US2393971A - Radio receiving system - Google Patents

Description

Feb I 5; 1946. H. cs. BUSIGNIES RADIO RECEIVING SYSTEM Filed March 22-, 1941 s Sheets-Shet 1 FIG.2.

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FeB. 5,1946.

. INVENTOR.

Feb. 5 1946. H. G. BUSIGNIES I RADIO RECEIVING SYSTEM 3, Sheets-Sheet 3 Filed March 22,, 1941 pal/2W6 ivy/6":

I l I 13 i l l l I I /coz/Pulve 672762-25 KECF/VE/P Patented Feb. 5, 1946 Henri G. Busignies, Forest Hills, N. Y., assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application March 22, 1941, Serial No. 384,670 7 In France December. 20,1940

14 Claims.

The present invention relates to a radio receiving system and more particularly to methods and apparatus for improving the signal-to-noise ratio in a radio receiver.

The causes of background noise in the input circuits of receivers are well known and consist primarily of the Johnson efiect or the effect due to thermal agitation in the component parts of the circuit and effects due to irregularities of electron emission from the cathode of the tube in the input circuit and ionization within the tube. Numerous precautions have been taken to reduce to a minimum the background noise in a good receiver. For example, a tube having a high value of amplification is employed in the input circuit and in the. plate circuit of this tube, a high quality circuit is inserted which enables one to use all of the amplification of the tube. In the grid circuit of the tube another high quality circuit is inserted which raises the value of the signal to a high level with respect to the noise caused within the tube. When all of these precautions are taken it is found generally that the tuned circuit of the grid causes anincrease of the background noise apparently due to the Johnson effect in this circuit.

It is known that the. ratio of the signal-to noise diminishes with the decrease of amplificattion of the first stage of a receiver and it is common, practice in circuits employing radio goniometers (where this ratio has a particular importance) to use the maximum possible of amplification in the first stage tube. It has been found that when the amplification of the tube is de-. creased, the noise decreases less rapidly than the signal and the receiver functionsas awhole less well than when the amplification is greater.

It is also known that the output power of. the noise increases proportionally to the band width of the receiver. Therefore, in a good quality receiver all the selectivity possible is utilized for a given modulated-signal.

The following table gives an example of: the signal-to-noise ratio that can be obtained under good conditions for a given selectivity:

Under the same conditions for a band width of one kilocycle the power ratio of signal-to-noise increases to 20, being in effect 16 db.

The signal-to-noise ratio in a receiver is evidently dependent on the amplitude of the high frequency voltage applied to the input circuit of the receiver. If'the magnitude of the high frequency voltage is increased, the signal-to-noise ratio is increased. Under many conditions the input signal can be increased by increasing the effective height of the wave collector or-antenna system. However, on aircraft it is diflicult to substantially increase the efiective height of the antenna system since the space available for the system is extremely limited. In direction finding systems employing loop antennas and goniometers on aircraft, an indication of direction is obtained by moving the goniometer or, in some cases, the loop antenna until a minimum signal is received. The precision of the indication depends on the signal-to-noise ratio. For very weak signals, the ratio determines the amount the loop or goniometer must be balanced to find both sides of the angle blurred by the noise. Also, when thereceiver is installed at a conside'rabledistance from the antenna system, the indication becomes less precise, due to the loss in the line between the'antenna and the receiver. The loss which results from transmitting the signal along a lineis definitive.

It is an object of my invention to improve the signal-to-noise ratio in a radio receiver.

It isanother object of my invention to improve the ratio of'signal-to-noise in receiving systems wherein the antenna system is at some distance from the receiver.

in accordance with one embodiment of my invention I provide-a coupling stage having a large amount of negative feed-back adjacent the receiving antenna. The transmission line of the receiving system is connected between a resistance or-impedance in the cathode circuit of the coupling stage and the input of the receiver.

