US2429734A - Automatic selectivity control in radio receivers - Google Patents

Description

Oct. 1947. J VERBEEK 7 2,429,734

AUTOMATIC'SEDECTIVITY CONTROL IN RADIO RECEIVERS Original Filed March 11, 1936 INVENTOR fang; yerzwk 7% w :AT'TORNEY Patented Oct. 28, 1947 s PATENT OFFICE AUTOMATIC SELECTIVITY CONTROL IN RADIO RECEIVERS Henri P. J. Verbeek, Eindhoven, Netherlands, assignor, by mesne assignments, to Hartford National Bank and Trust Company, Hartford,

001111;, as trustee Original application March 11, 1936, Serial No.

68,172. Divided and-this application November 13, 1941, Serial No. 418,930.

April 13, 1935 In' Germany 2 Claims. (01. 250-20) This invention relates to a circuit arrangement for use in radio receiving sets and for the purpose of automatically controlling the width of a frequency band transmitted by a tuned high or intermediate frequency amplifier. This application is a division of my application Serial No. 68,172,

filed March 11, 1936.

For the purpose of obtaining an optimum reproduction by radio receiving sets of speech or music transmitted by a broadcasting stationit is desirable that on either side of the carrier wave a frequency band of about kilocycles should be evenly amplified by the tuned high and/or intermediate frequency amplifier of the set. Since, however, with the present day distribution of wave lengths of the broadcasting, stations the carrier wave frequencies are spaced about 9 kilocycles apart, the radio receiving sets are so arranged that they only transmit at the expense of the quality of reproduction a frequency band of about 4 kilocycles on either side of the carrier Wave frequency.

Now, all transmitters are not received with identical intensity so that when receiving from transmitters the intensity of which is substantially higher than that of the neighboring transmitter it is possible for the band width of the high and/or intermediate frequency amplifier to be increased without the risk of troublesome interference phenomena occurring. It has therefore previously been proposed to provide radio receiving sets with a band width adjustment which either is operated by hand or is controlled automatically in accordance with the intensity of the signal received. Such an automatic band width adjustment is obtained, for example, by the automatic volume control which controls the amplification of the set in accordance with the amplitude of the carrier wave received acting simultaneously on the coupling of the high and/or intermediate frequency band filter, for example by thecoupllng of the coils of a tuned band filter comprising two inductively coupled oscillatory circuits being increased with increasing signal intensity so that the band width let through increases. These well known automatic band width controls have however, the disadvantage that the band width let through only depends on the intensity of the signal received and is wholly independent of the intensity of the neighboring transmitter. This leads to interference disturbances when the neighboring transmitters are received with sufficient intensity. According to the invention, this disadvantage is obviated by automatic control of the band width in accordance with the carrier wave amplitude of the neighboring transmitter.

In order that the invention may be clearly understood and readily carried into effect two circuit arrangements in accordance therewith will now be described more fully with reference to the accompanying drawing. Figures 1 and 2 disclose two embodiments of the invention, and only so much of the receiving circuit as shown for a proper understanding thereof.

