Time-delay-triggered TRAPATT oscillator having delay line with progressively increasing impedance

A time-delay triggered TRAPATT oscillator in which the delay line impedance progressively increases from the diode end to a trigger element. Thus, all reflections except those from the trigger element are negative-going and help to reduce the rise in diode voltage between trigger pulses.

BACKGROUND OF THE INVENTION 
This invention relates to a time-delay-triggered TRAPATT oscillator 
arrangement comprising a TRAPATT diode, a length of delay line connected 
across the diode and a trigger element in the form of a step transition 
from a high to a low impedance at the end of the delay line remote from 
the diode. Such an arrangement is described by W. J. Evans in "Circuits 
for high-efficiency avalanche-diode oscillators", I.E.E.E. Trans. MIT-17, 
1060, (1969). However, it has been reported (J. E. Carroll, "The use of 
pseudo-transients in the solution of the Evans TRAPATT circuit", 
Proceedings of the 8th International MOGA Conference, Amsterdam, 1970; J. 
E. Carroll and R. H. Crede, "A computer simulation of TRAPATT circuits", 
Int. J. Electron. 32, 273 (1972)) that successful operation of this 
circuit depends markedly on a number of parameters, and the complexity of 
the circuit (which comprises series tuners in the form of transmission 
lines of fairly critical lengths and impedances) is a considerable 
hindrance to the design and construction of simple TRAPATT oscillator 
modules. In particular, since the Evans circuit relies for its operation 
upon repeated triggering of the diode by pulses reflected from the 
junction of the delay line and the filter, then reflections from within 
the filter itself or from circuit elements beyond the filter can cause 
unwanted, spurious triggering. As can be appreciated from FIG. 6 of the 
Evans paper such reflections from within the filter can be caused by large 
impedance mismatches between the successive portions of the filter which, 
in the arrangement described with reference to FIGS. 4 to 6 of the Evans 
paper is in the form of a coaxial line with tuning sleeves. 
Thus, this conventional TRAPATT oscillator circuit suffers from the 
following disadvantages. 
Since each transition from a high impedance to a low impedance transmission 
line (as one moves away from the TRAPATT diode) is capable of generating a 
trigger voltage from a single diode-stimulus, multiple triggering is 
possible. Since each of these multiple trigger pulses can compete to 
control the oscillation frequency, coherence requires one trigger to 
achieve dominance. 
Little, if any, serious investigation appears to have been carried out on 
the practical problem of preventing the diode voltage from exceeding 
avalanche breakdown between trigger pulses, particularly during the 
transient phase of the TRAPATT mode, that is to say during the time period 
between the first TRAPATT pulse and the cycle in which coherence is 
established. Increased lumped local capacitance and a trigger-line with 
single impedance stop have been found to be advantageous from experience 
but it is likely that such voltage suppression has, in the main, been 
inadvertently achieved by reflections from discontinuities between low 
impedance and high impedance transmission lines from components between 
the TRAPATT diode and the radio-frequency load. 
Hitherto it has been thought that TRAPATT oscillator circuits merely 
provided the necessary steady state impedances at the fundamental and 
harmonically related frequencies, but in practice it is thought that it 
also provided the voltage-steps for suppressing the diode voltage between 
trigger pulses. Thus, to achieve coherence to accommodate this 
unanticipated role the many elements of the Evans circuit almost always 
require empirical adjustment. Failure to prevent the diode voltage 
exceeding avalanche breakdown between triggers will lead to different 
diode states prior to each trigger pulse and hence preclude coherence. 
SUMMARY OF THE INVENTION 
The object of this invention is two fold: first to inhibit unwanted 
triggering by reflections from discontinuities and thus to increase the 
effectiveness of the single trigger reflection from the high-to-low 
impedance step at the end of the transmission line, and secondly to 
suppress the diode voltage during the recovery period in the transient 
phase. 
According to the invention a time-delay-triggered TRAPATT oscillator 
arrangement comprises a TRAPATT diode, a length of delay line connected 
across the diode and a trigger element in the form of a step transition 
from a high to a low impedance at the end of the delay line remote from 
the diode, and is characterised in that the delay line impedance increases 
progressively from a lower impedance at the end nearer to the diode to a 
higher impedance at the end remote from the diode. 
The delay line may be stepped that is to say it may comprise a plurality of 
sections of increasing impedance from the diode end. 
Alternatively it may be tapered that is to say its impedance increases 
smoothly from the diode end. 
Alternatively it may be partly stopped and partly tapered. 
Where the arrangement is in coaxial form the stepping and/or tapering may 
be applied to the inner conductor or the outer conductor or both.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, which is not drawn to scale, a TRAPATT oscillator 
arrangement comprises a center conductor C of constant diameter and an 
outer conductor, of which the Figure shows diagrammatically only the 
outline of the inner surface, which is of stepped formation and can 
conveniently be regarded as being formed of sections 1, 2 . . . 7 
respectively of lengths L1, L2 . . . L7 having respective characteristic 
impedances, not indicated on the Figure, Z 1, Z 2 . . . Z 7. 
