US2428011A - Receiver for time or duration modulated electrical pulses - Google Patents

Description

, Spt, 30, 1947. P. K. CHATTERJEA ETAL 2,423,011

RECEIVER FOR TIME 0R DURATION MODULATED ELECTRICAL -PULSES Filed A ril 21, 1943 2 Sheets-Sheet 1 Inventory D.

By Attorney p 30,194? P. K. CHATTERJEA ETAL 2,428,011

' RECEIVER FOR TIME OR DURATION MODULATED ELECTRICAL PULSES Filed ,April 21, 1945 2 Sheets-Sheet 2 Patented Sept. 30, 1947 OFFICE RECEIVER FOR THVIE OR DURATION MODULATED ELECTRICAL PULSES Prafulla Kumar Chatterjea and Charles Thomas Scully, London, England, assignors to Standard Telephones and Cables Limited, London, England, a British company Application April 21, 1943, Serial No. 483,866 In Great Britain April 21, 1942 9 Claims. 1

The present invention relates primarily to time modulated electrical puls transmission systems and is principally concerned with the correction of distortion due to fading, interference and like causes. It is however also applicable more generally to transmission systems which involve amplitude limiting, such as, for example, frequency modulation systems, and the like.

The pulses employed in pulse transmission systems should ideally have vertical sides and square tops, but although substantially perfect pulses may be generated at the outset, they are liable to be distorted while passing through the various processes of transmission and reception, and this is particularly likely when the band width occupied by the channel has to be restricted. The distortion introduced in this manner is also made worse by fading, and by interference which may be picked up anywhere in the system. The present invention is concerned principally with means for reducing the effects of the distortion which has been caused in th ways just described.

According to the invention there is provided a receiving arrangement for an electrical pulse transmission system comprising means for limiting the amplitude of the received pulses between two amplitude levels differing by a predetermined amplitude, and means for varying the said levels in accordance with the power of the incoming si nals. 7

According to another aspect, the invention consists in a receiving arrangement for an electrical pulse transmission system subject to interference, comprising limiting means for selecting a portion of the amplitude of the received pulses, and means for fixing the mean level at which the portion is selected in accordance with thelevel of the interference. Y

The invention will be more clearly understood from the following detailed description and by reference to the accompanying drawings in which Figs. 1 to 4 are explanatory diagrams of time modulated pulses; and i t Fig. 5 is a schematic circuit diagram of an embodiment of the invention.

In order to make the principles of the invention clear, an explanation of the effects of distorted pulses will first be given, together with a general description of the manner in which these efiects may be minimised according to the invention.

Referring to Fig. 1, there is shown a series of ideal time modulated pulses, which it will be seen have substantially vertical sidesandsquare tops. When these pulses are passed through a system fecting the horizontal scale. actual cutting level in the receiver is fixed, the

[2 of limited frequency band-Width, the pulses may be distorted as indicated in Fig. 2, the sides becoming inclined and the tops and bottoms round-- ed or wavy. The effect on the sides of the pulses is substantially to increase the width at the base of each pulse and to decrease the width of the top, or in other words the sides of the ideal pulse appear to pivot about the middle points, thus producing the sloping pulses. A line a, b drawn half way up will be divided into sections having practically the same lengths as the original time modulated pulses of Fig. 1. The distorted pulses shown in Fig. 2 are obtained at the receiving end, and it has been found that substantially undistorted time modulated pulses may be obtained by cutting from the pulses a narrow slice mn, pq centred along the line a, b V

by means of a double limiting device; and by subsequent amplification, the slopes of the sides of the pulses so obtained may be so steepened that they become substantially vertical.

When demodulating the pulses according to the prevailing practice, it is usual to cut out a narrow slice at a fixed level which is independent of the received amplitude, and possibly also means may be rovided whereby the cutting level can be changed from time to tim to adapt the receiving arrangements to various conditions. This however does not take into account two sources of distortion, namely fading and noise,

- the effects of which will now be described.

