System for maintaining phase coincidence between a carrier wave and sidebands produced by a transmitter

To maintain a cophasal relationship between two concurrently transmitted waves of the same high frequency derived from a common pilot oscillator, one of these waves being modulated by a low-frequency signal, a phase shifter in cascade with the modulator is controlled by a phase discriminator including a comparator with inputs receiving the two high-frequency waves. An analog multiplier driven by the low-frequency modulating signal converts an error signal from the phase comparator into a unipolar pulsating voltage which, upon integration, yields a control voltage adjusting the phase shifter to correct any relative phase displacement.

FIELD OF THE INVENTION 
The present invention relates to a system for controlling the relative 
phase of two high-frequency oscillations, i.e. a carrier wave and 
associated sidebands, produced by a transmitter. 
BACKGROUND OF THE INVENTION 
Some radio transmitting systems employ the possibility of adding in space 
the signals concurrently sent out by two separate transmission sources. In 
order that the resulting field be able to represent the sum of the fields 
of the two transmission sources, it is essential that the phases of the 
high-frequency waves constituting the carrier wave and the sidebands, for 
example in the case of radio-navigation signals, correspond to each other 
with fidelity. The radio-navigation transmitting systems known as V.O.R. 
or I.L.S. employ phase-controlling devices. Such devices such as that 
shown in FIG. 1 comprise a pilot oscillator 1 emitting simultaneously over 
two amplifying channels a signal whose frequency is equal to the frequency 
of the carrier wave. An amplifier 2, possibly including an 
amplitude-modulating stage, delivers the carrier wave in the first channel 
whereas a modulating amplifier 3 preceded by a phase-shifting device 4 
delivers a second signal whose frequency is identical to that of the 
carrier wave and whose phase is fixed with respect to the latter. A phase 
comparator or mixing stage 5 connected to the outputs of the amplifiers 2 
and 3 delivers, after filtering of the difference signal, a continuous 
voltage or error signal which exactly measures the phase difference 
between the high-frequency signals delivered by the two amplifiers. The 
error signal, after amplification by a direct-current amplifier 6, is used 
to control the analog phase shifter 4 and to lock the signals delivered by 
the amplifier 3 in phase with those delivered by the amplifier 2. The 
high-frequency output signal of amplifier 3, phase-controlled by the 
carrier wave S.sub.1 issuing from amplifier 2, is fed to a sideband 
generator 7 controlled in a conventional manner by a low-frequency 
modulation signal BF to produce a sideband signal S.sub.2. 
Such devices permit a stabilization of the phase of the high-frequency 
signal S.sub.2 only if the sideband generator 7 has a very high intrinsic 
stability since that generator is not included in the regulating loop. 
This intrinsic stability of the sideband generator requires an operation 
of the latter at a high level. The incorporation of the sideband generator 
in the regulating loop cannot be achieved directly with such devices, the 
phase of the high-frequency signals S.sub.2 being inverted by .pi. every 
semi-period of the low-frequency modulation signal BF. The direction of 
the phase drift can therefore not be determined. 
OBJECTS OF THE INVENTION 
An object of the present invention is to provide improved means for 
controlling the relative phase of a carrier wave and an associated 
sideband oscillation of the same frequency. 
Another object of my invention is to provide a sideband generator which has 
a very high phase stability with respect to the carrier wave while 
operating at a low level. 
SUMMARY OF THE INVENTION 
A system according to my present invention includes phase-comparison means, 
such as the aforementioned mixer stage, with inputs connected to the two 
channels serving for the transmission of the two high-frequency waves 
between which a substantially cophasal relationship is to be maintained, 
the comparison means thus receiving the carrier wave from the first 
transmission channel and the associated sideband oscillation from the 
second transmission channel to produce an error signal proportional to the 
relative phase drift. This error signal, changing in polarity between 
successive half-cycles of the low-frequency signal fed to the modulator in 
the second channel, is supplied together with the modulating signal to a 
polarity inverter which converts the error signal into a unipolar control 
voltage varying in magnitude and sign with the detected phase drift. A 
phase-shifting device in one of the transmission channels, specifically in 
the channel containing the modulator which generates and possibly 
amplifies the sideband oscillation, corrects the phase drift in response 
to this control voltage. 
The system according to my invention is particularly applicable to the 
phase control of radio-navigation signals sent out, for example, by V.O.R. 
or I.L.S. transmitters.

