Switches for radio frequency signals must be effective to both isolate radio frequency signals applied to an input of the switch when the switch is in an'off state, whilst providing an efficient conduit for the radio frequency signals when the switch is in an'on'state. Radio frequency switches must obviate a first technical problem of effectively isolating the input of the switch from the output when the switch is in the'off'state. A second technical problem with current radio frequency switches is that they do not facilitate switching from the'on'state to the'off state in a time which is fast enough to allow only a predetermined quantity of radio frequency energy to pass to the output.

An example of a known radio frequency switch is illustrated in the schematic diagram shown in Figure 1. In Figure 1 a radio frequency switch 1 has an input 2 and an output 4 and a control input formed between two further terminals 6 and 8. The radio frequency switch 1 is comprised of a semiconductor diode 10 connected between the input 2 and output 4 via decoupling capacitors 12, 14. The control input terminals 6,8, are connected across the cathode and anode respectively of the diode 10 via first and second radio frequency chokes 16,18 respectively. In operation the known radio frequency switch shown in Figure 1 prevents signals applied to the input terminal 2 from reaching the output terminal 4 whilst the diode 10 is in an off state. The off state is therefore achieved by applying dc voltage between control input terminals 6,8 to the effect that diode 10 is reverse biased. Correspondingly, radio frequency signals applied to the input terminal 2 will be conveyed to the output terminal 4, when the diode 10 is conducting which is achieved by applying an appropriate DC voltage between the input control terminals 6,8 which has an effect of forward biasing the diode 10.

A disadvantage with this known form of radio frequency switch is that when very high frequency signals are applied to an input of the switch, the radio frequency chokes 16,18 may not be effective to prevent radio frequency signals from passing from the input terminal 2, through the radio frequency chokes 16,18 via the terminals 6 and 8 to the output 4 of the switch 1, bypassing the diode 10.

Thus for signals in the range between 1 to 10 GHz, this arrangement would not be satisfactory. Alternatively increased radio frequency isolation could be achieved by increasing the inductance of the radio frequency chokes 16,18. However, this would have a disadvantage in that the time taken for the diode 10 to conduct will increase with the effect that a time taken for the switch 1 to change from an'off to the'on'state will increase. As such the known radio frequency switch 1 may not be used to switch a radio frequency signal of a substantially short duration between the input 2 and the output 4.

The aforementioned disadvantages of isolating a high frequency signal between an input and an output of a radio frequency switch and arranging for the switch to change from an'off to an'on'state quickly, represents a technical problem which is addressed by the present invention.

According to the present invention there is provided a radio frequency switch which operates to communicate a radio frequency signal from an input of the switch to an output of the switch in accordance with a control signal, said radio frequency switch comprising a semiconductor diode coupled between the input of the switch and the output of the switch and a digital logic means having a first output generating a first DC voltage (+V) and a second output generating a reverse of the first DC voltage (-V) and an activating input coupled to the control input of the radio frequency switch, wherein the first output of said digital logic means is coupled to a cathode of the diode and the second output of said digital logic means is coupled to an anode of the diode and the first voltage (+V) and the reverse of the first voltage (-V) have an effect of reverse biasing said diode when said digital logic means is in a first state and consequent upon a change in the control signal fed to the activating input of said digital logic means, said digital logic means changes to a second state in which said first output generates the inverse of the first voltage (-V) and said second output generates the first voltage (+V) thereby forward biasing said semiconductor diode and connecting the input of the switch to the output of the switch.

By using a digital logic means to reverse bias the diode when the switch is in the off state and to forward bias the diode when the switch is in the on state, radio frequency isolation may be effected whilst providing a fast switching time as a result of the comparatively fast operation of the digital logic means. The digital logic means may be comprised of digital logic gates which may be fabricated in accordance with emitter coupled logic. The digital logic means may be a flip-flop.

The flip-flop may be a D-type flip flop.

