacitor with the input of I (n) in the video signal V (n) is present. - First stage of a shift register R 2 connected

The output signal V '(n) of the comparison arrangement is so that the voltage jump that occurs when the flip-flop B 2 FO is tipped over by the delay arrangement L1 slightly delays the input of an Imring so that it is ensured that the pulses in the shift register R 2 has the consequence. Gate circuit Pi is opened before the first pulse io. The shift register R 2 has the same structure at the gate circuit Pl. The gate circuit like the shift register .Rl, so that everyone in the PI remains open until the bistable flip-flop B 1 first stage input pulse is reset after 30.3 ^ s on off by another signal that appears from the transition of the last stage. Since the Kippschaleiner OR circuit Ul comes. As a result, the tong B 2 is triggered by the coincidence of the pulses I (n% transmission of unwanted response signals to 15 Flq (n- 1) and F2q (n- 1), which prevents terminal 4. The conditions for the shift register R 2 at its output a down-the-opening and shutting of the gate Pl are pulse from which is α in phase with the pulse F 2. This reference to the diagrams of Fig. 2 pulse is investigated in a delay circuit L 2 gebis 7 below. slightly delayed, so that shortly after the identification

F i g. 2 shows the case that two synchronized 20 ftzierungsimpuls SPI appear at the input of the OR circuit response pulse groups in the sampling periods (n- 1) Vl . The output of this OR circuit is and η appear. The line 0 represents the time origin with the reset input of the flip-flop B 1, which is given by the synchronization signal. so that this pulse blocks the gate circuit P1. The video signal Vq (n-1) contains two frame blocks.

impulseFl q (n-1) and F2q (n-1) and a special 25 This blocking the. TorschaltangPl at the end of a special identification impulse, denoted by SPI - every regular response impulse group prevents the net. The same response pulse group appears through the in FIG. 4 individual interference synchronous to this in the video signal V (n) of the sampling pulses as well as the pulses from the unsynchronized period (n). The response pulse groups generated by the comparison arrangement P 0, the signal V '(n) emitted as an isolated one, coincides with the signal V (n) 30 interference pulses which are identical in the sampling period. The diagram Sp 1 shows the open ode (η— 1) that have been recorded, however, when the gate circuit P1 was in position. At terminal 4, no response pulse group appears, a video signal appears to be linked to the video signal. The output signal of the ' Shift register Ä2

V (n) is identical. is also dem. via an OR circuit U2

3 shows the case that in the sampling period η 35 the reset input is supplied to the flip-flop B 2, so a response pulse group occurs which is set with the response pulse recorded in the sampling period (n-1) which is also returned to the idle state. Resetting the flip-flop B 2 group is not synchronized. can also be produced by the synchronization signal

The video signal V (n) does not contain a pulse which will call that the other input of the OR is in phase with the frame pulse Flq (n-1). The 40 circuit! 72 is supplied from the terminal 3. TorschaltangPl does not open, whereby all off In Fig. 5 the case is shown that two

input pulses of the comparison arrangement PO under-response pulse groups overlap each other. It pushes to be. The output signal Sn then has the following options: the value 0. The arrangement thus eliminates all Fi g. 5 a shows the case that initially a coincidence

Pulses of the unsynchronized response pulse between two frame pulses Fl q (n-1) groups, even if a random coincidence should occur between and F2q (n-1) of the sampling period (n-1) and a 'see two pulses the sampling period η is determined (what in

F i g. 4 shows the case that, according to the rear frame, the previously explained "manner indicates the presence of a menu pulse F 2 (and possibly the associated regular response pulse group in the scanning period identification pulse SPI, if any) and the opening of the gate circuit P1 distance , which has a duration of 20.3 μ & a response-), and that then at a distance of pulse group is different, both in the video less than 20.3 μβ a further coincidence between signal Vq (n-l) as well as in the video signal V (n) two frame pulses F'lq (n-T) and F'2q (n- 1) an interference pulse appears which does not belong to any response group as well as to a pulse of the sampling periods determined due to the coincidence of these 55. This is a criterion for the fact that in the subsequent interference pulses a corresponding pulse appears in the period η two mutually overlapping Ant output signals V '(n) of the comparison arrangement PO. Word pulse groups are present In order that the transmission of this pulse is desired, the gate circuit P1 is only prevented after Be-Terminal 4, the gate circuit P1 must block the end of the second response pulse group after the transmission of the last valid response. Whether the second response pulse group will be blocked. This is done by borrowing it depends on whether the signal has the arrangement described below. V (n) the dashed rear frame

The OR circuit Ul of Fig. 1, which the pulse appears.

