GB2193400A

GB2193400A - Attenuators

Info

Publication number: GB2193400A
Application number: GB08610534A
Authority: GB
Inventors: Roderick Leonard Walla Stevens; John Richard Cobden Snow
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1986-04-30
Filing date: 1986-04-30
Publication date: 1988-02-03
Anticipated expiration: 2006-04-30
Also published as: GB8610534D0; GB2193400B

Abstract

An attenuator comprising a plurality of impedance elements, such as resistors R1, R'2, R''2, coupled via a plurality of mixers M1 to M3 operated as switches between the input and output terminals of the attenuator, means being provided for selectively applying control signals 20, 22, 24 to the mixers to connect into circuit selective impedance elements so as to define between the input and output terminals a selected impedance. The mixers can be in the form of a diode bridge (Fig. 3) gated on or off by a control signal at (I). <IMAGE>

Description

SPECIFICATION Attenuators The present invention relates to an attenuator and more particuirly to an r.f. switching attenuator.

Known r.f. switching attenuators are expensive as they may comprise couplers, switches, attenuates and other r.f. components.

One of the objectives of the present invention is to provide an inexpensive r.f. switching attenuator which can serve as an r.f. switching modulator for switching the output of a signal generator between predetermined amplitude levels.

According to the present invention there is provided an attenuator comprising a plurality of impedance elements coupled via a plurality of mixer elements between the input and output terminals of the attenuator, means being provided for selectively applying control signals to the mixer elements to connect into circuit selective impedance elements so as to define between the input and output terminals a selected impedance.

In a preferred embodiment, the attenuator is an r.f. attenuator for use as an r.f. switching modulator and the plurality of mixers are used selectively to switch into circuit the impedance elements, thereby to produce a modulation waveform which is switchable between a plurality of amplitude levels.

In a preferred embodiment at least some, and preferably all, of the impedance elements are resistors, The present invention will be described further, by way of example, with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of an attenuator according to one embodiment of the present invention Figure 2 illustrates in graphical form the output power from the attenuator of Fig. 1 and Figure 3 is a circuit diagram of a mixer used in the attenuator of Fig. 1.

Referring to Fig. 1 an attenuator comprises three resistors R1, R2 and R2" coupled via three mixers M1, M2 and M3 between the input and output terminals of the attenuator. The three mixers M1, M2 and M3 are operated as ON/OFF switches and are driven from logic control signals derived from a switch control.

An embodiment of the mixers M1, M2 and M3 is illustrated in Fig. 3, each mixer comprising a diode gate arrangement coupled by input and output transformers to respective input and output terminals of the mixer. The logic control signals are applied to a control terminal I to either forward or reverse bias the diodes so that the mixer behaves as an ON/OFF switch.

The attenuation levels of the attenuator are set by the resistors R1, R2' and R2", the series resistors R2, and R2" are equal and have a combined resistance R2. It is advantageous to split R2 into the two equal resistors R2, and R2" on either side of the mixer M3 as this prevents the output of the mixer M1 driving directly into the mixer M3 and similarly the output of the mixer M2 is not connected directly to the output of the mixer M3. The attenuator has four settings which will hereinafter be referred to as the "OFF" setting, the "ATTENUATION ONE" setting, the "ATTENU ATION TWO" setting and the "ON" setting.

In the "OFF" setting the mixers M1, M2 and M3 are all OFF. There is therefore no signal path from the input to the output of the attenuator. In practice this is limited by the isolation of the mixers Ml, M2 and M3 and the output power from the attenuator is substantially zero, level A in Fig. 2.

In the "ATTENUATION ONE" setting the mixer M1 is driven into its ON state by a positive pulse from the switch control along a line 20 to the control terminal I of the mixer M1. The mixers M2 and M3 are maintained in their OFF state. At this setting there is a signal path through the mixer M1 and the resistor R1 to the output of the attenuator. Assuming no insertion loss in M1 the attenuation is given by the formula Zo/(R1+Zo) where Z0 is the impedance seen by the output of the attenuator. The attenuation Zo/(R1+Zo) can be preset therefore by a suitable choice of the value of the resistor R1. The output power from the attenuator is illustrated as x3dB, level B in Fig.

2.

In the "ATTENUATION TWO" setting both of the mixers M1 and M3 are driven into their ON state by positive pulses simultaneously applied from the switch control along the line 20 and a line 24 to respective control terminals I of the mixers M1 and M3. The mixer M2 is maintained in its OFF state. At this setting there is a signal path through mixers M1, M3 and the resistors R2, and R2" as well as the signal path through the mixer M1 and the resistor R1. The attenuation is given by the formula Zo/(R1/R2+Zo) assuming no insertion loss in the mixers M1 and M3. The output power from the attenuator is illustrated as (x2+x3) dB, level C in Fig. 3.

In the "ON" setting the mixers M1, M2 and M3 are all driven into their "ON" state by positive pulses simultaneously applied from the switch control along the lines 20, 24 and a line 22 to respective control terminals I of the mixers M1, M3 and M2. At this setting the signal path is straight through from the input to the output of the attenuator via the mixers M1 and M2. The output power is illustrated in Fig. 2 as level D and is equal to (x1 +x2+x3)dB.

In the embodiment described above there are four possible attenuation settings dependent on appropriate selection of the control signals to the mixers. In other embodiments of the present invention more preset attenua tion settings may be produced by the inclusion of extra mixers and resistors.

It will be appreciated that accuracy of the attenuation produced is dependent on the stability of the input impedance Z0 (normally 50 ohms) of the equipment following in the system. One application of the attenuator is that of an inexpensive modulator which can be used to switch the output of a signal generator between predetermined amplitude levels.

Claims

1. An attenuator comprising a plurality of impedance elements coupled via a plurality of mixer elements between the input and output terminals of the attenuator, means being provided for selectively applying control signals to the mixer elements to connect into circuit selective impedance elements so as to define between the input and output terminals a selected impedance.

2. An attenuator as claimed in claim 1 wherein at least some of the impedance elements are resistors.

3. An attenuator substantially as hereinbefore described with reference to the embodiments of the present invention.

4. An attenuator substantially as hereinbefore described with reference to, and as illustrated in, Fig. 1 of the accompanying drawings.

5. A system in which an attenuator as claimed in any one of claims 1 to 4 has its output coupled to the input of electrical equipment the output signal of which it is desired to modulate.

6. A system as claimed in claim 5 wherein the electrical equipment is a signal generator.