In accordance with another embodiment of my invention I provide in a receiving system a coupling stage having a large amount of negative High fre- Selectivity 1L2 3; Egg Type of signal Signal to noise'ratio at low frequency output grid Micmvoltr 2kc.6 dbdown... 0.7 to 0.9-... Modulated 30per cent, 10 db-being 6 milliwatts noise and milllw-atts. 4 km-20 db down" with 400 cycles/sec. signal.

% be obtained.

, wi h the line 2. the input circuit of the coupling stage and the iquency of the input signal.

triode, as shown, other types of tubes may be In accordance with another embodiment of my invention, a.circuit having a low, value. of Q,

where Q represents the ratio of the reactance of the circuit to the resistance of the circuit, as a high frequency transformer giving anincrease 1 of impedance, is inserted in either the coupling stage or the input of the receiver.

By suitably stage described above.

adjusting the value of Q or the transformation ratio in the manner described in connectionwith j the detailed description of the invention an im-' proved signal-to-noise ratio inthe receiver can My invention will be more clearly understood I 1 by referring to the following particular description made in connection with the accompanying drawings, wherein:

Fig. '1 illustrates a coupling stage, having a large amount of feedback, the ,input stage of a receiver and a transmission line interconnecting these two stages;

antenna system to the receiver because of the impedance transformation effected by coupling stage. The mere inclusion of an ordinary preamplifier adjacent the antenna system would not produce the results obtained by the coupling While it is true that the coupling stage does not compensate for the transmission circuit loss, the signal at the input of'the receiver is greater than what it would be with ordinary transmission methods and no coupling stage, and hence the signal-to-noise ratio in the receiver is improved. An ordinary pre-amplifier would increase the signal at the receiver, but at the same time it would introduce noise which would offset the advantage of increased signal. The coupling stage of the present invention has a large amount of negative feedback and hence does not introduce noise into the system due to noise within the coupling stage itself. Also the coupling stage, due to the feed back does not amplify in voltage the received 1 Fig. 2 illustrates a modification of the circuit of Fig. 1 including a circuit having a low value 1 of Q; t

1 :7 Fig. 3 is a diagram used in describing the 1 operation of the apparatus of the invention;

7 Figs. 4 and 5 show the application of my in- 1 vention to radio compass systems employing loop antennas.

InFig. 1 there is showna coupling stage comprising a triode L1 whose grid may be coupled to any radio frequencysource, such as a loop .an-

tenna. The resistance I in the cathode circuit of the tube L1 acts as a load for the tube L1 and 1 acoupling resistance for the transmission line 2. Preferably, the transmission line 2 is a low impedance line such as a coaxial line, and the impedance I'is adjusted to match the impedance of magnitude of'the impedance I being controlled decoupling condenser may be included in series Except for the impedance in anode supply, the grid of tube L1' is coupled directly to the plate of the tube.

and this large amount of feedback makes the I signal voltage produced'across the resistance l less than the signal voltage applied to the grid.

1 In other Words, the output signal of the stage is less than its input'signal but the output signal is across a much lower impedance.

There isno voltage amplification, but substantial power am- Instead'of the resistor. l, shown'in the cathode 3 circuit of the tube L1, a circuit comprising a re- 1 sistor and a series condenser and inductance in 1 parallel therewith, may be inserted in the cathode.

circuit to provide a selective effect at the fre- A1so,--instead of the employed, such as pentode tubes.

The circuit shown in Fig. 1 improves'the transmission of the high frequency signal from the the line or alternatively the impedance of the line is adjusted to match the impedance I, the

by the type of tube L1 employed. If desired, a"

This means I that there be large amount of negative feedback between the plate circuit and the grid circuit signal before transmission to the receiver, but instead it merelyfimproves the transmission ef ficiency. It should be understood 'that other methods of obtaining negative feedback may be employed, but I prefer. the method shown due to the ease with which large amount of correctly phased negative feedback may be obtained.

