CA1213646A

CA1213646A - Microwave reception chain comprising a device for direct microwave demodulation

Info

Publication number: CA1213646A
Application number: CA000450199A
Authority: CA
Inventors: Jacques Bursztejn; Michel Part
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1983-03-25
Filing date: 1984-03-22
Publication date: 1986-11-04
Also published as: JPS59181864A; EP0120786A1; FR2543379B1; JPH0666820B2; EP0120786B1; FR2543379A1; US4559499A; DE3467672D1

Abstract

ABSTRACT OF THE DISCLOSURE

The microwave reception chain according to the invention comprises a direct microwave demodulation device comprising a demodulation circuit associated with an oscillator operating at the microwave carrier frequency, controllable to coincide in phase with the carrier received. The demodulation circuit comprises a separator coupled to the input for the received signal and connected to two symmetrical mixers which respectively receive the carrier from the oscillator and the same carrier shifted through 90° which are available at the output of a coupler having a phase shift of 90°, the outputs of the mixers being connected to low-pass filters restoring the demodu-lated digital sequences. The reception chain comprises an automatic gain control circuit in the base band and a phase calculator receiving the demodulated digital sequences.

Description

.
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
The invention relates to the field of radioelectric transmi~sion of digital data and relates more particularly to a reception chain comprising a direct microwave demodulation device.

2. DESCRIPTION OF THE PRIOR ART
The usual microwave reception chains compri~e, in series after the reception antenna, a microwave band filter followed by a low-noi~e amplifier, then by a mixer performing a transposition into inter~e~iate frequency by means of a local reception o~cillator. The chain furthermore compri~es an intermediate frequency preampli-fier, cell~ for correcting the group propagation time, and an intermedia~e frequency amplifier. The circuit for demodulation of the intermediate frequency ~ignal then permit6 restoring the ~ignal in a base band, the coded signal being restored after processing of the ~ignal in the ba~e band. In ~uch reception chains, the double transposition of microwaveg into intermediate freguency and then from the intermediate frequency into the ba6e band, lead~ to utili~ing a whole array of intermediate frequency circuit~, with a local oscillator in particular.
A de~cription ha~ beén given in the FUJITSU period-2S ical, vol. ll, No. 4 of December 1975 KAWASAKI, page~63 to 67, in an article titled: ~Synchronou~ pha~e demodulator~ for high ~peed quadrature PSK tran~mi~ion ; ~ ~yQtems" of a microwave reception cha~n of a ~ore ~imple ~tructure which make~ it po~ible to pérform a direct demodulation on a received microwave ~ignal, with a 1213~

smaller number of element~, the intermediate frequency circuits being omitted.
In this arrangement, no means are provided for maintaining a constant level at the output of the reception chain. However it i~ essential to incorporate - such a level control.
One-obvious solution t~ this problem consists in providing an automa$ic gain control upstream 4f the demodulator, for the microwave signal received. However, a solution of this kind leads to making use of variable microwave "PIN" diodes, which are interposed between microwave amplifiers. Furthermore, the dynamic of the input signal being of the order of 55 dB, it i~ neces~ary tha~ the dynamic of the control circuit should be at least 55 dB, which in a ~olution of this kind requires a costly circuit to be located upstream of the demodulator.
SUMMARY OF THE INVENTION
The problem resolved by the pre~ent invention consists in the provi~ion of a direct demodulation microwave reception-chain which, in association with other functions~ comprises the automatic gain control function, the elements needed to establish this function being conventional commercial circuits which enable a reduction in the cost of the reception chain as a whole whilst retaining its performance.
Accordingly, the present invention provide~ a microwave;reception ohain comprising a circuit for direct demodulation of a modulated signal resulting from mixing two carrier~ in quadrature modulated by digital signals .. _ .. ... . . . . .. . .

