GB1567932A - Two way radio systems - Google Patents

Description

(54) IMPROVEMENTS IN TWO WAY RADIO SYSTEMS (71) I, WILLIAM JOSEPH MUL LARKEY, of 9, Elgin Avenue, Garswood, Ashton-in-Makerfield, WN4 ORH, formerly of 52 Peveril Close, Whitefield, Lancashire, a British subject, do hereby the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to two way radio systems and, in particular, to an improvement in the system disclosed in my copending Application No. 51490/74, (Serial No. 1,503,318) hereinafter referred to as the parent patent.

The parent patent is concerned with the problem of a local transmitter adversely affecting the operation of a receiver. Such problems arise particularly on board ships, where it is not possible to separate the transmitter and receiver by any great distance.

The parent patent solves this problem by providing apparatus for use in a two way radio system including: a syllable detector for detecting radio frequency signals with speech modulation originating at the local transmitter; means for deriving a control signal therefrom; and an attenuator located in the aerial feed to the receiver, controllable in response to said control signal for attenuating said signals for the duration of the speech transmission, so as to reduce the signal level of said signals at the receiver input.

It is thus an object of the present invention to provide additional protection to the receiver, particularly against signals containing a large amount of energy.

According to the present invention, there is provided apparatus as claimed in Claim 4 of the parent patent, wherein the improvement consists in additionally providing in the aerial feed to the receiver a resistive device whose resistance changes as the signal increases, in order to reduce the level of a large signal induced in the aerial before it is fed to the receiver.

Additionally, the apparatus may be used in a two way radio system in which the signals are not simultaneous, and thus covers such transmissions as, for example, "SITOR", in addition to speech signals.

In one form, the resistive device is located between the aerial and the attenuator and in series therewith, the resistance thereof increasing as the signal increases. Alternatively, the resistive device is connected at one end to the junction between the aerial feed and the attenuator, and at the other end to earth, the resistance thereof decreasing as the signal increases.

Preferably, two resistive devices are included, one being connected in series between the aerial and the attenuator, and the other being connected to the junction between the first device and the attenuator and earth.

The first resistive device may comprise a filament light bulb having a tungsten filament, whilst the second resistive device may comprise at least two diodes connected in parallel but in opposite senses. A resistor of small value may be inserted in series with the parallel arrangement of the diodes.

The present invention will now be described in greater detail, by way of example, with reference to the accompanying drawing, wherein: Figure 1 is a circuit diagram of one preferred form of apparatus for operating a two way radio system by reducing the signal level of signals propogated by a local transmitter for their duration; and Figure 2 is a circuit diagram of the first and second resistive devices shown in the circuit of Figure 1.

Referring to the drawing, between an aerial 10 and an input terminal 12 to the receiver (not shown) there is provided a first resistive device 11, a second resistive device 13, an attenuator 14, a syllable detector 16 and a shaping circuit 18. The first resistive device 11 and the attenuator are connected in series between the aerial 10 and the receiver input terminal 12. To the junction between the first resistive device 11 and the attenuator 14 is connected, firstly, one end of the second resistive device 13 and, secondly, the input to the syllable detector 16. The output from the syllable detector 16 is connected to the input of the shaping circuits 18, which include both extending circuits and time constant circuits.

The output from the shaping circuits 18 is connected to the controlling inputs of the attenuator 14, the detail of which is shown in Figure 2 of the parent patent. The other end of the second resistive device 13 is connected to earth.

Referring now to Figure 2, the first resistive device comprises a light bulb 30 having a tungsten filament, and the second resistive device comprises eight silicon diodes 32, 34 and a 50 ohm resistor 36. The diodes are arranged such that the diodes 32 will conduct positive signals to earth above a threshold voltage determined by the inherent bottom bend characteristics of the two diodes connected in series, and the diodes 34 will conduct negative signals to earth above a threshold voltage determiend by the inherent bottom bend characteristics of the two diodes connected in series. The junction between all series pairs of diodes 32, 34 are commoned, in order to ensure that the difference of the individual diodes are evened out.

In operation, if an input signal Vi, which is supplied from the aerial 10, is small, the light bulb 30 has a low resistance, and the diodes 32, 34 all have a high resistance, so the signal passes straight through the first resistive device with little or no attenuation, the output Vo being fed to the attenuator 14 and the syllable detector 16.

If, on the other hand, the input signal Vi is large, the light bulb 30 will pass a large current, and as its filament heats up, so the resistance increases, reducing the size of the signal. If, in addition, the voltage at point 38 is instantaneously above a certain threshold, either the diodes 32 or 34 will conduct, depending on the instantaneous value of the voltage at the point 38, and thus carry the majority of the large input signal Vi to earth via the resistor 36.

The threshold of conduction of the diodes 32 and 34 is determined by the forward bias required to make them conduct, and the size of the resistor 36 can be arranged such that the wanted smaller signals do not cause the diodes to conduct.

These two resistive devices 11 and 13, when combined with the attenuator 14 and its associated controlling circuits 16 and 18, ensure that however large the unwanted signals in the aerial feed to the receiver caused by the local transmitter are, the receiver is fully protected and cannot be overloaded, since the attenuator 14 provides additional attenuation over and above that provided by the resistive devices 11 and 13.

In many applications, a pair of diodes connected in reverse parallel mode will suffice, in view of the fact that the attenuator 14 will provide further attenuation of the unwanted signals. Moreover, if the input signal Vi is extremely high, it will cause the light bulb 30 to fuse, thus protecting the receiver.