Blocking the TorschaltangPl triggers, on the one hand, Fig. 5b shows the case that in the off

connected to a terminal 3, to which the sync sampling period η no pulse is applied in coincidence with the chronizing signal in each sampling period, pulse F'lq (n- 1) occurs. This means that although and on the other hand with a time control arrangement, which in the sampling period (n-1), but not in the Abdas blocking of the gate shift Pl a little after the sampling period η two overlapping response pulse

Claims (9)

groups are present, or that the pulses F'lq (n-1) and F'2q (n-1) are interference pulses. In this case it is desirable to block the gate circuit after the end of the first response pulse group. To achieve these effects, the arrangement of Fig. 1 contains a second gate circuit P2, the signal input of which is connected to the output of the coincidence circuit Π and whose control input is connected to the output of the bistable multivibrator circuit B 2, while its output is connected to the clear input of the shift register R 2. The gate circuit P 2 is thus opened at the first coincidence determined by the coincidence circuit T1. It then lets through the signal emitted by the coincidence circuit T1 when the next coincidence is detected if this signal appears before the reset of the flip-flop B2, i.e. less than 20.3 \ is after the first coincidence signal (case of FIG. 5 a). The signal emitted by the gate circuit P 2 then resets all stages of the shift register R 2 to zero, with the exception of the first stage, which brings it to state 1, which means that a pulse is recorded in phase with the pulse F'lq (n-1) in the shift register R 2. The shift register R 2 then triggers the blocking of the gate circuit Pl only after a time of a little more than 20.3 μβ after the occurrence of the pulse F'l q (n-l), ie. after the end of the second determined response pulse group. In the case of Fig. 5b, however, no coincidence is found during the opening time of the gate circuit P 2, so that the signal recorded in the shift register R2 and corresponding to the pulse Flq (n-1) is not deleted and the gate circuit P1 is blocked at the end of the first response pulse group. The circuit of FIG. 1 also contains a second coincidence circuit T 2 with three inputs. The first input is connected to the output of a negation circuit C, the input of which is connected to terminal 2, so that it emits the signal F (b) which is complementary to the video signal V (ri). The second input is connected to the output of a delay arrangement £ 3, the input of which is also connected to terminal 2 and which gives signal V (n) a delay of 20.3 μ5. Finally, the third input is connected to the output of the shift register R 2. The coincidence circuit Γ2 · determines three times the coincidence between the "pulse F2q (n-1), a pulse of the signal V (n) delayed by 20.3 μβ and a pulse of the signal V (ri). The coincidence between the pulse F2q (n-1) and the signal V (n) delayed by 20.3 μβ is equivalent to a coincidence between the frame pulse JFI ^ («- 1); ufid to a-: pulse I (ή). However, this coincidence occurs at the point in time in which the rear frame pulse F2n should appear in the signal V (n). However, if the signal V (n) has no frame pulse 2n, there is a pulse in the complement signal V (ri) at the same time a signal at terminal 5 when the pulse F 2 η is missing and a coincidence between the frame pulse Fl q (n-1) and a pulse I (n) has been established. The output signal at terminal 5 is used to to release the processing circuits connected to terminal 4 immediately, "if due to the lack of the rear Ra hmenimpulses F2 (n) indicates that the received response pulse group is garbled. This avoids an unnecessarily long occupancy of these evaluation circuits and indicates that the received response pulse group has been mutilated. The condition for the delivery of a signal to the terminal 5 exists, for example, if in the case of FIG. 5 a the dashed pulse in the signal V (n) is missing. Another possibility is shown in FIG. 6 shown. In this case there is a complete response pulse group in each of the signals Vq (n-1) and V (ri); however, these response pulse groups are offset from one another in such a way that a comparison is not possible. However, the response pulse group in signal V (n) leads so that one of its pulses I (n) coincides with the leading frame pulse FIq (n-1). This initially simulates the start of a complete response pulse group, and some coincident pulses can appear in the signal Sn. However, since there is no longer a pulse in the signal V (n) in coincidence with the rear frame pulse F2q (n-1), at this point in time an output pulse appears at the coincidence circuit T 2, which indicates the invalidity of the previously received response pulse group and enables the processing circuits . The reverse case, in which the response pulse group V (n) lags behind the response pulse group V (n-1), is shown in FIG. 7 shown. As can be seen immediately, in this case there is no coincidence between the front frame pulse FIq (n-1) and a pulse I (n), so that the coincidence circuit Tl already prevents the mutilated response pulse group from being evaluated. ■ Patent claims: 1. Secondary radar system with an interrogation station sends out characterizing interrogation signals in periodic sequences and the answering stations upon receipt each interrogation signal one of the relevant Interrogation