1213fi46 ` ` -scrambled so as not to compri6e long series of ~ne or zeros, of which the input is coupled with the xeceived ~ignal input, associated with an oscillator operated at the carrier frequency of the received ~ignal, controllable 5 60 that-.the phase of the carrier cominq from the oscill-ator coincides with the phase of the signal received and supplying demodulated digital signal~, and furthermore comprising a phase calculator having two inputs coupled to the output for demodulated digital signal~, of which the output provides a signal as a function of the phase difference between the received carrier and the local carrier, this output being connected to the control input of the oscillator via a loop integrator filter, wherein the arrangement also comprises means for automatically controlling the level of the demodulated signals X~t) and Y~t), compri~ing wide-band amplifier~ whose signal inputs are coupled to the outputs of the demodulation circuit, and which comprise an automatic gain control input, the outputs of these amplifier~ being connected to the inputs of a control circuit compri~ing integrating means, and of which the output~ are connected to the control inputs of the amplifiers.
BRIEF DESCRIPTION OF THE DRAWINGS
Thé invention will be better understood and other feature~ will appear from the following description given with reference to the accompanying drawings, in which:-~ Figure 1 is a block diagram of-one embodiment of a - reception chain in accordance with the invention, : Figure 2 is a block diagram of the demodulation device, , , ~-;
:,.....

- 1213~i4~;

Figufe 3 is a circuit diagram of one of the AGC
amplifiers with its control circuit, Figure 4 is an explanatory diagram, Fogure 5 is a diagraM of the phase calculator utilised in the reception chain according to the invention, and Figures 6 and 7 are explanatory tables.

DESCR~PTIO~ OF THE PREFERRED ENBODIME~T
The sphere of the invention is the radioelectric transmission of digital data, the transmission being performed by modulation of a microwave (SHF) carrier wave which may be represented by the combination of two independently modulated carrier waves in quadrature. A
linear modulation device has been described in French paten'c no. 2,476,947 filed February 22, lg80 in the applicant's name, in which the modulation is performed directly on the SHF carrier, without transposition into an intermediate frequency. ~ro this end, the digi~al modulation device utilised is linear for the modulation signal appiied to it.

The object of the present applicat~on is a recep~ion chain incorporating direct microwave demodulation, integrating all the func~ions necessary for processing modulated microwave signals, such as result for example from this linear digital modulation device Such a direcc microwave demodulation raises a definite number of problems in respect of the demodulation system as such, that is to say the mixer, as well as of the reception chain as a whole. As a matter of fact, selection of the kind of mixer utilised, and the ~ . , . ,, . ~

~213646 characteristic~ of the signals applied to the same, and consequently the processing operations required, are interdependent.
Figure 1 illustrates the block diagram of one embodiment of the reception chain according to ~he invention. The input ~upplied with the received ~ignal SR is connected to the input of a low-noise amplifier 1 whose output is connected to the input:for a modulated signal S of a demodulation circuit 2~ In the embodiment de~cribed, the received signal consists of two carrier waves in phase quadrature which are modulated ~y two ~ynchronous digital signals and form a modulated signal having four phase conditions. This circuit equally comprises an inpu~ for a local oscillation signal LO
connected to the output of a voltage-controlled oscillator

3. At its two outputs, this mixer supplies the signal6 P and Q of modulation in the base band. The ~ignals P
and Q are fed respectively to the input of two video-frequency preamplifiers, 4 and 5. The outputs of these two preamplifiers are respectively connected to the inputs of the two low-pass filter circuits 6 and 7 whose outputs are connected to the inputs of the automatic gain control amplifiers 8 and 9. The outputs of these amplifier~
which deliver the demodulated digital sequences X(t) and Y(t) are connected to the inputs of two output amplifiers 13, 14 and to the inputs of an automatic gain control - circuit 10 for the amplifier~ 8 and 9. The outputs of these amplifiers 8 and 9 are equally connected to the inputs of a phase calculator circuit il who~e output pro~ides an error signal e(t~ fed to the input of a loop .

, .

