US2393921A - Radio telegraph receiving arrangement - Google Patents

Description

Patented Jan. 29, 1946 nanro TELEGRAPH RECEIVING ARRANGEMENT Frederick Percival Mason, Croydon, England, as-

signor to Creed and Company Limited, Croydon, Surrey, England Application March 30, 1943, Serial No. 481,105 In Great Britain May 8, 1942 Claims. (cl. 178-88) In'any telegraph system it is desirable that the receiving apparatus should be provided with a substantially constant magnitude of signal, since it is common experience that apparatus of any kind may be adjusted to operate well at one valueof input but will give unsatisfactory performance with either a greater or lesser input.

Telegraph receiving apparatus of the electromagnetic class will give satisfactory performance over a range of input ofthe order of :1, while recorders of the chemical paper type have rather less tolerance.

It is, therefore, evident that in wireless telegraphy, where the received signal may vary to an extent of 40 db, i. e. a ratio of 10,000: 1, the signal will be of satisfactory strength for a small part of the transmitting period only.

This situation is generally recognised, and many solutions of the problem of maintaining constant the output ofa radio receiver have been propoud. The majority of these solutions, however, are suitable only for modulated carrier transmissions, and fail to operate correctly under keyed-"carrier conditions. I v

The object oi this invention is to provid an automatic volume control especially suitable for keyed-carrier wireless telegraph. Throughout this specification A. V. C. denotes automatic volume control.

As in known systems, the general process is that of converting part of the amplified signal ener y into grid bias for the amplifying valves. The novel feature of the method is the provision of positive, rapid adjustment of the control voltage during a marking period. and complete maintenance of the control voltage during a spacing interval.

Fig. 1 shows one embodiment of my invention using a mechanical relay.

Fig. 2 shows another embodiment of my invention using electronic relay devices.

The principle of operation may be understood by reference to Fig. 1. in which a radio receiver I has an output path 2 which devides to feed signals to a converter 3 and a utilisation device 4. Device l is shown, fo simplicity, as an electromagnet provided with an armature 5, which it is assumed is utilised to eifect recording of the message in any well known manner. The armature 5 also carries a contact 6, which is electrically connected to the output of converter 3 by conductor l. A stationary contact 8 is connected to the ungrounded plate of a condenser 9; the same plate is connected by conductor to to the A. V. C. busbar ll of the receiver I. By A. V. C. busbar is meant the line carrying the negative au. tomatic control bias to the portion or portions of the receiver upon which such bias operates, for example the grids of one or more tubes.

Upon the receipt of a signal, device 3 is energised and conductor I impresses a voltage on contact 6 proportional to the strength of the signal. Also, magnet l is energised, attracting its armature 5 to effect recording of the signal, at the same time closing contacts 6 and 8. Condenser 9 is immediately charged to the potential of conductor l, and the gain of receiver l is immediately adjusted to suit the received signal strength. Operation during a marking condition is thus identical with that of a fast automatic volume control of normal type.

Reversion to the spacing condition causes conductor 1 to resume zero potential, but at the same time magnet 4 is de-energised, armature I deop- At each subsequent marking signal, the-charge on condenser 9 is readiusted,'at the beginning of the signal. to a value corresponding to the strength of the signal.

Thus, a fast-acting A. V. C. is provided which does not lose control during spacing periods. It

will be seen that directly the signal has faded to below a strength sumcient to operate the electromagnet, no further adjustment of the control voltage is desirable, so that there is no attempt on the part of the receiver to increase its amplification to such an extent that noise operates the magnet 4. To this end, the receiver no-signal sensitivity would be adjusted so that noise does not operate magnet 4. It is also desirable that converter 3 is non-responsive to signals of noise magnitude, and for this purpose the converter may consist of a biased diode and load circuit of usual type. Alternatively, the lower plate of condenser 9, instead of being grounded, may be taken to an adjustable negative potential; this method would automatically ensure that the receiver nosignal sensitivity and the converter characteris-' tic were correct.

The device may be applied at a radio receiving station in the following ways:

1. It maybe incorporated in an amplifier designed to be connected to an existing radio receiver (such as, for example, the amplifiers used in the tape facsimile system or the Morse recorder).