A modification and preferred embodiment of my invention is shown in Fig. 2 In this figure, the, coupling stage comprises'a tube L3, cathode resistance I, and dropping resistors 5 and 6, which supply a positive potential to the'grid i. As in Fig. 1, the tube L3 ofFig. 2 may also .be a triode. A transmission line '2 coupled to the resistance 1 by the coupling condenser 19 couples the coupling stage to the receiving stage, A coupling circuit having a low value of Q is designated generally at 8. This low Q circuit comprises a coil 9 and a condenser in parallel therewith designated l0. transmission line 2 to the coil 5 and theinput circuit of the receiver.' h I The introduction of the low Q circuit 8 in the transmission circuit between the coupling stage and the receiver produces an, unusual and important effect. The curves shown in the diagram of Fig. 3 will help to describe this'effect. With the oscillator 12, which produces a signal of about 500 kilocycles modulated 30% with a 400 cyclenote, as a signal source the characteristics shown in Fig. 3 were obtained. Curve l illustrates the voltage on the-grid of receiver tubeiLz V,

with the oscillator connected directly thereto. The voltage measured at this grid was found to be .70 microvolt for a signal-to-noise ratio of 10 db at the receiver output. Throu'ghout' the tests the signal-to-no'ise ratio at the output of the receiver was maintained constant at 10 db.

With no Q circuit. 8 in the input'circuit' of the tube L2 (with only a direct high frequency connection between the resistance l and thegrid of tube L2) the Voltages requiredon'thegrid of the tube L3 with varying values of cathode re-' circuit of Fig. 2 without'a 'lowQ circuitin the transmission circuit produced a loss in the signal:

7 'to noise ratio ofabout 4 to 5 db.

The curves of Fig. 3 show the magnitude of the A coupling coil ,ll .couples the;

lustrate the improvements given by the invention for a given value of signal-to-noise ratio.

When the signal voltage required at theqgr'id of tube L3 is less for a given signal-to-noise ratio at the output of the receiver, the transmission of the signal voltage between the antenna and the receiver is more efficient'. With a more efiicient transmission of the signal andv a sub-- stantially constant background noise the ratio of signal-to-noise in the receiving system increases for a given signal.

With the insertion ofa circuithaving a Q of about 4 and a cathode resistance of about 200 ohms for a coupling tube type 1852,'the transmission efiiciency is considerably increased over the value obtained when the Q circuit is omitted as shown by curve 3. At this. point the input signal necessary to produce an output having a signal-to-noise ratio of db is approximately 1.1 microvolts less than the'signal necessary when no Q circuit is provided in the-transmission circuit between the coupling tube and the receiver input. With an increase of the Q to a value between 5 and 6 the amount of input signal necessary is again reduced. However, if the Q is further increased his found by experiment that no further benefit is obtained, and instead, the background noise introduced bythe first tube of the receiver begins to decrease the ratio of the signal-to-noise. Therefore, to effect an improvement in the signal-to-noise ratio, I prefer to insert circuits in the transmission circuit having avalue of Q of approximately 1.0 to 10.