~2i364Çi ' 6 integrator filter 12 whose output is connected to the frequency contrcl input of the voltage-controlled oscillator 3.
The operation of this reception chain i~ as follows:
' the received signal which passe~ through the low-noise amplifier 1 i~ able to maintain the noise factor at a low value, for example 2.5 dB. The mixer 2 i~ a double ~ymmetrical linear mixer of which a more detailed diagram will be given in the following and which make~ direct delivery of the two demodulated signals in the ba~e band;
the videofrequency amplifiers have a gain of say 10 dB
and precede the signal filters; the autom~tic gain control amplifiers 8 and 9 have a fixed gain to which may be added a controllable.gain variation, for example fixed of 20 dB and variable of S0 dB which may be varied via the control input; the signals fed to the pha~e calculator 11 and to the'output amplifier~ 13 and 14 con~eguently have a constant peak amplitude.
~iqure 2 is a block diagram of the demodulation circuit 2. For a si~nal comprising two carriers modulated in guadrature, the demodulation device 2 compri~es: a separator 20 connected to the input for the signal S, and enabling the input ~ignal to be divided into two, the two,output~ of the separator being connected to the inputs of two identical mixers 21 and 22. The local oscillation ~ignal LO input is connected to the input of a 3 dB coupler 23 whose outputs are phased shifted 90 'with ~espect to each other. The two outputs of this 3 dB coupler 23 are connected to the second inputs of the two linear mixers 21 and 22. The f,requenc~ of the local ~,, ~ .. .. . . .

~2~36~

oscillator i6 equal to the carrier frequency of the signal received. The mixers 21 and 22 consequently provide, ater filtering in the low-pas~ filters 24 and 25 and to the extent to which the phase difference between the carrier 5 signal and the local oscillatic~n ~ignal iB zero, the demodulated signals P and Q.
The signal received has the form:

Z(t) z P (t) cos [~ot ~ ~(t)] in which ~(t~ is the modulation phase: ~(t) = ~ + k (t)~C .
Z (t) = X (t) sos ~ot ~ Y (t) sin ~ (t), by separation into two carriers in quadrature.
The local oscillator has the ~ame frequency as the signal and has the form L = 1 cos ~t. After the coupler 23, the local carriers in quadrature are:
Ll = - cos ~ot L2 =~r- ~in ~ot In the first mixer 21, the signalresul~in~ from the mixing action is a modulation product having a second degree component on each of the diodes. The low-pass filtering action permits restoring the digital ~dulation signal in the base band into cos ~ ~t). Similarly, the signal resulting from the mixing action in the ~econd mixer 22 and after low-pass filtering, permits restoring the digital signal in the base band into sin ~ (t).
This circuit may resemble a microwave direct modula-tion circuit constructed according to the same technology, such as is described by way o~ example in the patent ~z1364Ei referred to in the forefoing. Nevertheless, the quality of demodulation is closely linked with the quality of the local oscillation signal and consequencly witn itS
phase accuracy and with the quality of the local oscillation coupler. This coupler should actually have a very high phase stability throughout a wide band and a very satisfactory balance between the two coupling branches. As a result, in a preferred form of embodimenc of the invention, the coupler has been produced on a different board than that carrying the other elements of the demodulation circuit, so that no imbalance may be caused between the two coupling branches.

With a circuit of this kind, the demodulation spectrum is very close to that transmitted.
Furthermore, the form of the demodulation spectrum is retained when the reception power fluctuates.

Nevertheless, the mixer output level is certainly a function of the power received. As a result, in order to maintain a constant level at the output of the reception chain, i'c is necessary to provide an automatic control for the gain of the reception chain.
This control is an important elemen'c of the receptlon chain according to 'che invention.

This gain control cannot be established by acting on the level of the local oscillato. 3. As a matter of fact, variations controlled at the level of the local oscillator cause substantial distortions of the demodulated signal spectrum. Moreover, the variation of the level of the outpu'c signal of the demodulator as a function of the oscillator level, is no~ linear.

~A
r ~213646 - -This necessitates the provi~ion of the automatic gain control either in the microwave region, by utilifiing an automatic gain control microwave amplifier, but this ~olution is co~tly as state~ in the foregoing, or in the base band (videofrequency).
In the reception chain according to the invention the automatic gain control is operated on the ~ignal demodulated in the base band. This is rendered possible by the fact that the modulation ~ignals resulting ~rom the coding action upon transmission, are signals processed by "scrambling~ that is to say interposition of ~1~ or n o n judicuously inserted into the digital ~equence in order to avoid long series of noughts or ones. The main component of the demodulated ~ignal i5 then characteristic of the variations of the power received. The corresponding "unscrambling" is performed upon reception, following demodulation. Conseguently, as shown in ~igure 3, the control circuit 10 comprises an integrator circuit incorporating low-pass filters, whose output ~ignals are characteristic of the variations in level of the demodula-ted signals. The variable qain amplifiers 8 and 9 must have a sufficient pass band. The a~plifiers selected in the embodiment illustrated are differentital amplifiers, model MC 1733, of very wide band (120MHz) 81. The same circuit is thus utilised for digital outputs of 8.5 or 34 Mbit/s. The gain contr~l is exercised by variation of the emitter resistors of the transistors of the~e differential amplifiers by means of FET transistorC 82 which act as variable resistors. Finally, for an output at zero impedance whil~t having a low load resistor,-the .