The semiconductor diode may comprise first and second pairs of semiconductor diodes each of which first and second pairs are connected in serial with the first pair being connected in parallel with the second pair, wherein the input of the radio frequency switch is formed at a connection between the first pair of diodes and the output of the radio frequency switch is formed at a connection between the second pair of diodes.

The first and second pairs of semiconductor diodes may be formed in separate semiconductor modules thereby substantially improving radio frequency isolation between the input and the output of the switch.

Embodiments of the present invention will now be described with reference to the accompanying drawings wherein, FIGURE 2 is a schematic block diagram of a first embodiment of a radio frequency switch, FIGURE 3 is a schematic block diagram of a second embodiment of the radio frequency switch, In Figure 2, a radio frequency switch 20 is shown to be comprised of a D- type flip-flop 22 which has a Q and a Q output which serve to generate DC voltages of +V and-V respectively when the control input (D) 24 is at a low DC voltage on the active transition of a clock signal CLK applied to clock input 38.

Connected between an input 26 of the switch 20 and an output 28 of the switch 20, is a diode 30, which is AC coupled to the input 26 and output 28 via capacitors 32, 34. The circuit is completed by biasing resistors R which arrange for the Q and Q outputs from the flip-flop 22 to be coupled to the anode and cathode of the diode 30. The biasing resistors R serve to maintain the diode 30 in reverse bias mode whilst the flip-flop 22 is arranged to be in a first mode where the Q output generates the positive DC voltage +V and Q output generates the negative DC voltage-V. It is a feature of this switch 20 that the flip-flop 22 is comprised of emitter coupled logic gates which provide the flip-flop with a switching time in the order of 50 psec. However, as will be appreciated other semiconductor active components and logic designs may also be used. When a switching pulse 36 is received and applied to the input 24 of the flip-flop 22, the flip-flop switches on the active transition of a clock signal CLK applied to clock input 38, such that the Q output is positive +V whilst the Q output is negative-V. This has the effect of forward biasing the diode 30, thereby causing the diode 30 to conduct, which in turn has the effect that the input 26 is connected to the output 28, thereby passing the signal from the input 26 to the output 28 whilst the control input 24 is high.

With this switch arrangement a switching time in the order of 50 pico seconds may be effected, such that the radio frequency switch 20 may operate to convey a radio frequency pulse with a duration in the order of 500ps with a distortion period caused by switching in the order of only 10% of this duration.

A further preferred embodiment of the present invention is illustrated in Figure 3 where parts also appearing in Figure 2 bear identical numerical designations. The radio frequency switch shown in Figure 3 is the same radio frequency switch shown in Figure 2 apart from the replacement of the diode 30 by two pairs of diodes 40,42,44,46. Furthermore, the input and output of the radio frequency switch which are connected to the two pairs of diodes. The diode 30 has been replaced by the two pairs of serially connected diodes 40,42,44,46, each of which pair is connected in parallel with the other pair and connected between the Q and Q outputs of the flip-flop 22. The input 26 of the radio frequency switch 20 is connected, via capacitor 32, between the junction between the first pair 40,42 of serially connected diodes and the output 28 of the radio frequency switch is connected, via capacitor 34, to the junction between the second pair 44,46 of serially connected diodes. The first pair of diodes 40,42 is formed in a first semiconductor package 50 whereas the second pair of diodes 44, 46 is formed in a second semiconductor package 48.

An effect of the arrangement of replacing the diode 30 with the first and second pairs of semiconductor diodes is that a radio frequency isolation between the input 26 and the output 28 of the switch is improved. Otherwise, operation of the radio frequency switch shown in Figure 3 is substantially in accordance with that described for Figure 2 and so will not be repeated here. A further improvement in isolation is effected by arranging for the first and second pairs of diodes to be fabricated in the separate semiconductor packages 48,50.

As will be appreciated by those skilled in the art, various modifications may be made to the aforementioned embodiments whilst still falling within the scope of the present invention.