Although I have disclosed a. circuit having a low value of Q in the preferred embodiment of my invention, it is possible to use other voltage step-up networks or devices such as wide band transformers to produce the results of my invention. That it is possible to use such networks will be apparent from a brief outline of the principle of my invention. Suppose, for example, that an ordinary amplifier stage is inserted between the antenna system and thereceiver. If there is no transmission line between the amplifier stage and the receiver input, the effect is the same as though another stage had been added to the receiver, and thus there is no improvement in the signal-to-noise ratio at the receiver output. Instead, there is aslight decrease due to the introduction ofadditional noise by the additional stage. If there is a transmission line beween the amplifier and the receiver input, then impedance m'atchingnetworks must be employed at the output of the amplifier and the input of th receiver. These networks introduce losses in the system, and although the signal voltage at the input of the receiver may be increased by this arrangement, the additional amplifier reduces the signal-to-noise ratiov at the input and the networks themselves may introduce noise and further reduce the ratio. When a coupling stage or amplifier having a largeamount of negative feedback is employed according to the teaching of the present j invention, there is an impedance match between the stage and the line with a small loss. in signal voltage and no increase of noise. In the previously. described arrangement using an ordinary amplifier, an impedance. match between th amplifier and the line required a considerable reduction ofthe signal voltage'on theline, and in order to compensate for this reduction, a line terminating network having a. large voltage step-up was required. The noise introduced in a system by these networks increases with the transformation ratio required. In the system of the. present invention, since there is substantially no decrease in the signal voltage on the line by the use of the negative feedback amplifier for impedance matching, a network having a small step-up may be employed to terminate the line. There is thus produced on the grid of the tube of the input stage of the receiver a signal voltage equal to or greater in magnitude than the signal voltage produced thereon by the above system without the use of any noise adding circuits. The output of the coupling stage being of low impedance (200 ohms, .for example) the noise made by thermal agitation in the coupling stage is only the noise corresponding to the small noise volt. age produced across this low impedance. A similar result; may also be produced in systems that do not require transmission lines, the negative feedback amplifier being directly coupled to. the step-up network in the receiver input. 7 It is also possible to further improve the signal-tor noise ratio by connecting a number of coupling stages of the type shown in Fig. 2 in, cascade with associated step-up networks. In this arrangement,. however, the improvement gained by the addition of each stage gradually decreases. When several stages are connected in cascade, they may all be connected before the transmission line or some may be connectedbefore the line and some after the line. i

The amount of step-up employed inthe system of the present invention is primarily determined by the impedance introduced by the step-up net-I work. It has been found that the impedance of the step-up network alters the impedancein the feedback path and the operation of the feedback amplifier, and in order to avoid the introduction of noise by the amplifier, the feedback,

must be large and be produced by a substantially resistive network. It is thus desirable to keep the step-up low, for example between 1.0and l0 and preferably between 3 and 7, the value of 5 being the optimum value.

A coupling coil may be used in place of, or in conjunction with, the cathode resistance 1. When this coupling coil is used, it is possible to couple the low Q circuit to the coupling stage end of the transmission line rather than to the receiving stageend. In this case, the transmission line 2 may be connected directly to the grid of the tube L2, or by means of a coupling impedance. However, with this modification, care must be taken to keep the impedance of the cathode circuit of tube L: at a low value, and hence, the coupling between the coupling coil in the cathode circuit of the tube L3 and the low Q circuit must be loose. Also, the coupling must be loose so that the low Q circuit will have substantially no effect on the negative feedback of the coupling stage. In the preferred embodiment, in order to avoid the coupling difficulties, the low Q circuit is inserted in the input stage of therece'iver. Also, toavoid anyharmful effect on the negative feedback of the coupling stage, instead of employing a value of which produce a maximum signal voltage, I prefer to use a value of Q equal to 5. In all cases, the value of Q is for the frequency of reception.

If desired, it is also possible to provide two lines between the coupling stage and th'e receiver input and. to connect these lines so that the coil I l is in' series withtheresistance I; In this case; 1 the circuit from cathode to' ground would .be as follows: from the cathode .to one end of'coil'j l. byone line; return from the other; end .of::.coil

ll..by the other line toone-iend of resistance" I;

and through resistance I to. ground; :Also; a decoupling condenser may be connected in par-J 1 allel with resistance l. i

-Bysuitablyadjusting the'Q of the circuit a j and the coupling between the coupling coil. and

the, circuit, the loss due to the transmission line vention is desirable in all'types of receiving systcms' and it isparticularly desirable in systems A loss in the? trans-- employing antennas of low effectiveheight, such 1 as radiogoniometer and radio compass systems. 5' In- Fig.4, an application "of iny invention to' a radio compass system is shown.

