US2375126A - Diversity radio receiver - Google Patents

Description

Mai' 1,- 1945- i R. c. M'ATHES I 2,375,126

' DIVERSITY RADIO RECEIVER Filed May 8, 19213 g 'Patented May l, 1945 DIVERSITY RADIO RECEIVER Robert C. Mathes, Maplewood, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 8, 1943, Serial No. 486,239

(Cl. Z50-20) 6 Claims.

This invention relates to radio receiving systems of the diversity type and particularly to oiversity systems for the reception of telegraph or similar signals of low frequency or slowly varying type, such as telautograph, facsimile or trie like.

vAn object of the invention is to overcome the effects of fading and particularly to avoid the eifects of large reductions in signal amplitude due to fading.

One general method that is used for overcoming the effects of fading in radio communication is in the use of diversity systems. 'ihe operation of such systems is based on the fact that radio waves of even slightly different frequencies, or radio waves from the same transmitter that arrive at a receiver by dierent paths are subjected to different fading conditions. The first type of system is known as a frequency diversity system. The second type may be a space diversity system, an angle diversity system or the like, depending upon what method is used for selecting the waves arriving over the different paths. In all systems the beneficial effects are achieved by combining the individual signals or by continuously selecting between them.`

A further object of the invention is to provide an improved method of combining the signals in a diversity receiving system. y

In a specific preferred embodiment of this invention the detected telegraph or similar signals from two diversity channels are caused to modulate carrier supplies from a single carrier source with one carrier supply shifted in phase by 120 degrees with respect to the other. The outputs of the two modulators are combined and detected. With this system the resultant signal will be equal to the two received signals as long as they are both of the same amplitude and will closely approximate the larger of the two received signals when they differ in amplitude.

The invention will be more fully understood by reference to the following detailed description in connection with the drawing in which:

Fig. 1 is a block schematic circuit diagram of one embodiment of the invention;

Figs. 2 and 2A are explanatory diagrams rep,- resenting time versus signal amplitude;

Fig. 3 is an explanatory Vector diagram; and

Fig. 4 is a block schematic circuit diagram of a second embodiment of the invention.

Fig. 1 shows, for the purposes of illustrating the invention, a radio receiving system for receiving a signal that has been transmitted as modulations of two radio carriers of slightly different frequencies. One such modulated signal is received by the antenna Il and amplified and detected in the receiver l2. Similarly, the second signal is received, amplied and detected by antenna 2| and receiver 22.

The two detected signals are combined in the respective modulators |3 and 23 with local carriers from a common oscillator 30. The carrier oscillations supplied to one of the modulators (r3) 1s shifted in phase by 120 degrees by being transmitted through the phase shifter 3l. The outputs of the two modulators are combined and supplied to a detector 32 preferably of the rectier type from which the required signal is obtained.

The outputs of the two modulators I3 and 23 which are preferably of the balanced type are combined through the hybrid coil 33 to avoid any possibility of interaction.

The operation of the system of Fig. l can probably be best understood with the aid of the diagrams of Figs.A 2 and 3. It should also be borne in mind that the effectiveness of a diversity system is based on the fact that, in general, the fading will only adversely alect the signal shape of only one of the diversity channels at any particular time. f

When nol fading is present the signal received over the two channels will have exactly the same shape as indicated by the full line curves i0 and 20 of Figs. 2 and 2A. Let us now suppose that fading conditions are such that the signal received in the receiver |I-|2 is not disturbed while the same signal as received over channel 2|-22 is distorted as indicated by the dashed curve 20. f

The effect of combining these two signals (I0 and 20') by means of the circuit of Fig. l may be determined by reference to the vector diagram of Fig. 3. In this diagram the vector OA represents the signal output from modulator I3 and the vector OB represents the signal output from the modulator 23 when the received signals are of the same amplitude, the phase difference being due to the action of the phase shifter 3| on the supplied carrier. In such a case the amplitude of the resultant (OC) of the combination will be equal to each of the originals (OA and OC). If the vector OB is reduced to zero, the amplitude of the resultant signal is unchanged, being equal to OA. For intermediate values of OB the end of the resultant vector OC will lie along the chord AC. The maximum deviation of amplitude of the resultant will be when OB is one half `OA and will amount to about 13 per cent, which would be negligible for telegraph signals and could be tolerated in most otherl types of code systems.

Bearing in mind these facts we see that the signal resulting from the combination of signal represented by the curve |0 and the signal with the large fade represented by the curve 20' will look like the signal of curve I0 with the two small fades represented by the dotted curves 4| in'relation to the curvel.'

While in the above explanation it has been assumed that fading conditions have affected the signal received at the antenna 2l and not that received at the antenna l l, it will be observed that the same effects are produced under conditions in whichv the signal received at the antennal I fades and not that received at antenna 2 l.

