US2171135A - Radio receiving circuit - Google Patents

Description

Aug. 29, 1939. ATKINSON 2,171,135

RADIO RECEIVING CIRCUIT Filed April 28, 1937 Alllll IIIVVI B FREQUENCY Y I 1 1 i A C INVENTOR A/OfL' A T/(I/VSON I BY ATTO R N EY Patented Aug. 29, 1939 UNITED sTA'rss PATENT OFFICE Electric & Musical England Industries Ltd., Hayes,

Application April 28, 1937, Serial No. 139,357

' In Great Britain May 5, 1936 2 Claims.

The present invention relates to wireless and like receivers, and is particularly concerned with short-wave receivers such as are employed, for example, for the reception of television signals.

Short-wave receivers suffer from the disadvantage that the frequency band which the receiver is tuned to receive variesor driftsin operation, the drift being" chiefiy due to various parts of the receiver Warming up on account of the heating effect of current flow, and resulting in the signal to be received being wholly or partly lost. The disadvantage manifests itself particularly in superheterodyne receivers, and originates chiefly in the local oscillator; in this case, the frequency band fed to the intermediate-frequency amplifier tends to drift away from the band to which the amplifier is tuned.

The disadvantage may be avoided by reducing the selectivity of the receiver, but this method has certain obvious undesirable features.

It has been found, however, that since the drift due to warming up usually predominates over that due to other causes, such as variations in the carrier frequency, the drift is usually always in one and the same direction. It is an object of the present invention to make use of this fact to provide a novel or improved receiver in which loss of a signal to be received, due to the frequency drift discussed can be avoided or reduced.

The present invention accordingly provides a wireless or like receiver adapted to pass a band of frequencies embracing the carrier frequency and side bands of a desired signal, and wide enough to permit the frequency drift likely to be encountered in practice, without substantial loss of signal, wherein the magnitude of a tuning indication derived from the receiver is a maximum at a frequency, in the neighbourhood of one limit of the pass band of the receiver, the arrangement being such that, in operation frequency drift tends to take place in a direction away from the said limit of the pass band and towards the other limit thereof.

The means for deriving the tuning indication may comprise a selective circuit passing a signal frequency band of substantially the same width as that passed by the receiver which exhibits a maximum of response at or near to one limit of its pass band, this limit being such that the frequency drift is towards the other limit. The selective circuit may be arranged in the signal transmission path. of the receiver, and the indication of maximum response may be obtained by listening to the reproduction of the received signals. In a receiver provided with automatic volume control, where an indication of maximum response could not be obtained by listening, a tuning indicator of any known form might be employed, and if desired the tuning indicator might be arranged in association with a selective 5 circuit outside the main transmission path of the receiver. In operation, the selective circuit of the receiver may be first tuned for maximum response, and the arrangement is conveniently made such that the maximum of response is of the order 10 of 4 to 6 decibels above the general response level, for example.

The present invention ensures that when the receiver is initially tuned in the normal manner that, after warming up and the consequent frequency drift have taken place, the carrier and side bands fed to the selective circuit occupy approximately the middle of the pass band of that circuit. The loss of signal level which results from the frequency drift may thus be made quite small, and circuits having nearly the maximum permissible selectivity may be employed.

A method of carrying the invention into practice will be fully understood from the accompanying description with reference to the accompany- 2:; ing drawing in which:

Fig. l is a block schematic diagram of a wireless receiver embodying the invention, and

Fig. 2 is a curve showing the pass band of a receiver.

Referring to Fig. 1, it will be seen. that the arrangement therein shown comprises an aerial I, signal currents developed in which are impressed on the tuned input of signal frequency amplifier 2, the output of which is passed to a frequency changing stage 3, in which signals of intermediate frequency are derived. These signals are applied over inductively coupled tuned circuits 4 and 5 to the input of intermediate frequency amplifier 6, from which the signals are fed over a single tuned circuit 1 to the input of a detector 8, in which the signals are brought into condition for operating a suitable receiving device such as a loud-speaking telephone in well known manner.