Referring to Figure 1, which illustrates the essential circuit arrangement, two amplifier valves I and 2 of an intermediate frequency amplifier comprising inductively coupled band filters 3 and 4 are illustrated. The band width may be adjusted by varying the coupling between the coils of the band filters in that they are pushed more or less towards each other. This control is effected by an electromagnet 5 and is shown schematically by the dotted line. The latter is controlled in the circuit arrangement of Figure 1 by the anode current of an amplifier valve 6, the grid voltage of which depends on the voltage drop across two series-connected resistances I and 8. The load resistances I and 8 are inserted respectively in the direct current circuits of two diode rectifiers 9 and Ill. The diode rectifiers 9 and ID are provided with the tuned input circuits I I and I2 respectively, having one terminal connected to the anode and the other terminal thereof connected to the cathode through the respective resistances 8 and I. The negative end of resistance 8 is connected to the control grid of amplifier 6, the negative end of resistance I is connected to the cathode of rectifier 9, and the cathode of rectifier I0 is connected to the cathode of amplifier 6. The latter connections are such that load resistances I and 8 are always in series between cathode and control grid of amplifier 6, the voltage developed across either resistance determining the bias impressed upon the amplifier grid 6. As shown the tuned circuits I I and I2 are coupled, for example, to the band filter 3 of the interme-' diate frequency amplifier. The circuit I I is tuned to a frequency exceeding by 9 kilocycles the intermediate frequency and the circuit I2 is tuned to a frequency 9 kilocycles below the intermediate frequency. The arrangement is adjusted in such manner that when a transmitter is not received, the coupling between the coils of the band filters 3 and 4 is a maximum and these band filters allow the passage of a frequency band of about 9 kilocycles on either side of the intermediate frequency. This condition is still unvaried on receipt of a transmitter having a materially higher carrier Wave amplitude than the neighboring transmitters since in this case in the intermediate frequency amplifier, when the neighboring transmitters diifer by 9 kilocycles from the transmitter received, the amplitude of the lateral band frequency which differs by about 9 kilocycles from the intermediate frequency proper is but small. In this case the alternating voltage supplied to the rectifiers 9 and I and the direct currents passing through these rectifiers are also small and thus a substantial variation of the current passing through the electro-mag-net is not brought about so that the adjustment of the band width is still unaltered.

If, however, a neighboring transmitter has a considerable intensity so that an intermediate frequency alternating voltage which differs from the intermediate frequency proper by about 9 kilocycles and ha a sufficiently high amplitude occurs, the grid voltage of the tube 6 becomes more negative due to the direct current passing through the rectifiers 9 and I0 so that the anode current of the tube 6 which passes through the electro-magnet decreases. The coupling between the coils of the band filters 3 and 4 is thus decreased and the width of the frequency band transmitted by the said band filters decreases unti1 a given position of equilibrium in which the interference noise of 9 kilocycles is no longer troublesome ensues.

Now, the various broadcasting stations are not spaced apart exactly 9 kilocycles. For this reason, it may be advantageous to tune the circuits II and [2 to frequencie differing from the intermediate frequency, for example, by from '7 to 8 kilocycles instead of by 9 kilocycles.

In the circuit arrangement shown in Figure 1 the arrangement for the automatic band width adjustment is controlled by the intermediate frequency amplifier. In the circuit arrangement shown in Figure 2 this control is effected from the low frequency part of a receiving set. In this circuit arrangement an intermediate frequency band filter I3, the band width of which is automatically adjusted by an electromagnet I4, is coupled to the second detector l5 of the set. The low frequency alternating voltage occurring across the resistance 16 controls the low frequency amplifier of the set via connecting conthe anode circuit of which includes the relay M. The filter 20 is so proportioned as to transmit only frequencies exceeding, for example, 6 kilocycles. The operation of thi circuit arrangement is identical with that of the circuit arrangement shown in Figure 1.

It is of course obvious that the circuit arrangement according to the invention is also applicable to high frequency amplifiers.

The invention is not limited to the described regulation of the width of the transmitted frequency band by control of the coupling factor of the band filter but also applicable to any other well known control of the band width.

What I claim is:

1. In a, modulated-carrier signal receiver, a band-pass selector for selecting a. desired signal comprising a carrier frequency and a band of modulation frequencies, means for deriving from said desired signal and an undesired signal on a carrier frequency near the desired signal carrier frequency a control signal comprising beat notes of said signals, and means responsive to the amplitude of said control signal for adjusting the Width of the band of frequencies passed by said selector inversely in accordance with said amplitude of said control signal.

'2. In a modulated-carrier signal receiver, a band-pass selector for selecting a desired signal comprising a carrier frequency and a band of modulation frequencies, means for deriving from said desired signal and an undesired signal on a carrier frequency near the desired signal carrier frequency a control signal comprising beat notes of said signals, and means responsive to the amplitude of said control signal for adjusting the width of the band of frequencies passed by said selector inversely in accordance with said amplitude of said control signal, said deriving means comprising a high pass filter adapted to pass frequencies over 6 kilocycles.

HENRI P. J. VERBEEK.

REFERENCES CITED The following references are of record in the file of this patent: 1

UNITED STATES PATENTS Number I Name Date 2,255,645 Bligh et a1 Sept. 9, 1941 2,017,523 Beers Oct. 15, 1935

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