At the upper end as viewed in the Figure this coaxial system is terminated 
by a transverse wall W between which and the end of the central conductor 
C is a TRAPATT diode T. 
The delay line, the length of which primarily determines the frequency of 
oscillation, and from the end of which remote from the diode T are 
reflected the pulses which act as trigger pulses, is formed of the 
sections 1 to 5 inclusive, while the step transition from high to low 
impedance at the junction of sections 5 and 6 forms a trigger element 
which reflects part of the pulse from the diode T back along the line to 
trigger the next pulse. 
Beyond the section 6 is a further section 7 which forms the output section 
of the oscillator arrangement and to which the load and the bias circuits 
for the diode would be coupled in the usual way. The length of this 
section is, of course, immaterial provided that the termination is a 
broad-band match that is to say a constant resistance network for example 
such as that disclosed in British Application No. 79-17969 filed May 23, 
1979, corresponding to U.S. Pat. application No. 151,218, filed May 19, 
1980. 
It will be appreciated that because the high-to-low impedance transition 
between sections 5 and 6 is the only one of its kind, then this is the 
only one in the oscillator arrangement which would invert the 
negative-going pulse from the diode so that a positive-going trigger pulse 
is returned down the delay line to the diode. Because all the other 
impedance transitions are low-to-high, reflections of the diode pulse are 
not inverted but are returned as a series of negative-going pulses which 
act to inhibit or suppress the rise in the diode voltage between trigger 
pulses and thus to stabilize the effect of the trigger pulses reflected at 
the trigger element formed by the impedance transition 5-6. 
Section 5 of the arrangement carries a polystyrene washer B some 3 mm long 
which is a good fit in the outer conductor and over the inner conductor C. 
The reason for providing this washer is purely mechanical, to add strength 
to the assembly. 
Section 6 of the arrangement was in the form of a brass slug S of annular 
cross section slidable within the continuous inner surface of sections 5 
and 7 and having spring contact fingers F which engage this surface: this 
enabled the length of section 5, and hence the fundamental frequency of 
oscillation to be varied. It was found that by the use of this 
construction the fundamental frequency could be varied between the 
approximate limits of 1.8 to 2.5 GHz while at the same time maintaining 
coherence. 
Examples and dimensions of some embodiments in coaxial line will now be 
given with reference to FIG. 1. In all examples the center conductor C was 
3 mm in diameter: in the following examples d is the inner diameter of the 
outer conductor. The frequency was 2.5 GHz in all cases and operation was 
pulsed with a duty cycle of 0.1%. 
EXAMPLE 1 
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Section Z(ohms) L(mm) d(mm) 
______________________________________ 
1 10 15 3.5 
2 15 9 4 
3 20 9 4.2 
4 30 9 5.0 
5 50 8 7 
6 10 13.5 3.5 
7 50 7 
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In this Example the mean current drawn by the diode during the pulse could 
be varied from about 2 amps to at least 4 amps and at the latter current a 
peak output power of 40 watts was reached. 
EXAMPLE 2 
Dimensions were as for Example 1 except that section 6 was 9 mm long. The 
output power was some 45 watts with an input pulse current of 4 amps. 
EXAMPLE 3 
Dimensions were as for Example 2 except that L2 was 9 mm and Z2 was 20 ohms 
so that in effect this was a four section line instead of a five-section 
line as in FIG. 1, since sections 2 and 3 of FIG. 1 were identical. This 
arrangement could only be driven up to 3.7 amps mean current during the 
pulse, but output powers of up to 50 watts were attained. 
EXAMPLE 4 
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Section Zohms Lmm 
______________________________________ 
1 10 6 
2 15 18 
3 20 9 
4 30 9 
5 50 9 
6 10 9 
7 50 
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Output powers of some 60 watts were attained with this arrangement, again 
with an input pulse current of 4 amps. 
EXAMPLE 5 
Dimensions were the same as for Example 4 except that Section 1 was 9 mm 
long and was loaded with a cross-linked polystyrene slug some 2 mm long, 
and peak outputs of over 60 watts were attained. 
EXAMPLE 6 
A fixed-frequency oscillator at 2.5 GHz was made to the same dimensions as 
those of Example 2 with the difference that the brass slug S forming the 
low-impedance section L6 was omitted and instead the center conductor C 
was formed with a portion 9 mm long and having an increased diameter of 6 
mm. 
FIG. 2 illustrates an embodiment of the partly stepped and partly tapered 
delay line, in coaxial form. This delay line comprises TRAPATT diode T, 
transverse wall W, center conductor C, constant impedance sections 11, 12, 
14, 15, 16, tapered impedance section 13 and polystyrene washers B1, B2. 
Although the embodiments described above are all in coaxial line 
configuration it will of course be understood that the techniques 
discussed in this specification are equally applicable to other forms of 
transmission line such as for example stripline. 
FIG. 3 illustrates an embodiment of the tapered delay line, in stripline 
form. This delay line comprises a TRAPATT diode T and an insulating layer 
I on a metallic ground plane M. One contact of the diode is directly 
connected to the ground plane M while the other contact is 
electrically-connected by wires BW to a strip conductor on the layer I, 
having tapered impedance section 21 and constant impedance sections 22 and 
23.