GB2194110A

GB2194110A - Reflex microwave amplifier

Info

Publication number: GB2194110A
Application number: GB08715971A
Authority: GB
Inventors: Colin Stuart Aitchison
Original assignee: ERA Patents Ltd
Current assignee: ERA Patents Ltd
Priority date: 1986-07-14
Filing date: 1987-07-07
Publication date: 1988-02-24
Also published as: GB8715971D0; GB8617080D0

Abstract

A reflex microwave amplifier apparatus comprises a broadband amplifier circuit (26) having an input (6) and an output (7); means (28) to feed into the input (6) of the amplifier circuit (26) a signal in a first frequency band; means (30, 32, 33) coupled to the output (7) of the amplifier circuit (26) to receive the amplified first frequency band signal and to produce therefrom a signal in a second frequency band spaced from the first frequency band; means (28) to feed the second frequency band signal into the input (6) of the amplifier circuit (28); and means (30, 36) coupled to the output (7) of the amplifier circuit (26) to receive the amplified second frequency band signal. Means (38, 39) may be provided to generate a signal in a third frequency band and to feed this signal into the input (6) of the amplifier circuit (26). The apparatus may be part of a double-superheterodyne receiver and in this case the first frequency band signal is a received X-band signal, the second frequency band signal is a first I.F. signal, which may be in the L-band and the third frequency band signal is a second I.F. signal. <IMAGE>

Description

SPECIFICATION Microwave amplifier apparatus This invention relates to broadband microwave amplifier apparatus, and particularly, but not exclusively, to a circuit including a broadband amplifier which effects a number of successive operations in a microwave receiver, for example in a receiver for receiving directbroadcast-by-satellite (DBS) signals.

Microwave receivers which operate in a double-superheterodyne configuration are known. Such receivers receive the transmitted microwave signal, amplify it, mix the amplified signal with a signal from a first local oscillator to produce a first intermediate frequency (I.F.) signal, amplify that l.F. signal, mix the amplified signal with a signal from a second local oscillator to produce a second l.F. signal, amplify that signal, and demodulate the amplified second l.F. signal, all in a succession of individual stages.

Such receiver circuits are complicated and require the use of an appreciable number of expensive microwave semiconductor devices.

It is an object of the present invention to provide a simplified microwave circuit.

According to the invention there is provided microwave amplifier apparatus comprising a broadband amplifier circuit having an input and an output; means to feed into the input of the amplifier circuit a signal in a first frequency band; means coupled to the output of the amplifier circuit to receive the amplified first frequency band signal and to produce therefrom a signal in a second frequency band spaced from the first frequency band; means to feed the second frequency band signal into the input of the amplifier circuit; and means coupled to the output of the amplifier circuit to receive the amplified second frequency band signal.

Preferably the means to produce the signal in the second frequency band comprises oscillator means, and means to mix the output of the oscillator means with the amplified first frequency band signal.

Preferably the means to feed the first frequency band signal and the means to feed the second frequency band signal into the input of the amplifier circuit together comprise a directional filter.

The means to receive the amplified second frequency band signal preferably comprises means to produce from said signal a signal in a third frequency band spaced from the first and second frequency bands, and to feed the third frequency band signal into the input of the amplifier circuit. Means is then coupled to the output of the amplifier circuit to receive the amplified third frequency band signal.

The apparatus may be part of a doublesuperheterodyne receiver; the first frequency band signal may be a received X-band signal, the second frequency band signal may be a first l.F. signal, which may be in the L-band and the third frequency band signal may be a second l.F. signal.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Fig. 1 is a schematic diagram of a distributed amplifier for use in the invention; Fig. 2 is a block schematic diagram of a conventional double-syperheterodyne receiver; and Fig. 3 is a block schematic diagram of a double-superheterodyne receiver incorporating the present invention.

High-frequency broadband amplifiers have conventionally included resonant circuits. Due to the use of resonant circuits, the bandwidth which can be achieved between the points which are, say, 3dB down relative to the peak has been quite limited, and a frequency band from, say, 10 MHz to 20 GHz cannot be achieved in such circuits.

More recently, however, amplifiers have been designed which do not use resonant circuits, and which, by use of semiconductor devices having a very high operating speed, can achieve the very wide frequency band mentioned above.

These amplifiers are "distributed" amplifiers, comprising a number of field effect transistors connected as shown schematically in Figure 1 of the drawings. The transistors are preferably GaAs MESFETS. The amplifier may comprise any desired number of these transistors such as the transistors 1 to 4 shown in the figure.