SPECIFIC DESCRIPTION 
According to FIG. 2, the device according to the invention comprises a 
pilot oscillator 21 delivering a signal whose frequency F.sub.o is equal 
to that of the carrier wave S.sub.1 to be emitted via a first transmission 
channel 11 including an amplifier 22; the latter, like amplifier 2 of FIG. 
1, could also have a modulating function. The pilot oscillator 21 delivers 
the same basic signal of frequency F.sub.o to a sideband generator 27 
through a phase shifter 24. The output 271 of generator 27 emits the 
sideband signal S.sub.2 on a second transmission channel 12. A phase 
discriminator 20 comprises two high-frequency input terminals 202 and 203, 
respectively connected by couplers 201 and 200 to channels 11, 12 and a 
control input 204 connected to a circuit delivering a low-frequency 
modulation signal A to a control input 272 of the sideband generator 27. 
The output terminal 205 of the phase discriminator is connected to the 
control input 241 of the phase-shifting device 24. The control of the 
phase discrimator 20 by the low-frequency modulation signal a permits, 
irrespective of the semi-period of that modulation signal, the 
determination in magnitude and sign of the phase shift between the two 
channels 11 and 12, the phase discriminator 20 producing at its output 205 
a voltage representing this phase shift in magnitude and sign and 
permitting the correction of that shift by means of device 24. 
According to the specific embodiment shown in FIG. 2, the phase 
discriminator 20 includes a mixer stage 25 whose input terminals 252 and 
253 are respectively connected to couplers 201 and 200. Each of these 
couplers comprises a sensor constituted, for example, by a resistive 
circuit or a parallel-band transmission line; the mixer stage 25 is 
constituted, for example, by a ring modulator. The output 251 of the mixer 
stage 25 is connected to a first input 281 of a multiplying device 28. The 
mixer circuit 25 further comprises a filter for delivering at its output 
251 an error signal B constituted by a periodic voltaage in the form of a 
square wave which represents the phase relationship of the two 
high-frequency oscillations S.sub.1 and S.sub.2. A second input 282 of the 
multiplying device 28 receives the modulation signal A fed to the sideband 
generator 27. The output 283 of the multiplying device 28 is connected to 
the control input 241 of the phase-shifting device 24 through an 
integrating circuit 29 for averaging a pulsating signal C delivered by 
device 28. This integrating circuit is, for example, constituted by a 
low-pass filter connected in cascade with a direct-current amplifier and 
generates a control signal D. 
The device operates in the following manner. The amplitude of the periodic 
signals indicated in FIG. 2 have been plotted in FIG. 3 against time. 
Graph (a) represents the modulation signal A fed to the sideband 
generator. Graphs (b.sub.1) and (b.sub.2) show the error signal B 
respectively appearing in the output 251 of the mixer stage 25 in the case 
of a phase lead and a phase lag of the carrier wave S.sub.1 with respect 
to the sideband oscillation S.sub.2, whereas graphs (c.sub.1) and 
(c.sub.2) show the pulsating signal C respectively issuing from the 
multiplying device 28 in the case of a phase lead and a phase lag of the 
carrier wave with respect to the sideband oscillation. FIG. 3(d) 
represents the response curve of the phase-discriminating circuit 
constituted by the mixer stage or phase comparator 25 and the multiplying 
device 28, with the magnitude of the integrated control voltage D plotted 
against the magnitude of the phase shift .phi.. This response is linear 
for a phase variation between +80.degree. and -80.degree. and results in 
the obtainment of a control voltage between +10 V and -10 V. The 
modulation signal A, shown as a sine wave in FIG. 3(a), is in phase with 
the error signal B whose amplitude represents the extent of the phase 
shift between the carrier wave and the sideband oscillation. The analog 
multiplication of the error signal B by the modulation signal A yields the 
unipolar pulsating signal C whose continuous component as obtained from 
integrator 29 represents the value of the phase shift in amplitude and 
sign. Thus, analog multiplier 28 acts as a full-wave rectifier which 
inverts alternate half-cycles of signal A and changes its amplitude 
according to the detected phase difference. The integrated and amplified 
signal D controls the phase-shifting device 24. 
According to FIG. 4a, the analog phase shifter 24 comprises two .pi. 
networks with shunt branches each constituted by a variable-capacitance 
diode or varactor 41 which is connected in series with an induction coil 
42 or 45. The branches of each network are interconnected by an induction 
coil 43. The anode of each varactor 41 is connected to ground or to a 
reference voltage of the device. The varactors 41 are biased by means of a 
choke coil 44. The phase-shifting control voltage applied to the input 241 
(FIG. 2) of the analog phase shifter 24 is superimposed on the biasing 
voltage of varactors 41 by an adder amplifier not shown. In the absence of 
a phase-shifting control signal the varactors 41 have a capacitance of 15 
pF. The outer shunt coils 42 have an inductance of 44 nH and the series 
coils 43 have an inductance of 60 nH. The middle shunt coil 45 has an 
inductance of 88 nH corresponding to twice that of each outer coil 42. 
FIG. 4b shows the response of the analog phase-shifting device 24 according 
to FIG. 4a. A linear phase variation between -40.degree. and +40.degree. 
is obtained for a control voltage between 7.5 V and 12 V. By way of 
example, the carrier-frequency generator 21 of FIG. 2 is constituted by a 
quartz oscillator having a high stability. This oscillator delivers a 
signal of frequency between 100 MHz and 300 MHz whereas the modulation 
signal A has a low frequency of 30 Hz in the case of a radio beacon of the 
V.O.R. type, and of 90 Hz or 150 Hz in the case of navigation signals of 
the I.L.S. type. 
By the present improvement I have been able to achieve, for a correction of 
phase within less than one degree, a substantial reduction of the input 
power of the sideband generator from a level of the order of 40 W in the 
prior-art system to 10 mW in the system according to the invention, which 
results in a reduced overall size and an increased efficiency.