, many radio-compass receiving installations,

the receiving loop'is locatedat'some pointrecoupling stage to the receiver by a low impedance 1 line. In Fig; 4a loop I3 tunable by a condenser I4 is coupled to the coupling stage. The loop l3 is located at a point remote from the receiver 1 and the coupling stage is mounted immediately j adjacent theloop. The remaining elements of Fig. 4' are the same as in Fig. 2. The loop I3 I 'mote from the receiver. In these installations, I propose tomount the'coupling stage immediately adjacent the receiving loop and toc'ouple the raise the impedance of the circuit preceding the. coupling stage in order to match the input impedance of the stage.

Itmay be. noted that whenigoniometers are employed in receiving. systems, a considerable transmission loss results for the coefficient of coupling of these. goniometers is not very high. As a result the chainof circuits comprising the goniometer, .does not have a highQ. Instead, these'circuits have a relatively low Q, and since it is an object of the present invention to include a circuit having a low Q in. the circuit after'the coupling stage, my invention is. especially appli-' cable to systems employing'goniometers, both because of compensation ior'transmission loss and because. of the co-operation between the coupling stage and the goniometer circuits.

By experiment, I have found that in the above system, a loss of 6 db in the goniometer can be replaced by at gain of 4 db when the circuit is arranged in accordance with the present invention. There is thus an increase of 10 db from the value of signals normally obtained. If these systems employ tuned loops, a further gain of from 6 to 8 db may also be'obtained, but of course,

this latter gain is not due to the circuit of the present invention. When the loops are tuned at a distance, much of the advantage obtained by tuning the loop is lost due to the transmission loss between the tuned loop and the input circuit of the receiver if a circuit of the present invention including a coupling stage is not employed.

' By experiment, Ihave found that in ground station receiving system installation. when the antenna system is 2'kilometers from the receiver the high frequency loss in the line is of the order of 6 db. -If,.as in the present invention, a circuit 7 having a Q of from 3 to -5 is inserted between the may, or may notbe tunable. When the loop 13 is tunable,as shown, the tuning may be per- 1 formed by known types of remote control mechanisms.

" Inradio-compass systems employing twoil'oops and a goniometer; the coupling stage may be connected in the receiving circuit immediately after the goniometer. In this case only a si'ngle'coupling stage is necessary: -Preferably,fhowever, two coupling stages are employed,'one in each line between a loop and its corresponding goniometer coil. 'Fig. 5 illustrates this latter arrangement, and in this figure, loops l5 and H which maybe tuned by condensers l6 and I8 respectively, are coupled to the goniometer by two coupling stages comprising tubes L1 and Li. Separate cathode resistances I and I are associated with the tubes L1 and L1. These coupling stages and the remainder of the circuit of Fig. 5 are the same asthe coupling stage and the transmission line and receiver circuit of Fig. 2. As explained in connection with the showing. of Fig. 4, the coupling stages are preferably mounted immediately adjacent the loop antenna. Also, al-

though I have shown the loop l6 and 1-1 as tunthe coupling stage maybe placed just ahead of the goniometer, but preferably the coupling stage is mounted adjacent the receiving antenna. In order'to obtain all of the benefits of the present invention, it is expedient in these systems to coupling stage and the receiverit is possible to compensate for this loss of 6 db, and it is possible in some cases to replace the loss of 6 db by a gain of some decibels. increase, of 6 db in the signalebove the" value normally obtained doublesthe range of the re: ceiving system without decreasing-the precision, it may readily be seen that the apparatus of the present invention is particularly desirable in direction finding systems. I Although I have described my invention as particularly applicable to direction finding systems, it will be understood that my invention may be used in other receiving-systems such as television receiving systems. Also, my invention is particu larly applicable to ultra-short wave systems. In these latter systems the input impedance of the first tube of the receiveris usually of low impedance. Also, the Q'of the'input circuit of these systems isgenerally'of a low'value. As pointed out .in connection with my invention, an improved signalto-noise ratio can'be obtained withthe circuitof my invention even though the input impedance of the receiver may be low.

