Microwave acoustic wave oscillator

The microwave oscillator is formed by a free oscillator 1 coupled with two surface or volume acoustic wave delay lines 9 and 10 connected in series or in parallel and operating by reflection. The first line imposes a delay 2.tau..sub.1, corresponding to one reciprocation, which is a multiple of the delay 2.tau..sub.2 imposed by the second line, and a=.tau.1/.tau.2 is preferably equal to 2 or 3. Application to oscillators operating at ultra-high frequencies, particularly in the L-band.

This invention relates to a microwave oscillator utilising the propagation 
of acoustic waves in delay lines operating by reflection. 
It is known from the prior art that a microwave oscillator can be produced 
by coupling a free oscillator, formed by an active quadripole connected to 
two passive networks such that this system enters into oscillation, with 
an acoustic wave-delay line operating by reflection which stabilizes the 
frequency of the free oscillator. 
An oscillator of the type in question is capable of operating at ultra-high 
frequencies and, more particularly, in the L band. In this frequency 
range, technological problems are involved in the production of 
electromechanical transducers of the interdigital comb type for surface 
wave delay lines and it is only volume wave delay lines, which use 
different transducers, that can be used on an industrial scale. Volume 
wave delay lines also have the advantage of ensuring better thermal 
stability than surface wave delay lines. 
The acoustic wave delay line operating by reflection only stabilizes the 
frequency of the free oscillator if the signals reflected by this delay 
line are in phase which the signals coming from the free oscillator. 
Since the delay corresponding to one reciprocation on the delay line is 
equal to 2.tau., the frequencies of the free oscillator at which stability 
is obtained are multiples of 1/2.tau. and are expressed thus: 
F=k.f=K/2.tau., where k is a positive integer. These frequencies represent 
the modes of operation of the microwave oscillator which are thus 
separated by a frequency band equal to 1/2.tau.. 
The delay line has an impedence variable as a function of frequency and the 
frequency of the free oscillator varies as a function of the load 
connected to it (Pulling effect). Thus, the association of these two 
effects enables the frequency of the free oscillator to be locked for a 
single mode of operation. 
The problem which then arises is reproducibly to select a single mode of 
operation of the microwave acoustic wave oscillator. This is because, when 
it is started up, the free oscillator supplies a frequency varying over a 
fairly wide band. Accordingly, it is possible for the free oscillator to 
stabilize for a mode of operation different from the mode of operation 
selected. 
If the delay 2.tau. corresponding to one reciprocation on the delay line is 
reduced, the frequency band 1/2.tau. separating two successive modes of 
operation increases, which contributes towards the selection of a single 
mode of operation, although at the same time the overvoltage coefficient Q 
of the microwave oscillator and, hence, its short-term stability, 
.DELTA.F/F per second, are reduced which is a disadvantage. 
The present invention relates to an acoustic wave oscillator capable of 
operating at ultra-high frequencies, particularly in the L band, of which 
a single mode of operation may be reproducibly selected without reducing 
the overvoltage coefficient and, hence, the short-term stability of the 
oscillator for the working point selected. 
The microwave oscillator according to the invention comprises a free 
oscillator coupled with two acoustic wave delay lines connected in series 
or in parallel and operating by reflection. The first line imposes a delay 
2.tau..sub.1, corresponding to one reciprocation, sufficient to obtain a 
high short-term stability for the working point selected, the second line 
imposes a delay 2.tau..sub.2, corresponding to one reciprocation, which is 
a sub-multiple of the delay imposed by the first line, enabling the 
frequency band separating two successive modes of operation to be brought 
to 1/2.tau..sub.2 =a/2.tau..sub.1, which contributes towards reproducibly 
selecting a single mode of operation. 
The microwave oscillator according to the invention also comprises means 
for starting up the feed of the free oscillator in three stages; these 
means enable the band of frequencies supplied by the free oscillator when 
it is started up to be reproducibly reduced and, hence, also contribute 
toward reproducibly selecting a single mode of operation.