WHAT I CLAIM IS: 1. Apparatus as claimed in claim 4 of the parent patent, wherein the improvement consists in additionally providing in the aerial feed to the receiver, a resistive device whose resistance changes as the signal increases, in order to reduce the level of a large signal induced in the aerial before it is fed to the receiver.

2. Apparatus according to claim 1, wherein the resistive device is located between the aerial and the attenuator and in series therewith, the resistance thereof increasing as the signal increases.

3. Apparatus according to claim 1, wherein the resistive device is connected at one end to the junction between the aerial feed and the attenuator, and at the other end to earth, the resistance thereof decreasing as the signal increases.

4. Apparatus according to claim 1, including two resistive devices, a first resistive device being connected in series between the aerial and the attenuator, and the second resistive device being connected to the junction between the first resistive device and the attenuator, and earth.

5. Apparatus according to claim 4, wherein the first resistive device is a light bulb having a tungsten filament.

6. Apparatus according to claim 4 or 5, wherein the second resistive device comprises at least two diodes connected in parallel but in opposite senses.

7. Apparatus according to claim 6, wherein a resistor of small value is inserted in series with the parallel arrangement of the diodes.

8. Apparatus as claimed in claim 4 of the parent patent, constructed substantially as herein described with reference to and as illustrated in the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. attenuator 14, a syllable detector 16 and a shaping circuit 18. The first resistive device 11 and the attenuator are connected in series between the aerial 10 and the receiver input terminal 12. To the junction between the first resistive device 11 and the attenuator 14 is connected, firstly, one end of the second resistive device 13 and, secondly, the input to the syllable detector 16. The output from the syllable detector 16 is connected to the input of the shaping circuits 18, which include both extending circuits and time constant circuits. The output from the shaping circuits 18 is connected to the controlling inputs of the attenuator 14, the detail of which is shown in Figure 2 of the parent patent. The other end of the second resistive device 13 is connected to earth. Referring now to Figure 2, the first resistive device comprises a light bulb 30 having a tungsten filament, and the second resistive device comprises eight silicon diodes 32, 34 and a 50 ohm resistor 36. The diodes are arranged such that the diodes 32 will conduct positive signals to earth above a threshold voltage determined by the inherent bottom bend characteristics of the two diodes connected in series, and the diodes 34 will conduct negative signals to earth above a threshold voltage determiend by the inherent bottom bend characteristics of the two diodes connected in series. The junction between all series pairs of diodes 32, 34 are commoned, in order to ensure that the difference of the individual diodes are evened out. In operation, if an input signal Vi, which is supplied from the aerial 10, is small, the light bulb 30 has a low resistance, and the diodes 32, 34 all have a high resistance, so the signal passes straight through the first resistive device with little or no attenuation, the output Vo being fed to the attenuator 14 and the syllable detector 16. If, on the other hand, the input signal Vi is large, the light bulb 30 will pass a large current, and as its filament heats up, so the resistance increases, reducing the size of the signal. If, in addition, the voltage at point 38 is instantaneously above a certain threshold, either the diodes 32 or 34 will conduct, depending on the instantaneous value of the voltage at the point 38, and thus carry the majority of the large input signal Vi to earth via the resistor 36. The threshold of conduction of the diodes 32 and 34 is determined by the forward bias required to make them conduct, and the size of the resistor 36 can be arranged such that the wanted smaller signals do not cause the diodes to conduct. These two resistive devices 11 and 13, when combined with the attenuator 14 and its associated controlling circuits 16 and 18, ensure that however large the unwanted signals in the aerial feed to the receiver caused by the local transmitter are, the receiver is fully protected and cannot be overloaded, since the attenuator 14 provides additional attenuation over and above that provided by the resistive devices 11 and 13. In many applications, a pair of diodes connected in reverse parallel mode will suffice, in view of the fact that the attenuator 14 will provide further attenuation of the unwanted signals. Moreover, if the input signal Vi is extremely high, it will cause the light bulb 30 to fuse, thus protecting the receiver. WHAT I CLAIM IS:

1. Apparatus as claimed in claim 4 of the parent patent, wherein the improvement consists in additionally providing in the aerial feed to the receiver, a resistive device whose resistance changes as the signal increases, in order to reduce the level of a large signal induced in the aerial before it is fed to the receiver.

2. Apparatus according to claim 1, wherein the resistive device is located between the aerial and the attenuator and in series therewith, the resistance thereof increasing as the signal increases.

3. Apparatus according to claim 1, wherein the resistive device is connected at one end to the junction between the aerial feed and the attenuator, and at the other end to earth, the resistance thereof decreasing as the signal increases.

4. Apparatus according to claim 1, including two resistive devices, a first resistive device being connected in series between the aerial and the attenuator, and the second resistive device being connected to the junction between the first resistive device and the attenuator, and earth.

5. Apparatus according to claim 4, wherein the first resistive device is a light bulb having a tungsten filament.

6. Apparatus according to claim 4 or 5, wherein the second resistive device comprises at least two diodes connected in parallel but in opposite senses.

7. Apparatus according to claim 6, wherein a resistor of small value is inserted in series with the parallel arrangement of the diodes.

8. Apparatus as claimed in claim 4 of the parent patent, constructed substantially as herein described with reference to and as illustrated in the accompanying drawing.