While I have described particular embodiments of my'in'vention for purposes of illustration, 'it will be understood that various modifications and adaptations thereof may be made within the scope of the invention. 'What isclaimediis: '1 1. A radio receiving systemcomprising an antenna system, a complete receiver, a couplingv stage comprising an amplifier having a large amount of n'egativefeedback coupled to said antenna system, and a'circuit having a low value of ratio .of'rea'ctanceto resistance at the operating frequency of said receiving system' 0011'- When one considers that an' nected between said coupling stage and said receiver.

2. A radio receiving system comprising an antenna system, a complete receiver, means for tuning said receiver to a desired frequency, a coupling stage comprising an amplifier having a large amount of negative feedback, means for coupling said coupling stage to said antenna system, and means for coupling said stage to said receiver comprising a voltage step-up network having a low value of step-up at said desired frequency.

3. A radio receiving system according to claim 2 wherein said means for coupling has a value of step-up between 1.0 and 10.

4. A radio receiving system according to claim 2 wherein said means for coupling has a value of step-up between 3 and 7.

5. A radio receiving system according to claim 2 wherein said means for coupling has a value of step-up equal to 5. o

6. A radio receiving system comprising an antenna system, a receiver, a plurality of coupling stages coupled to each other in parallel and to said antenna system and said receiver, each of said stages comprising an amplifier having a large amount of negative feedback, coupling means for coupling each of said stages to each other comprising a voltage step-up network having a low value of step-up, and coupling means for coupling one of said stages to said receiver comprising a voltage step-up network having a low value of step-up.

7. A radio receiving system comprising an antenna system, a receiver, a coupling stage com-' prising an amplifier having a large amount of negative feedback, means fo coupling said stage to said antenna system, and a transmission circuit between said coupling stage and said receiver comprising a resistance operatively connected in said amplifier, a transmission line coupled to said resistance, a circuit having a low value of ratio of reactance to resistance operatively connected in said receiver, and means for coupling said transmission line to said circuit having a low value of ratio of reactance to resistance.

8. A radio receiving system comprising an antenna system, a coupling stage comprising a vacuum tube having a cathode, a gridoand an anode, a low impedance connected in series with said cathode, an anode circuit, a grid circuit and means for feeding energy from said anode circuit to said grid circuit in a negative sense, a receiver comprising 'a vacuum tube having a grid, a cathode and an anode and a grid circuit comprising ,a circuit having a 10W value of ratio of reactance to resistance, means for coupling the grid circuit of said coupling stage to said antenna system, a transmission line, means for coupling said transmission line to said low impedance, and means for coupling said transmission line to the grid circuit of said receiver.

9. A radio receiving system according to claim 6 wherein said coupling stage is mounted adjacent said antenna system.

10. A radio receiving system according to claim 6 wherein said antenna system comprises a loop antenna.

11. A radio receiving system according to claim 6 wherein said grid circuit of said receiver comprises a condenser and coil connected in parallel with each other, the ratio of reactance to resistance of said circuit being equal to 4.

-12. A radio receiving system comprising a first antenna system, a second antenna system, a goniometer, a receiver, transmission circuits for coupling said antenna systems to said goniometer and for coupling said goniometer to said receiver, a coupling stage in at least one of said transmission circuits comprising an amplifier having a large amount of negative feedback and a circuit having a low value of ratio of reactance to resistance in at least one of said transmission circuits.

" tems and Said goniometer and each of said stages is mounted adjacent its respective antenna system and said circuit having a low value of ratio of reactance to' resistance is'operatively connected in the input of said receiver.

14. The method of increasing the signal to noise ratio in a receiver using a coupling stage including an impedance element and having a large amount of negative feedback, and a transmission line which comprises feeding a received signal to said coupling stage, tapping off said signal from-said stage at a reduced amplitude from said impedance element and transmitting the reduced signal to said receiver over said transmission line, the impedance in said stage matching the impedance of said line.

HENRI G. BUSIGNIES.

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