US2297470A - Sensitivity control for radio receivers - Google Patents

Description

P 1942- e. MONTI GUARNIERI 1 2,297,470

SENSITIVITY CONTROL FOR RADIO RECEIVERS Filed April 27, 1940 J. amB

Patented Sept. 29, 1942 SENSITIVITY CONTROL FOR RADIO RECEIVERS Gaetano Monti Guarnieri, Milan, Italy; vested in the Alien Property Custodian Application April 27, 1940, Serial N0. 332,047 In Italy May 12, 1939 2 Claims.

This invention relates to means for the sensitivity control in radio receiver sets and comprises sensitivity control means adapted to keep the signal/noise ratio at a high value throughout the sensitivity control range in respect of the noise which develops in a radio receiver set as an effect of the operation of electron tubes thereof, as well known.

It is known that in radio receiver sets the electron tubes and the circuits in which they are operative, develop perturbations inherent to their operation, said perturbations causing a noise which affects the set operation.

Said noise depends on two actions, that is: (1) on the heat effect in the circuits which generates electromotive forces depending on an electron unbalance in said circuits which in turn depends on the temperature, said elfect being substantially constant so long as the impedances of the circuit elements are unaltered; (2) on the shot effect arising in electron tubes and due to uneven and intermittent emission of electrons by the tube cathodes, this effect varying with the square root of the plate circuit current in the concerned tube.

In radio receiver sets provided with sensitivity control when the set sensitivity is adjusted to a high value, the signal/noise ratio is also high and accordingly the noise in the output has an admissible value, but when the sensitivity is decreased said ratio decreases because while the signal is :2

frequency circuits are unchanged whilst the shot :2

effect decreases in a ratio with the square root of the signal only. Consequently when the receiver sensitivity is decreased the signal action in the set network decreases to a larger extent than the noise and the set operation is affected.

This invention is directed to remove the above outlined objection; in this invention the adjustment of the set sensitivity is effected, assuming its highest value as the starting point, by reducing first the sensitivity of the stage following the first one or ones and by reducing the sensitivity of the first stage or stages only when the signal is so strong as to require a reduction of the sensitivity of said first stage or stages; also, to avoid overloading of the respective tube or tubes.

The signal/noise ratio is thus held unchanged at its high value in the first stages of the set and the amplification of said noise is reduced in the subsequent stages; on the occurrence of very 55 strong signals the reduction of the input stages sensitivity which acts adversely in respect of the signal/noise ratio, is accompanied by such a reduction in the gain in the subsequent stages that the noise has practically no effect on the set output.

This invention also includes a device comprising a single sensitivity adjusting means operative on all the stages to be controlled, said means actuating jointly, the sensitivity controls belonging to the first stage or stages and to subsequent stages respectively to provide for the desired effect.

An embodiment of this invention is illustrated diagrammatically in the single figure of the annexed drawing in connection with a superheterodyne radio receiver of which only the essential parts are shown.

In said figure, I designates a radio frequency amplifier tube energised by the antenna-earth circuit 2; 3 denotes the mixer tube energised by the output of tube I by means of a transformer 4 and by a local oscillator circuit which is shown by a block 5, it being of any known preferred type. The tube 3 energises by means of the transformer 6 the two intermediate frequency tubes 1, 8 which are cascade-connected by means of a transformer 9, the tube 8 feeding the circuit iii of a second detector; the subsequent circuits of the set are not shown since theey may be of any preferred type and are not concerned with the present invention.

The control grids of the tubes l, 3, l, 8 are selfbiased by the voltage drop developed by the plate currents of the respective tubes across resistors Tl, 1'3, 11 and r8 connected across the tube cathodes and earth.

In the above outlined arrangement the set sensitivity is adjusted by controlling the grid bias in the several tubes, this control being effected by changing the operative value of the self bias resistors TI, r3, r1, 18.

To embody the present invention in a circuit of the above described class, the control of the resistor rl of the tube 1 is made separate with respect to the control of the resistors r3, 1''! and 18 of the other tubes 3, l, 8; accordingly when the sensitivity is adjusted the values of resistors r3, r1, 18 only may be changed and the value of the resistor TI on the contrary may be left unaltered to secure the above stated conditions.