The amplifer basically comprises two microwave transmission lines; a "gate" transmission line in which the gate electrodes of the transistors 1 to 4 are connected, and a "drain" transmission line in which the drain electrodes are connected. The source electrodes are connected to a "ground" line 5, which is common to the two transmission lines and to a microwave signal input 6 and a microwave signal output 7. The gate electrodes of successive transistors are interconnected via inductors 8, and the drain electrodes are interconnected by similar inductors 9. Inductors 10 and 11 are provided in the end sections of the lines.

The gate transmission line is terminated at its right-hand end by a resistor 12 equal to the characteristic impedance of the line, which may be, for example 50Q. The resistor 12 may be referred to as the "idle gate load".

The microwave signal is fed into the input 6 via a resistor 13 equal to that characteristic impedance. The drain transmission line is terminated at its left-hand end by a resistor 14 equal to the characteristic impedance of that line; again 50Q, for example. The resistor 14 may be referred to as the "idle drain load".

The signal output 7 is connected to a load circuit (not shown), which will have an input impedance equal to the characteristic impe dance of the drain transmission line. A d.c.

source 15 is connected to the line 5, and is connected to the junction of the resistor 14 and the adjacent inductor 11, via a low-pass filter 16, to provide bias for the drain electrodes.

In use of the amplifier, a microwave signal fed into the input 6 is amplified by the successive transistors 1 to 4. A respective travelling wave passes along each of the gate and drain transmission lines, and if each line is correctly terminated the gain of the amplifier will be substantially independent of the signal frequency. A gain of some 6dB is obtained over a very wide frequency band.

Fig. 2 shows a conventional double-superheterodyne microwave receiver of the type mentioned above. The receiver includes a tuned microwave amplifier 17 which receives from an antenna 18 a modulated X-band signal having a carrier frequency of approximately 12 GHz. The X-band signal is amplified and is fed to a first mixer 19, which also receives a signal from a first local oscillator 20. The signals are mixed, to provide sum and difference frequencies. The difference signal, at a frequency of approximately 1 GHz, i.e. in the Lband, is amplified in a tuned first l.F. amplifier 21. The amplified signal is fed to a second mixer 22 where it is mixed with a signal from a second local oscillator 23 to provide sum and difference signals. The difference signal, at a frequency of approximately 70 MHz is amplified in a tuned second l.F. amplifier 24 and is fed to a demodulator 25.The modulation signal thereby obtained is fed to further video or audio stages (not shown), as required.

It will be apparent that the amplifiers 17 and 21 both involve the use of expensive microwave semiconductor devices. For example, each of those circuits may include four such devices. Furthermore, each of the amplifiers 17, 21 and 24 involves the use of tuned circuits.

By use of the present invention, a doublesuperheterodyne receiver which achieves the same effect as the above-described known receiver, but in a simplified manner, can be obtained, and the invention will now be described, by way of example, in relation to such simplified receiver.

Referring to Fig. 3, the input 6 of a wideband distributed amplifier 26, as described above with reference to Fig. 1, is connected via a directional filter 28 to an antenna 29 which receives the X-band signal. The X-band signal- is amplified by approximately 6dB by the amplifier 26 and is fed from the output 7 into a second directional filter 30. The filters 28 and 30 present the required matching impedance to the gate and drain lines of the amplifier 26. Capacitors 27 and 45 in series with the filters 28 and 30, respectively, and capacitors 43 and 44 in series with the idle gate load 12 and the idle drain load 14, respectively, prevent the flow of a direct current into the respective components 28, 30, 12 and 14.The values of the capacitors 27 and 45 are selected so that their reactance over the whole of a frequency range of at least 70MHz to 12GHz is negligible compared with the respective gate and drain line load impedances, so that they do not interfere with the proper matching of those lines. The reactance of the capacitors 43 and 44 is negligible compared with the respective line impedance at the upper end of that frequency range.

The filter 30 passes the amplified X-band signal, via a line 31, to a mixer 32, where it is mixed with a signal from a local oscillator 33. Sum and difference signals around 23GHz and 1GHz, respectively, produced in the mixer, are fed to an input 34 of the directional filter 28, which passes only the 1GHz signal to the input 6 of the amplifier 26. That signal is amplified and is fed from the output 7 to the directional filter, which feeds the amplified 1GHz signal, via a line 35, to a low-pass filter 36 having a cut-off frequency just above 1GHz, which filters out any spurious frequencies. The filtered 1GHz (L-band) signal is fed via a line 37 to a second mixer 38, where it is mixed with a signal from a tunable second local oscillator 39 to produce the difference signal at 70MHz. This signal is fed to the input 6 of the amplifier 26 and is amplified therein.The amplified signal is fed from the output 7 to the filter 30 and is passed through the filter 36 to a low-pass filter 40 having a cut-off frequency just above 70MHz, which filters out the first l.F. signal at IGHz leaving only the 70 MHz second l.F. signal.