In the above arrangement the tuned circuits 4 and 5 are arranged in accordance with known practice to pass a Wide enough band of frequencies on either side of the intermediate carrier frequency to enable the signal side band frequencies to be transmitted to the amplifier 6. For example, in a normal broadcast receiver, the width of the pass band required in a receiver might be of the order of ten kilocycles per second. The tuned circuit 1 in the output of the amplifier 6 is arranged to resonate so as to produce a peak to the side of the pass band opposite to that towards which the intermediate carrier frequency tends to drift, due to the change in the condition of the frequency changer 3 as the receiver warms up, as clearly shown in Fig. 2.

In Fig. 2 frequency is represented horizontally and the gain of the system is represented vertically. The receiver may be represented to pass frequencies between the limits represented by the points A and B on the base of Fig. 2, which might for example, represent frequencies displaced by five kilocycles per second on either side of the frequency of one megacycle per second. The separation of the points A and B is mainly determined by the choice of the coupled tuned circuits 4 and 5 of Fig. 1. According to the invention, a peak in the gain or the response of the receiver is arranged to occur at a frequency corresponding to C in Fig. 2, for which frequency the receiver may have gain or response of for example, the order of from four to six decibels of the general response level in the pass band. This peak in the response is produced by the tuned circuit 1 shown in Fig. 1

With the above described arrangement, when the receiver is tuned in, the maximum signal strength will be obtained at the point C, and assuming that the receiver is tuned to this point, then as the receiver warms up, the frequencyof the intermediate signal carrier will move from the frequency represented by C, towards the higher end of the pass band, to the frequency indicated by D, in Fig. 2, which may, for example, be about five kilocycles per second higher than the frequency represented by C. This change brings the frequency of the intermediate frequency carrier into approximately the middle of the pass band of the receiver so as the receiver warms up, the shift of the intermediate carrier frequency which results, does not cause the signals reproduced by the receiver to fade or dey crease in volume to any noticeable extent.

In cases where the receiver has an automatic volume control, the control voltages might be obtained for example, from the detector stage 8, and there would be provided a tuning indicator to indicate the amount of volume control applied to the receiver. It will be seen that the amount of this control, and consequently the indication of the tuning indicator would be a maximum when the receiver is tuned to derive from the received signal an intermediate frequency repre sented by the point C of Fig. 2. Consequently, when the receiver is initially tuned before it has warmed up, it would be tuned in such a way as to allow for the frequency drift of the intermediate frequency carrier due to the warming up.

In cases where the automatic volume control voltage is derived from the output of intermediate frequency amplifier 6, it would be necessary 4 toinsert in the circuit of the tuning indicator device some means by which its response is given a peak corresponding to the peak at the point C in Fig. 2, so that the tuning indication given by the indicator will be a maximum at this point.

I claim:

1. In a superheterodyne receiver, a tunable high frequency circuit and a local oscillator which combine to produce a resultant intermediate frequency, the local oscillator having a tendency to drift in frequency during the warming up period thereby causing displacement of the intermediate frequency, a pair of coupled circuits capable of passing with uniform amplification a band of frequencies wide enough to accommodate the displacement, and a single sharply tuned circuit which combines with the pair of coupled band pass circuits to produce a characteristic response curve which is peaked at only one end, the tunable high frequency circuit being tuned to said peak frequency.

'2. In a superheterodyne receiver, a tunable high frequency circuit and a local oscillator which combine to produce a resultant intermediate frequency, a pair of coupled circuits fixedly tuned to said intermediate frequency and capable of passing with uniform amplification a band of frequencies which has the intermediate frequency as its mid-frequency, the local oscillator having a tendency to drift in frequency during the warming up period thereby causing displace- NOEL AT INSON.

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