2. It may be incorporated in the receiver itself.

3. It may be incorporated in an amplifier, as described in (1) with the addition of a control ing out the principle of the device is by way of v illustration only, and in cases where the contacts 6 and 8 of Fig. l are undesirable or non-existent, then an arrangement as shown in Fig. 2 may be adopted. The operation is then completely independent of the nature of the receiving device 4.

The operation of the circuit of Fig. 2 is as follows:

The output of the radio receiver I is conveyed over conductors 2 and 3' to the utilisation device 4', the nature of which is immaterial. A transformer 6' is provided with a primary winding 5', two secondary windings l and 8', and a further center-tapped'secondary winding 9'.

Winding l" is provided with a load circuit comprising rectifier l6. resistor l8 and condenser 20, such that a unidirectional voltage is developed across resistor l8 when an alternating voltage.

appears in winding I. t

The resistor 18 is connected in series with a battery 26, and the series circuit thus formed is connected between the grid and cathode of triode ll in such a way that battery 26 normally biases tube l 4 to cut oil, but the voltage developed across resistor l8 overcomes this bias and renders the tube conductive.

Winding 8' co-operates with rectifier ll, condenser 2l, resistor l9 and battery 21, to control triode [5 in a similar manner. Thus, when a tone output occurs on conductors 2 and 3', tubes I4 and I5 are rendered conductive.

Winding 9' is arranged to co-operate with rectifiers l and II, condenser l2 and resistor l3, to produce a unidirectional voltage proportional to the amplitude of the signal on conductors 2 and 3'. The time constant of this circuit, and those of the circuitscontrolling tubes M and ii, are of the order of one-tenth of a dot interval.

One end of resistor I3 is grounded, and the other, the negative end, is connected to the anode of tube I l and the cathode of tube IS. The cathode of tube l4 and the anode of tube ii are connected together and to the ungrounded electrade of condenser 22. This electrode is also connected to the automatic volume control busbar I l of the radio receiver l. The other electrode of condenser 22 is grounded via the adjustable potential divider 24, over which a P tential drop, negative with respect to ground, is produced by battery 25.

Since, during a marking interval, as has already been shown, a negative -potential appears at the ungrounded end of resistor-I3, and tubes l4 and I are rendered conductive, current flows from resistor l3 to condenser 22, or vice versa, according to whether the potential of the unoi the radio receiver I. it is evident that, during a marking period, a fast-acting automatic volume control is exerted on the outputsignals supplied to device 4'.

During a spacing interval, since tubes l4 and I! are rendered non-conductive, the charge on condenser 22 is not allowed to adjust itself to equalise the potential existing between itself and resistor l3. The potential of automatic volume control busbar l I is thus substantially maintained at the value determined during the previous marking interval, and the gain of the receiver is maintained constant. If it were not for this feature, the gain would readjust itself to a high value such that the background noise is amplified suillciently to constituting a marking signal.

v In practice, it is desirable to provide a-high re sistance leakage path for condenser 22, such as resistor 23, in order that, in the event 01a sudden drop in signal strength of considerable magni-- tude, the charge on condenser'22 can readjust itself in any desired period to increase-the gain of the receiver. In such a case, if the decrease in signal strength occurred "during a spacing interval, and was of such a magnitude that the tubes I4 and I5 failed to be energised, the con-' denser 22 has no opportunity to readjust its charge until the signals again reach their former strength. Since this period may be of anyduration, the device '4' may not be actuated for long periods; thedesirability of the leakage path 23 is thus demonstrated. v I

The leakage path 23 results in the receiver gain being increased to maximum after any desired period, so that during a long spacing interval the background noise might become efiectively a.

marking signal. To overcome this difliculty, a control such as potential divider may be provided, this being adjusted so that the background 4 noise is just prevented from operating device 4'.

grounded electrode of condenser 22 is higher or General advantages .0! A. V. C. are- I 1. Much greater range of input signal magnitude allowable for satisfactory operation, as compared with a system not employing A. V. C.

2. Efiective signal-to-noise ratio is greatlyim-v proved at high signal values, 1. e., the signal to noise ratio which exists is always taken advan-,

tage of. If A. V. C. is not used, thesignal-tonoise ratio is always equal to that obtaining during the reception of the weakest usable signal.