In Fig. 3, A1 shows one of the Fig. 2 pulses as received, taken to represent any time modulated pulse, the line a, b representing the centre line of the slice which the receiver is adjusted to cut out, and which, as already explained, it has been found should be half way up. p, This centre line will for convenience be called the cutting level. If the signals are subject to fading, this pulse may be'received sometimes with greater amplitude (as at B1) or with smaller amplitude (as at C1). The effect of the fading will be chiefly to change the vertical scale of the pulses as drawn, without af- If therefore, the

relative level will be too low for the B1 pulse and too high for the Cl pulse, so that the corresponding derived square time modulated pulses will be respectively too long and too short. These derived pulses are shown at A2, B2 and C2 in Fig. 3 for the three received pulses, respectively.

If, however, the cutting level be made variable so that it is always-substantially half way up the pulse irrespective of the amplitude, all the derived pulses will have the same length, so that the distortion produced by the fading will have been the cutting level is made variable under the con-- trol of the received power so that when the signal amplitude varies, the cutting level is automatically varied accordingly so that undistorted derived pulses are obtained. irrespective of the fading. The details of this. arrangement will be explained later on.

When excessive interference is being receivedwith the pulses (assuming there is no fading),

considerable distortion may be produced This will be obtained as shownat,- B. Ifthe cutting.

level be raised to c, d (Fig. 4, A). the peaks due to the interference will be avoided, and pulsesas shown at D will be obtained. Since, however, the cutting level has been raised, thesepulses will be to some extent distorted since their lengths are now incorrect. An improvement will have been obtainedbecause generally the distortion produced by the interferencewill be considerably worse than that-produced-by changing the level.

It will be evident that the addition of interference'will be equivalent-to increasing the receivedpower, so that if-the receiver has been designed according to-the invention to deal with fading in the, manner eXplainetLit will also raise the cutting level WhBII'GXCGSSlVBfHOiSB is present. It willalso be clear that the raising of the cutting levelcan be made just sufficient to avoid the interference, so that thedistortion which is unavoidably associated with the raising of the level w l be always aum n m.

, I t -w ill be evident also, thatthe arrangement ofthe invention will operate as described to obtain the best possible result when both interferenceand; fading are simultaneously; present, since the correction for bothof'these' operates through themedium of; thereceived power.

The details-of an" embodiment of the invention are shownin the-Iona of a schematic circuit in Fig. 5. Radio frequency wavesmodulated with the time modulated pulses are appliedto. the primary winding; L3 of a transformer ,T. These waves are applied from the secondary winding L1 to a diode valve Vz where-they;arerectified, the

time modulatedpulses bein fiPtained-as volt I age variations across'the loadresistance Re. The

radio frequency; components; are removed by means of the smoothing circuitconsisting of the condensers C 2 and C3 and the resistance R4.

The pulses in the load Rs are applied through a condenserCi to thegrid of the first limiter valve V3.

beyond the cut-off by connecting thecathode to the junction of resistancesR-z and R11 connected across this supply l." C7 is the usual by-pass condenser for the cathode, and-R is-the grid resistance.

The 'plate of this, valve is connected through aresistance R9 to the positiveterminal of-the high tension supply; andqthis valve isploiassed;

the second limiter valve V4 through the condenser C9, the plate of V4 being connected to the high tension supply through the resistance R12. This-valve is also biassed by connecting the cathode to the'iresistances Rs ahdiRm connected across the supply, Cs being the by-pass condenser. The output is taken from the plate of V i through a condenser Cs.

A variable bias for the valve V3 is developed across the resistance Re connected in series with the control grid. This bias is obtained from the incoming, signals which are applied to the rectifier Vii from the secondary winding L2 of the transformer T. R1 is the load resistance for V1, and C1, C5, R2 is a-smoothing circuit having a time constant greater than the period of one cycle ofthe lowest signal frequency present. Accordingly a continuous negative bias is produced across Re which varies with the general level of the received signal, and is applied to the grid of V3 in addition to the-bias obtained from the cathode resistance R7.