sends out corresponding response pulse group, the at least two in a predetermined Contains spacing of successive frame pulses, between which depending on the to transmitted information code pulses can be present at predetermined pulse locations, with one in the receiving station for the response pulse groups installed device for the suppression of interfering signals and response pulse groups, which are not synchronized with the transmitted query signals, with a memory for temporary storage of the received and demodulated response pulse groups and with a comparison arrangement, which each received video frequency Reply pulse group at the same time as that received in the previous interrogation period and stored response pulse group corresponding to the same interrogation manner Delay by one polling period is supplied, and the only those pulses received Reply pulse group lets through, at the corresponding pulse positions of the in the preceding Query period received response ': - - "- ■ :: ■ ·) 809 519/224 word pulse group are also present, characterized by a coincidence circuit (Γ1) which receives the response pulse groups fed to the comparison arrangement (PO) at two inputs and the response pulse group received in the previous interrogation period at a third input with a lead corresponding to the spacing of the frame pulses and, in the event of coincidence three input signals emits a control signal that triggers the transmission of the output signals of the comparison arrangement (FO) to the evaluation circuits, and by a time control circuit (B 2, R2, L2), which is restarted by each control signal and after one of the duration of one Response pulse group corresponding time since the delivery of the last control signal blocks the transmission of the output signals of the comparison arrangement (PO) to the evaluation circuits. 2. Secondary Radärranlage according to claim 1, characterized by a second coincidence circuit (T 2) which at one input the negated response pulse group received, at the second input the delayed by the gap.der two frame pulses received response pulse group and at the third input a by the distance signal appearing two frame pulses after the output of the last control signal _; receives and when the three input signals coincide, it emits an output signal which enables the evaluation circuits. 3. Secondary Radäranlage according to claim 1 or 2, characterized by an arrangement (P 2), which resets the timing circuit to its initial state when after a Control signal another control signal follows at a time interval that is smaller than the interval is the frame pulse. 4. Secondary radar system according to claim 3, characterized in that the output of the comparison arrangement (PO) is connected to the signal input of a gate circuit (Pl), the control input of which is connected to the output of a first bistable multivibrator (B 1), the setting input of which is connected the output of the first coincidence circuit (Tl) and its reset input are connected to the output of the time control circuit (B2, R2, P2, L2). 5. secondary radar system according to claim 4, characterized in that the timing arrangement a second flip-flop (52), a second gate circuit (P 2) and a "shift S. Register (R2) contains that the setting input of the second bistable multivibrator (B 2) and the signal input of the second gate circuit (P 2) are connected to the output of the first coincidence circuit (Γ1), that the output of the second bistable multivibrator (B 2) is connected to the control input of the second gate circuit (P 2) and via a differentiating element to the input of the first stage of the shift register (R 2) so that the output of the second gate circuit (P 2} is connected to the clear input of the shift register (R2) , and that the reset inputs of the two bistable multivibrators (B 1, B 2) are connected to the output of the shift register (R2). 6. Secondary radar system according to claim 5, characterized in that an OR circuit (171, U 2) is inserted between the output of the shift register (R 2) and the reset inputs of the bistable flip-flops (B 1, B 2) and that the synchronization signal of the radar system is fed to a second input of each OR circuit (Ul, U2). 7. secondary radar system according to claim 5 or 6, characterized in that a small-sized delay arrangement (L 2) is inserted between the output of the shift register (R2) and the reset input of the first bistable multivibrator (51). 8. secondary radar system according to claim 5 ,. Referring back to Claim 2 via Claims 4 and 3, characterized in that the output of the shift register (R2) is connected to the third input of the second coincidence circuit (T 2}. 9. Secondary radar system according to one of the preceding claims, characterized in that the response pulse group received in the preceding interrogation period is entered with a lead corresponding to the distance between two frame pulses in a further shift register (Rl) with the capacity of a response pulse group, and that an input of the comparison arrangement (PO) and an input of the first coincidence circuit (Tl) are connected to the last stage of the further shift register (J? 1) and the third input of the first coincidence circuit (Tl) to the first stage of the further shift register (Rl) . Considered publications:
Electronic Engineering, 33 (1961), 401
up to 420. For this purpose 2 sheets of drawings 809 519/224 3.68 © Bundesdruckerei Berlin