3. Manual volume control eliminated.

Advantages of the proposed system over existing systems 1. Greater range of input signal strength allowconnected in said gain control circuit of said re ceiver, means including two tubes for connecting said potential deriving means to said condenser. 7

said tubes forming two connecting channels with the plate-cathode circuit of one tube connected reversely to the plate-cathode circuit of the other tube, and a pair of separate circuits connected respectively to the grids of the two tubes to maintain the tubes conductive for the entire marking period of the signal and to make said tubes nonconductive, during each spacing period of the signal, each of said separate circuits including a resistor shunted by a condenser and coupled to the output of said receiver through a rectifying device, whereby the potential on said condenser which controls the gain of said receiver is controlled by the strength of the incoming signal and is maintained constant during the spacing periods or the signal.

2. In a telegraph receiver which has an A. C. signal circuit and a gain control circuit and is adapted to receive signals composed of marking periods and spacing periods, an automatic volume control device comprising: potential-deriving means to derive a potential proportional to the strength of the received signals; a condenser connected in said gain control circuit; electronic means including two elements each having an anode-cathode .current path and control electrode means to control the current flow therethrough, one of said elements beingreversely connected with respect to the other element so that said elements form two conducting channels between said potential-deriving means and said condenser, said channels being conductive when said elements are rendered conductive and being nonconductive when said elements are rendered nonconductive; means for applying a bias to the control electrode means of said elements whereby said elements are rendered non-conductive during the spacing periods of the signal; and circuit means including rectifying means coupling the control electrode means of said elements to the signal circult to app y a potential in opposition to said bias potential thereby to render said electronic valves conductive during each marking period.

3. In a telegraph receiver which has a gain control circuit and is adapted to receive signals composed of marking periods and spacing periods, an automatic volume control device comprising: potential-deriving means to derive a potential proportional to the strength of the received signals; electronic means including two elements each having an anode-cathode current path and control electrode means to control the current flow therethrough, one of said elements being adapted to receive signais composed oi marking periods and spacing periods, an automatic volume control device comprising: potential-deriving means to derive a potential proportional to the strength of the received signals; a condenser connected in said gain control circuit; electronic means including two elements each having an anode-cathode current path and control electrode means to control the current flow therethrough, one of said elements being reverseiy connected with respect to the other element so that said elements form two conducting channels between said potential-deriving means and said condenser, said channels being conductive when said elements are rendered conductive and being non-conductive when said elements are rendered non-conductive; and control circuit means connected to the control electrode means or said elements to render said elements conductive for each entire marking period and to render said elements nonconductive during each spacing period, said control circuit means including resistor means coupled through rectifying means to the signal circuit to impress direct current potentials on said control electrode means during the entire marking periods whereby said elements are rendered conductive.

5. In a telegraph receiver which has an A. C. signal circuit and a gain control circuit and is adapted to receive "signals composed of marking periods and spacing periods, an automatic volume control device comprising: potential-deriving means to derive a potential proportional to the strength oi the received signals; a condenser connected in said gain control circuit; electronic means including two elements each having an anode-cathode current path and control elecreversely connected with respect to the other the received signals and during each spacing period said elements are rendered non-conductive and the potential in said gain control circuit is maintained substantially constant.

4. In a telegraph receiver which has an A. 0. nal circuit and a gain control circuit and is ing periods.

control electrode means of said elements to impress a positive bias on the control electrode means during each marking period and to im press a negative bias on said control electrode means during each spacing period, each of said circuits comprising, a battery and a resistor connected in series across a condenserwhich is connected between the cathode and the control electrode means so that the battery tends to impress a negative bias on said control electrode means,

and rectifier means connected to impress a potential across said resistor in opposition to the potential of said battery with said rectifier means deriving its current from thesignal circuit, whereby said condenser is directly connected to said potential-deriving means during each marking period so that the condenser assumes the desired gain-control potential and whereby said condenser is disconnected from said potential-deriving means during each spacing period with the result that the gain-control potential is maintained substantially constant during normal spacmnnnarcx PnacrvAL MASON.

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