The pulses (Fig. 2) which are obtained across the resistance R5 are applied in a positive sense to the control grid of V3, and assuming first of all that the incoming signal level is steady, the resistance R1 will beadjusted until the cut-ofi bias of the valve is overcome when the amplitude has reached the level p, q. The valve then becomes conducting and the plate voltage falls. This will cause the valve Vi (which is normally conducting) to be cut ofi at the level m, n, which is determined by the setting of resistance Re, which will be adjusted until a slice of the desired thickness is obtained, centered on either side of the line a, b.

If the incoming signals are subject to fading, so that their amplitude becomes reduced, for example, the variable bias voltage across Rs will be correspondingly reduced, so that the first cut p, q produced by the valve V3 takes place at the desired lower level. Similarly when the signal amplitude increases, the cut occurs at a higher level. The width of'the slice; being determined bythe resistance Rs, will be the same whatever the cutting .level.

' Since the effect of interference on the valve Viwill be substanti'ally'the same as an increase in signal level, it will be evident that the cutting level limitin'g the} effective-portions of the received pulses to those betweentwo amplitude levels difiering by a predetermined amplitude, and

meansfor automatically varying'the mean level of the selected portion of saidpulses in accordance with the power-of-the incoming signals.

2. A receiving arrangementflforan-electrical pulse transmission system subject to interference, comprising limiting means for selecting a portion of the received pulses between predetermined amplitude limits, and means for automatically fixing the mean level at which the portion is' selected in accordance with the level of the interference,

An-arrangementaccording to claiml' comprising arectifier'for deriving from the incomingsignals a continuous potential depending upon table circuits will'easily be devised'by' those the signal power, said limiting means including an amplitude limiting valve having a control grid, and means for applying the continuous potential to the control grid of said amplitude limiting valve. 4

4. A receiving arrangement for a constant amplitude electrical pulse transmission system comprising a first amplitude limiter valve adjusted to cut off below a predetermined pulse amplitude level, a second amplitude limiter valve coupled in tandem with said first valve and adjusted to cut 01? above a predetermined pulse amplitude level higher than the cut off level of said first valve, an input circuit, means for rectifying pulse modulated carrier waves received in said input circuit and for applying the rectified pulses to said first limiter valve, means for deriving from said input circuit a variable direct current voltage, means for applying said direct current voltage as a variable control grid bias to said first limiter valve, and means to maintain a normal bias for said limiter valve such that the valve will only operate above the level of noise and interference signals.

5. A receiving arrangement for a constant amplitude electrical pulse transmission system comprising a first amplitude limiter valve including a control grid and adjusted to cut off below a predetermined pulse amplitude level, a second amplitude limiter valve coupled in tandem with said first valve and adjusted to cut off above a predetermined pulse amplitude level higher than the cut oil level of said first valve, an input circuit, a first diode detector coupled to said input circuit and having a first load resistor, means for applying the voltage developed across said load resistor in a positive sense to the control grid of said first limiter valve, a second diode detector coupled to said input circuit and having a second load resistor, a time constant circuit, and means for applying the voltage developed across said second load resistor over said time constant circuit and in a negative sense to the control grid of said first limiter valve.

6. A receiving arrangement according to claim 5 in which said first and second limiter valves are each provided with an adjustable cathode lead biassing resistance.

7. A receiving arrangement for an electrical pulse transmission system comprising means responsive to changes in voltage between predetermined amplitude levels on the leading and trailing edges, respectively, of received pulses, and means automatically to vary the position of said levels with the amplitude of received pulses.

8. A receiving arrangement according to claim 7, in which the position of said levels is maintained at substantially half the amplitude of the received pulses.

9. A receiving arrangement according to claim 7, in which the position of said levels is maintained above the level of the interference.

PRAFULLA KUMAR. CHATTERJEA. CHARLES THOMAS SCU'LLY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,175,270 Koch Oct. 10, 1939 2,265,883 Applegarth, Jr. Dec. 9, 1941 2,271,203 Okrent Jan. 27, 1942 2,216,582 Barton Oct. 1, 1940 2,308,639 Beatty et a1. Jan. 19, 1943 2,061,734 Kell Nov. 24, 1936 2,113,214 Luck Apr. 5, 1938 2,323,880 Carnahan July 6, 1943 2,324,275 Becker July 13, 1943 2,277,000 Bingley Mar. 17, 1942

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