The filtered l.F. signal is fed to a demodulator 41, and the resulting demodulated signal is fed out over a line 42.

The directional filters 28 and 30 can be of the type such as is used for feeding the Xband signal and the first local oscillator signal to a mixer in a conventional receiver, such as the mixer 19 of Fig. 2. The mixer 32 is preferably an FET mixer. The local oscillator 39 is preferably tunable for enabling a specific channel to be selected from the many channels carried in the X-band.

It will be apparent that the X-band signal, the L-band signal and the 70 MHz signal are all amplified by successive passes through the single untuned amplifier 26, so that the circuitry of the amplifiers 21 and 24 of Fig. 2 is dispensed with.

As mentioned above, the X-band signal is amplified by some 6dB in the amplifier 26.

The L-band and 70MHz signals would also be amplified by about the same factor, but for the fact that the value of the capacitor 43 connected in series with the idle gate load 12 and the value of the capacitor 44 connected in series with the idle drain load 14 are selected such that, aithough those capacitors have negligible reactance at the X-band frequency, they have, at 70 MHz, quite an appreciable reactance compared with the respective line load impedance. Those capacitors will, therefore, cause a considerable mismatch in the gate and drain lines. This means that almost complete reflection of the energy in those lines will occur, so that the input voltage to be amplified is effectively doubled.

This results in an overall amplification of approximately 18dB in the 70MHz signal. At 1GHz, i.e. the first l.F. signal frequency, the capacitors 43 and 44 will have an intermediate reactance value, and so will cause some mismatching and consequential reflection of the energy. The signal to be amplified will therefore be increased to some extent, and an amplification factor somewhere between the 6dB value for the X-band signal and the 18dB value for the 70MHz will be achieved, for example 12dB. The overall gain of the system will not be 6+6+6=18dB (less some insertion loss for the two mixers) as would be expected, but 6+12+18=36dB (less that insertion loss).

It should be noted that a distributed amplifier, similar to the amplifier 26, could also be used for the microwave mixer 32. In that case, the X-band signal and the signal from the local oscillator 33 would both be fed to the gate line or the drain line, and mixing would be effected therein by virtue of the non-linearity of the active components.

Although the invention is described above in relation to an X-band double-superheterodyne DBS receiver in which the single amplifier 26 is operative to amplify the received signal and two l.F. signals of widely different frequencies, it will be apparent that it could be used for other purposes. Indeed, the invention may be used in any application where signals of different frequencies are to be amplified. Any number of passes through the amplifier may be made, provided that the frequency separation between the successive signals is adequate to enable the filters to discriminate between the desired and spurious signals.

Claims

1. A microwave amplifier apparatus comprising a broadband amplifier circuit having an input and an output; means to feed into the input of the amplifier circuit a signal in a first frequency band; means coupled to the output of the amplifier circuit to receive the amplified first frequency band signal and to produce therefrom a signal in a second frequency band spaced from the first frequency band; means to feed the second frequency band signal into the input of the amplifier circuit; and means coupled to the output of the amplifier circuit to receive the amplified second frequency band signal.

2. A microwave amplifier according to claim 1, in which the means to produce the signal in the second frequency band comprises oscil lator means, and means to mix the output of the oscillator means with the amplified first frequency band signal.

3. A microwave amplifier according to claim 1 or 2, in which the means to feed the first frequency band signal and the means to feed the second frequency band signal into the in put of the amplifier circuit together comprise a directional filter.

4. A microwave amplifier according to any one of the preceding claims, in which a directional filter is coupled to the output of the amplifier circuit, the directional filter providing as one output the amplified signal in the first frequency band and as another output the amplified signal in the second frequency band.

5. A microwave amplifier according to any one of the preceding claims, in which the means to receive the amplified second frequency band signal comprise means to produce from the said signal a signal in a third frequency band spaced from the first and second frequency bands, and to feed the third frequency band signal into the input of the amplifier circuit, and in which means are then coupled to the output of the amplifier circuit to receive the amplified third frequency band signal.

6. A microwave amplifier according to claim 5, in which the means to produce the signal in the third frequency band comprises another oscillator means, and means to mix the output of the other oscillator means with the amplified second frequency band signal.

7. A double-superheterodyne receiver comprising a microwave amplifier according to claims 5 or 6, in which the first frequency band signal is a received X-band signal, the second frequency band signal is a first l.F.

signal and the third frequency band signal is a second l.F. signal.

8. A double-superheterodyne receiver according to claim 7, in which the amplified second frequency signal is passed through a low pass filter before beign fed to the means to produce a signal in the third frequency band.

9. A microwave amplifier substantially as described with reference to Figures 1 and 3 of the accompanying drawings.

10. A double-superheterodyne receiver substantially as described with reference to Figures 1 and 3 of the accompanying drawings.