Further the adjustment of the resistor TI on one hand and of the resistors 1-3, r1, r8 on the other one may be intercoupled so that the sensitivity of the input tube l is also reduced after the sensitivity of the tubes 3, I, 8 has been reduced through a suflicient extent.

For such a purpose in accordance with this invention the resistor H is earthed through a variable resistor II and the resistors r3, 11, 18 are interconnected in parallel and subsequently to earth through a single adjustable resistor I2; the sensitivity of the tubes 3, I, 8 may thus be changed to the desired extent by adjusting the resistor I2 and subsequently the sensitivity of the input tube I may be adjusted by adjusting the resistor II.

The adjusting means for the resistors II and I2 are with advantage intercoupled to provide for securing automatically the desired conditions in respect of all the concerned tubes by means of a single control device.

In the illustrated embodiment the resistor TI is connected by means of a lead I3 to a tap I4 adjustable along the earthed resistor II and the resistors r3, r1, T8 are connected .by means of the leads I5, I6 with a movable tap H which cooperates with the earthed resistor I2 and said tap I4 and I! are coupled with each other as shown diagrammatically at IS.

The resistor II includes on a portion of the path of the coupled movable taps I4, II in register with the earthed end portion of the resistor I2 a non-resistant or short circuited section as shown at H.

Assuming the coupled taps I4, I! to be located on the earthed ends of the respective resistors II, II and I2, said resistors are inoperative and the several tubes I, 3, I, 8 have their maximum sensitivity, the bias of the respective grids being a minimum.

On the coupled taps I4, I'I being shifted from the earthed ends of the respective resistors II, II and I2, in the first portion of their adjustment that is in the positions thereof along the inoperative section II' of the resistor II, an increasing portion of the resistor I2 is made operative; the sensitivity of the tubes 3, I, 8 is thus reduced because the resistances of the respective plate circuits and the bias of these tubes are increased but the total resistance of the cathode circuit of the tube I and the sensitivity thereof remain unaltered.

Consequently the high signal/noise ratio is held unchanged in respect of the tube I while the sensitivity of tubes 3, I, 8 is reduced and accordingly the gain they introduce in the output of tube I and the amplification of the noise are reduced; on the manipulation of the taps I4, IT in connection with an input signal which is so strong as to require that the sensitivity of the tube I be reduced also to avoid overload thereof, the tap I4 reaches the operative section of the resistor II and inserts an increasing portion thereof in the plate circuit of the tube I while the tap I! by its cooperation with the resistor I2 further reduces the sensitivity of the tubes 3, I, 8 and the gain they apply to the output of the tube I and consequently to the noise.

Under these circumstances the signal/noise ratio is increased also in the first tube I but in this portion of the adjustment range the large reduction of the amplification of the following tubes 3, I, 8 and respective stages deprives the noise of any substantial action on the set output.

Of course the arrangement above described in connection with the tube of a first stage may be provided in a number of subsequent input end stages should it be necessary to secure a large gain and a large signal/noise ratio in the set at the same time.

This invention has been described in its embodiment in a network in which the sensitivity control is effected by changing the grid self bias of the amplifier tubes but it may be embodied in any suitable manner in networks in which the sensitivity control is effected by difierent means.

What I claim as my invention and desire to secure by United States Letters Patent is:

1. In sensitivity control means for radio receiver sets, an input section including electron tube amplifier means and a subsequent section including electron tube amplifier means fed by said input section amplifier means, a cathode resistor for tube grid bias in said input section, a separate cathode resistor for tube grid bias in said subsequent section, means for adjusting said input section cathode resistor to a certain extent and means for adjusting said subsequent section cathode resistor to a larger extent with respect to said first named extent, said adjusting means for said input section cathode resistor being operative to increase the operative resistance value of the respective cathode resistor after said adjusting means for said subsequent section cathode resistor have been operated to a portion of the respective extent to increase the operative resistance value of the respective cathode resistor.

2. In sensitivity control means for radio receiver sets, an input section including electron tube amplifier means and a subsequent section including electron tube amplifier means fed by said input section amplifier means, a cathode resistor for tube grid bias in said input section, a separate cathode resistor for tube grid bias in said subsequent section, means for adjusting said input section cathode resistor to a certain extent and means for adjusting said subsequent section cathode resistor to a larger extent with respect to said first named extent.

GAETANO MONTI GUARNIERI.

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