:. .

~2~3646 - io -output consist~ of a transi~tor 83 connected as an emitter follower, followed by a capacitor ~4 in series with a 50 ohm resistor.
As for the phase calculator, this has the task of producing a stable phase reference so that the different phase changes carrying the information may be extracted in correct manner by means of the demodulation circuit.
In the particular case of direct microwave demodulation, the restoration of a carrier by multiplication is certainly precluded. Furthermore, the sy~tems of the demodulator-remodulator type, are also excluded in view of their cost and difficulty of production.
Consequently, in the reception chain according to the invention makinq use of direct demodulation, the phase calculator operates on the basis of a COSTAS loop demodula-tion system. In this kind of system, the error voltage e needed to block the phase loop is obtained directly from the de dulated sequences X(t) and Y(t). As a matter of fact, it is possible to obtain from these two signals, an error signal characteristic of the phase difference ~ between the carrier received and the local oscillation signal. An example of ~uch a loop utilising a phase calculator of the so-called "sin 4~n type is utilised in a demodulation device described in the paper by FUJITSU referred to in the foregoing.
In a preferred embodiment of the demodulation device, a more simpie calculator of the "sin 4~ prefix" type is utilised, ~ being the phase difference between the local carrier-provided by the oscillator 3 and the carrier received.

~z~

The diagram in Figure 4 conveys an understanding of the operation of this calculator which is illustrated in particular in Figure S.
Z(t) i~ the composite signal resulting from combining S the two carriers modulated in quadrature upon transmission Z(t) z P (t) [cos (~ot ~ ~(t)] .
Let x(t)andy~t) be the modulation signals on these carrier~ upon ~ing tran~mitted, then x(t) = y(t) in the example shown, and the modulation phase ist z ~. Let X~t) and Y(t) be the signals demodulated upon reception on the carriers in quadrature coming from the local oscillator, ann ~ the phase differen~e between the carriers upon transmi~sion and reception. An error function is calculated to evaluate this phase shift. If S(t) = X(t) ~ Y(t) and D(t) = X(t) - Y(t), it is possible to show that the function X(t).Y(t).S(t).D(t) = ~ 1/2 P 4(t) sin 40 .
Consequently, the function has an amplitude variable as a function of time and equally varies as a function of ~ following sin 4~. Based on this function , it is possible to obtain an error signal by observing that sin 4a changes sign when ~ passes from one ~L sector to an adjacent fiector, in the figure defined by orthogonal axes representing the carriers transmitted in quadrature, yielding the values x(t) and y(t), and the bisectors of these axes corre~pond to the modulation phases upon transmission. Let A,B,C... H be these sectors in Figure

4.
~he ~rror function e(t) = sign (~1/2 P4 (t) sin 4~) is positive in the ~ector~ A,C,E,G and negative in the ~ectors B,D,F, and H.

12~364~

This siqn i~ the result of the product of the signs of the different components X(t),Y(t),S(t) and D(t). It would thus be possible to produce the calculator by utilising multipliers. ~wever, the error vcltage which would be obtained w~uld have an amplitude varying with the amplitude of the incoming signal.
In a preferred embodiment of the invention, the calculator adopted is less complicated and permit~
obtaining an error voltage independent of the amplitude of the incoming signal.
As a matter of fact, the table of Figure 6 shows the sign of the product X(t).Y(t).S(~).D~t) as a function of the signs of the different components. Some impossible combinations are denoted by annI". For example, when X
and Y are positive, their sum cannot be negative.
Similarly when X and Y are negative, their sum cannot be positive. If X is positive and Y negative, the difference D =X-Y cannot be negative, etc....
Figure 7 illustrates the result of the logic "OR
EXCLUSIYE" function of the variables sign X, sign Y, sign S and sign D, where the values of thevariables and of the function for "positive sign" are denoted by "1" and the same values for ~negative sign" are denoted by "on. Upon comparing these two tables, it is observed that for the possiblé products of the variables sign X, sign Y, sign S and sign D, the "OR EXCLUSIVE" function is identical to the sign function of the product. This explains the structure of the phase calculator 11 illustrated in Figure 5.
The signal~ X(t) and Y(t) at the input of the ~1213~