In these various Figures, the same references denote the same elements. 
FIG. 1 is a basic diagram of a microwave oscillator corresponding to the 
prior art. This oscillator comprises a free oscillator 1 formed, as 
mentioned above, by an active quadripole, for example a microwave 
transistor, connected at its input and output to two passive networks. The 
output network comprises an impedance matching circuit 2 and coupler 3 
connected unidirectionally to the load impedance 4 and bidirectionally to 
an acoustic wave delay line 5 which operates by reflection. The input 
network establishes the oscillation conditions. 
FIG. 2 is a diagram enabling the working point of the oscillator shown in 
FIG. 1 to be determined. 
On the Smith abacus shown in thin lines in FIG. 2, the variation in the 
coefficient of reflection of the delay line 5 as a function of the 
frequency is represented in thick lines by a series of concentric circles 
corresponding to the equation A.e.sup.-j.2.tau...omega., where A is a 
constant and .omega. is the pulsation of the free oscillator. This 
representation is valid in approximate terms, particularly in the case of 
a frequency variation .DELTA.F around the frequency F of the 
ultra-high-frequency oscillator such that .DELTA.F/F.ltoreq.0.01. 
Also shown in chain lines on the Smith abacus is the Rieke diagram 8 which 
represents the frequency evolution of the free oscillator. The Rieke 
diagram is made up of isofrequencies, the various points of an 
isofrequency corresponding to the various loads which have to be connected 
to the output of the free oscillator for it to operate at a constant 
frequency. 
Since the influence of the load 4 is considered to be negligible, the 
working point of the microwave oscillator is the point of intersection of 
the circle 6 and the isofrequency 7 corresponding to one mode of 
operation, the only stable working point A being that for which the 
frequency variations on the circle 6 and the Rieke diagram are oppositely 
directed. 
The modes of operation are separated by a frequency band equal to 1/2.tau. 
which is small by comparison with the frequency F of the microwave 
oscillator. On the other hand, the impedance of the delay line is 
substantially periodic with a period of 1/2.tau.. The microwave oscillator 
may thus stabilize for a mode of operation difference from the mode of 
operation selected. 
If the delay 2.tau. due to one reciprocation on the delay line is reduced, 
the frequency band 1/2.tau. separating two successive modes is increased, 
which contributes towards the selection of a single mode of operation, 
although at the same time the impedance variation corresponding to one and 
the same frequency variation is reduced, which corresponds to a reduction 
in the overvoltage coefficient Q of the ultra-high-frequency oscillator 
and hence to a reduction in its short-term stability .DELTA.F/F per 
second. 
In the same way as before, the acoustic-wave microwave oscillator according 
to the invention comprises a free oscillator formed by an active 
quadripole connected to two passive networks such that this system enters 
into oscillation. This free oscillator is coupled with two acoustic wave 
delay lines which are operating by reflection and reproducibly select a 
single mode of operation. 
The modes of operation of the microwave oscillator being the frequencies at 
which the microwave signal coming from the free oscillator is in phase 
with the signal reflected by the acoustic wave delay lines. 
The first line imposes for one reciprocation a delay 2.tau..sub.1 
sufficient to obtain a high overvoltage coefficient and, hence, high 
short-term stability of the microwave oscillator at certain points of the 
band of frequencies separating two modes of operation, from which points 
the working point will be selected. 
The second line imposes for one reciprocation a delay 2.tau..sub.2 which is 
a sub-multiple of the delay imposed by the first line, enabling the 
frequency band separating two successive modes of operation to be brought 
to 1/2.tau..sub.2 =a/2.tau..sub.1. 
The two acoustic wave delay lines are connected in series or in parallel. 
They may be surface or volume acoustic wave delay lines. 
For one reciprocation, the first line imposes a delay 2.tau..sub.1 
sufficient to obtain a high overvoltage coefficient and, hence, a high 
short-term stability of the microwave oscillator at certain points of the 
frequency band separating two successive modes of operation, from which 
points the working point will be selected. 
For one reciprocation, the second line imposes a delay 2.tau..sub.2 which 
is a sub-multiple of the delay imposed by the first line, thus enabling 
the frequency band separating two successive modes of operation to be 
brought to 1/2.tau..sub.2 =a/2.tau..sub.1, a=.tau.1/.tau.2 preferably 
being equal to 2 or 3. 
FIG. 3 shows one particular embodiment of an oscillator according to the 
invention. Two delay lines 9 and 10, for example of the acoustic volume 
wave type, are connected in parallel and separated from the coupler 3 by 
an impedance matching circuit 11, their delays .tau..sub.1 and .tau..sub.2 
being such that .tau.1/.tau.2=a, a being a positive integer greater than 
1. 
FIG. 3a is similar to FIG. 3 but shows the delay lines 9 and 10 connected 
in series. The circuit drawing is otherwise the same as that of FIG. 3, 
and therefore the complete circuit is not repeated. 
FIGS. 4 and 5 show two diagrams illustrating the variation in the 
coefficient of reflection of the delay line as a function of the frequency 
in the case where a=.tau.1/.tau.2=2 and in the case where a=3. In FIG. 4, 
two delay lines .tau..sub.1 =1 .mu.s and .tau..sub.2 =0.5 .mu.s are used 
whilst, in FIG. 5, two delay lines .tau..sub.1 =1 .mu.s and .tau..sub.2 
=0.33 .mu.s are used. In both cases, the mode of operation selected is 
F=1500 MHz, the frequency band separating two successive modes being 
1/2.tau..sub.1, i.e. 1 MHz and 1.5 MHz, respectively. 
The frequencies are calibrated by using a discontinuous representation of 
the coefficient of reflection. In both cases, a frequency band of 25 KHz 
separates two consecutive points of these diagrams. 
It can be seen from these diagrams that the impedance variation 
corresponding to one and the same frequency variation is not constant, as 
is the case when a single line is used, so that the overvoltage 
coefficient and the short-term stability vary as a function of the 
frequency. Accordingly, it is sufficient to fix the working point in the 
zone where this impedance variation is at its greatest. 
The amplitude of the reflection coefficient is not constant and varies 
periodically, depending on whether the signals coming from the two lines 
are in phase or out of phase. 
The microwave acoustic-wave oscillator according to the invention also 
comprises means for starting un the feed of the free oscillator in three 
stages: 
the free oscillator is first fed for a period sufficient to enable the 
active quadripole which it comprises to reach its operating temperature; 
the feed is then interrupted for a period short enough to prevent the 
active quadripole from cooling, but long enough to allow relaxation of the 
microwave oscillator; 
feeding of the free oscillator is then resumed, the transient variation in 
the feed voltage from OV to the operating value taking place with a steep 
slope and being reproducible. 
This starting up of the free oscillator makes it possible to reduce the 
band of frequencies supplied by the oscillator on start-up and renders it 
reproducible. 
The means for starting un the free oscillator thus contribute towards 
reproducibly selecting a single mode of operation. 
FIG. 6 is a circuit diagram of one particular embodiment of the means for 
starting up the free oscillator. Two monostable circuits 12 and 13, for 
example of the TTL-type, are used. These monostable circuits are used to 
work out the periods corresponding to the first two stages of the starting 
up of the feed of the free oscillator. 
Means 14 enable the free oscillator 1 to be fed with a clearly defined, 
reproducible transient variation in the feed voltage. The means 14 may be 
formed by a power transistor 15 and an RC circuit 16 intended to establish 
a clearly defined reproducible transient variation in the feed voltage of 
the free oscillator.