While the invention has been described with reference to Fig. 1 as applied to a frequency diversity system, it is equally applicable to space, angle or similar diversity systems where the two received signals are modulations of the same carrier frequency. Fig. 4 shows a modification of the system of Fig. 1 in which there is embodied a further refinement for eliminating the residual reactiony of fading, inherent in the former. t

The system of this figure is general similar to. that of Fig. l and similar circuit elements have been ygiven the same reference numerals. It diiers from that of Fig. l in the'use of a variable phase shifter 42 in place of the xed phase shifter 3l andthe addition of a ratio control device Sii connected through a shaft 54 to the variable phase shifter 42.

The ratio control device 50 comprises two magnetic coils 5| vand 52 mounted at an anglenwith respect to each other and a magnetic armature 53 fixed tothe shaft 54. rlhe signal input to the modulator I3 is also fed to thecoil 5l and the signal input'to `modl'llator 23 is ied to coil 52. If there is no signal received in antenna `2| (amplitude of B signal is zero) the armature 53 will take the position indicated by the dashed 'line 55. Similarly, if no signal is received in the antenna Il th armature 53 will assume the position indicated by the dotted line 56.. When the two signals are of equal strength the armature 53 will assume the intermediate position in which it is shown. For intermediate values of relative signal strengths lthe armature 53 ywill take corresponding positions between the limits 55 and The variable phase shifteris arranged so that when the armature 53 is in this intermediate p0- sition it produces a phase shift of 120 degrees.

`As the armature 53 moves to other positions it operates through the shaft 54 to vary the phase shift so as to maintain the amplitude of the .resultant combined signal. equal to the amplitnde of the larger signal.

lIhis action may be understood by reference to the vector diagram of Fig. 3 aswill no w be explained.

As was pointed out abovein connection with the system of Fig. l, when the two signals (OA and OB) are equal and are combined with a phase difference of 120` degrees the resultant OC=OA=OB. Now, assuming that the signal received in the antenna 2l is` less than the signal OA (received in antenna H.) and equal to 0B' the problem is to determine the angle AOB at which the signalsV must be combined to make the resultant OC=OA. t Let R= length of vector OA, and XR=length oi vector OB'.

The chord AC=OB=XR.

The angle ACD is 90 degrees.

Accordingly DC' is perpendicular to OB', OB'be- `ing parallel to AC'.

Since OCB' is an isoceles triangle,

Then,

0 Xreug Sm elfe And angle B'OA=90 degreeS-i-arc sine X/Z.

'When X=1, B'OA=120 degrees, and

When X=0, BOA=90 degrees.

. reduced to zero. For intermediate values of relative signal strengths the phase shift should be degrees plus the arc whose sine is X/2, where X is the fraction that one signal is o the other.

What is claimed is:

1 ,In a radio receiving system, two,..d.iversit branches, a source of local carrier oscillations, a modulator for modulating oscillations from said source with signals from `each of said branches, a phase shifter included in the circuit from said oscillator to one of said modulators so vthat the oscillations supplied thereto are degrees out of phase with the oscillations supplied tothe other of said modulators at least when the signals from said branches are substantiallynof the same amplitude, and means for combining the outputs of said modulators.

2. in a radio receiving system, two diversity branches, a source of local carrier oscillations, a modulator for modulating oscillations from said source with signals from each of said branches, a phase shifter included in. the circuit from said oscillator to one of said modulators so thatv the oscillations supplied thereto are of the order of 90 to 120 degrees out of phase with the oscillations supplied to the other of said modulators, and means for combining the outputs of said modulators.

3. In a radio` receiving system, two diversity branches, a source of local carrier oscillations, a modulator for modulating oscillations from said source with signals from each of said branches, means for producing a xed phase shiftof 120 degrees between the oscillations supplied to one of said modulators and the oscillations supplied to the other of said modulators, and means for combining the outputs of said modulators.

4; In a radioreceiving system, two diversity branches, a source of local carrier oscillations, a modulator for modulating oscillations from said source bysignals from each of said branches, a vvariable phase shifter included in the path. from said. source to one of said modulators, means for combining. the outputs of said modulators, and means forcontrolling the phase shift produced by said phase shifter in. accordance with the relative amplitudes of the signals in said branches.

5. A system in accordance with claim 4 in which the last-mentioned means produces a phase shift of 120r degrees when said signals are of. equal amplitude. v

6. A radio receiving system in accordance with claim 4 in wlfiich said last-mentioned means produces a phase shift that varies between 120 degrees when the signals in said branches are. of

-eq-ua amplitudey and 90 degrees when the ampli- ROBERT C. tabernas.

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