calculator 11 are connected on the one hand to the two inputs of an adder 111 whose output provides the summated signal 5(t) = X(t) + Y(t), and on the other hand to the inputs of a ~ubtractor 112 whose output provides the difference signal D(t) = X(t) - Y(t). The inp~t~ X(t),Y(t) ~d the outputs S~t) and D(t1 are connected to the inputs of threshold circuits 113,114,115 and 116, respectively.
The outputs of these threshold circuits provide "high"
and n low" logic signals, depending on whether the signs of the signals fed to their inputs is positive or negative.
The outputs of these threshold circuits are grouped in pairs and connected to the inputs of two logic "OR
EXCLUSIVE" gates 117 and 118, whose outputs are connected to a third logic "OR EXCLUSIVE" gate }19. The output of the gate 119 forms the output of the calculator and supplies the error voltage e(t) at the logic "high" or ~low" levels depending on whether the product, and consequently sin 4~, is positive or negative. This calculator has been produced by the logic emitter coupling (LEC) technology, but it would equally be possible to produce the same by TTL technology.
When the carriers transmitted and received are in phase with the local carriers, the output signal of the calculator e(t) eventually has clearcut transitions during the transitions of the signals X(t~ and Y(T). By contrast, when a phase error appears between the carriers received and the local carriers, "peaks" will appear in the calculator output signal, which indicate the phase changes from one modulation ctate to another.
As stated in:the foregoing, the output of the lZ~;36~ .

calculator ll is connected to the input of a loop integra-tor filter 12 which integrates these ~peaks~ to produce the error voltage applied to the control input of the voltage-controlled oscillator.
The invention is not restricted to the embodiment described and illustrated.
In particular, the embodiment described in the fore-going has been tested for a transmission output of 34 MBits/s, on a carrier frequency of 2 GHz modulated in 4 phases. Any other combination i8 possible, and in particular, the same reception chain may receive lower outputs, for example 8 MBits/s. The carrier frequencies may also be of optional value. It is equally possible to adapt the reception chain for an "agile" frequency system, lS that is to say able to operate at different carrier frequencies, or for a spectrum expansion system.
Furthermore, the system has been described in the foregoing for a signal modulated over four phase conditions.
This example is obviously not restrictive and the reception shown described is applicable to the reception of any digital modulation signal which may be put in the form of two carriers in phase quadrature, in particular those resulting from modulation comprising discrete states having 8 phases or 16 phases (16 QAM); The demodulated digital sequences may have more than two levels.

Claims

1. A microwave reception chain comprising a circuit for direct demodulation of a modulated signal resulting from mixing two carriers in quadrature modulated by digital signals scrambled so as not to comprise long series of one or zeros, of which the input is coupled with the received signal input, associated with an oscillator operated at the carrier frequency of the received signal, controllable so that the phase of the carrier coming from the oscillator coincides with the phase of the signal received and supplying demodulated digital signals, and furthermore comprising a phase calculator having two inputs coupled to the outputs for demodulated digital signals, of which the output provides a signal as a function of the phase difference between the received carrier and the local carrier, this output being connected to the control input of the oscillator via a loop integr-ator filter, wherein the arrangement also comprises means for automatically controlling the level of the demodulated signals X(t) and Y(t), comprising wide-band amplifiers whose signal inputs are coupled to the outputs of the demodulation circuit, and which comprise an automatic gain control input, the outputs of these amplifiers being connected to the inputs of a control circuit comprising integrating means, and of which the outputs are connected to the control inputs of the amplifiers.

2. A microwave reception chain according to claim 1, wherein the phase calculator is a loop calculator establishing the sign function of the product X(t).Y(t).
S(t).D(t), in which S(t) and D(t) respectively are the sum and the difference of the digital signals X(t) and Y(t).

3. A microwave reception chain according to claim 2, wherein the calculator comprises a summator and a subtra-ctor, threshold circuits and a logic circuit establishing the "OR EXCLUSIVE" function of the output signals of the threshold circuits.