Waveguide Gunn diode oscillator with harmonic tuning

The oscillator comprises a main cavity housing a Gunn diode and auxiliary cavity branching laterally and allowing only inlet of the harmonic components. The auxiliary cavity is provided with harmonic tuning means for varying the reactance of the auxiliary cavity to the harmonic components. The load impedance seen by the Gunn diode is thus varied.

This invention relates to a new type of waveguide Gunn diode oscillator 
with harmonic tuning. 
Waveguide Gunn diode oscillators have for some years been of fundamental 
importance in modern electronic technology, in which they are used as 
tunable microwave sources. Other important reasons for this success are 
their low cost, high reliability and great constructional simplicity. 
One simple method of constructing oscillators of this type is to mount the 
Gunn diode on a cylindrical peg disposed perpendicular to the wider side 
of the waveguide. Fur tuning purposes, a mobile short-circuiting piston 
can be provided, or, as in the case of commercial oscillators, a system 
comprising a dielectric bar and a load coupling iris, or alternatively 
electronic devices such as a varactor. 
The constructional simplicity of these oscillators is contrasted by the 
considerable complexity of their electromagnetic behaviour. This 
operational complexity is largely due to the existence of a series of 
limitations on the performance obtainable with this type of oscillator. In 
this respect, the facility for obtaining a good power level, effective 
electronic tuning, low frequency modulation noise and good operating 
frequency stability with temperature variation is often compromised by 
certain non-linear phenomena, the mechanisms of which have up to now been 
largely unknown. 
A theoretical and experimental investigation, which for reasons of brevity 
is not reproduced here, has enabled the cause of all the aforesaid 
limitations to be identified as the interaction, due to the non-linearity 
of the diode, between the harmonic frequency components and the 
fundamental frequency component of the signal present in the oscillating 
cavity. 
The theoretical investigation has shown that by suitably adjusting the load 
at the harmonic frequencies, and in particular at the second harmonic 
frequency, the power level, the electronic tuning characteristics (tuning 
obtainable by adjusting the voltage either across the diode or across a 
varactor) and the temperature stability of the oscillator operating 
frequency can be simultaneously optimised. It is also possible to show 
that the same type of adjustment can be used for reducing the frequency 
modulation noise level of the oscillator. 
As the normal waveguide components are designed to function correctly at 
the fundamental frequency, it is extremely improbable that load conditions 
at the harmonic frequency which allow optimum operation are obtained when 
the oscillator is connected to normal circuits. It is therefore necessary 
to provide a structure which allows the load at harmonic frequencies to be 
easily varied in a manner practically independent from the load at the 
fundamental frequency. This adjustment must be able to be made over the 
entire tuning band of the oscillator, it being often advantageous to be 
able to make this band very wide. 
The oscillator according to the invention satisfies all the aforesaid 
requirements. It comprises, as is usual, a main cavity extending over the 
entire length of the waveguide and housing a Gunn diode mounted 
perpendicular to the direction of extension of the waveguide, and means 
for adjusting the tuned frequency of said main cavity, and further 
comprises an auxiliary cavity branching laterally from said main cavity 
which is of such dimensions as to prevent inlet of the fundamental 
component of said tuned frequency but to allow inlet of the harmonic 
components, said auxiliary cavity housing harmonic tuning means arranged 
to vary the load impedance seen by the Gudd diode at the harmonic 
frequencies by varying the reactance of said auxiliary cavity to the 
harmonic components. 
In other words, the oscillator according to the invention is based on the 
concept that by associating, with a normal resonant cavity of a waveguide, 
an auxiliary cavity capable of receiving only the harmonic frequency 
components of the signal present in the main cavity, and by suitably 
varying the reactance of said auxiliary cavity to the harmonic components, 
it is possible to vary the load impedance seen by the Gunn diode at the 
harmonic frequencies alone, while the load at the fundamental frequency 
remains unaltered and corresponding to the design load.

The oscillator shown in FIGS. 1 and 2 comprises a waveguide 1 containing a 
main resonant cavity 2 of constant rectangular cross-section. Said main 
cavity, which is closed by a mobile short-circuiting piston 3 at the 
opposite end to the connection end for the load (not shown), houses below 
a column 4 a Gunn diode 5, which is polarised by a low-pass filter 6. 
Parallel to the diode 5 and in its same transverse plane with respect to 
the extension of the waveguide, there is also disposed in the cavity 2 a 
column 7 having the same dimensions as the column 4. 
An auxiliary cylindrical cavity 9 provided with a mobile tuning piston 10 
communicates with the main cavity 2 by way of a restricted passage 8. The 
transverse dimensions of the cavity 9, possibly in combination with those 
of the passage 8, in which case completely clear, are chosen to prevent 
inlet of the fundamental component of the signal present in the main 
cavity 2, while allowing inlet of the harmonic components. An antenna wire 
11 passes through the communication passage 8 to electrically connect the 
column 7 to the wall of the auxiliary cavity 9. 
In operation, whereas the short-circuiting piston 3 (FIG. 2) enables the 
tuned frequency of the main cavity 2 to be adjusted, the harmonic tuning 
piston 10 enables the reactance of the auxiliary cavity 9 to only the 
harmonic components of the signal present in the main cavity 2 to be 
varied with the length of the auxiliary cavity 9, the fundamental 
component being prevented from entering the auxiliary cavity 9 as stated 
heretofore. This is equivalent to presenting to the double gate system 
constituted by the two pairs of facing surfaces A--A' and B--B' (FIG. 3) a 
reactance capable of assuming all possible imaginary values over the range 
(-j.infin., +j.infin.). The result is a corresponding variation in the 
load impedance seen by the Gunn diode 5 at the harmonic frequencies. 
The equivalent electrical circuit of the system shown in FIG. 1 is 
represented in FIG. 3, where X indicates the variable reactance deriving 
from the auxiliary cavity 9, P the double gate system A--A', B--B', C the 
protection capsule for the Gunn diode 5, and S the actual semiconductor 
disposed inside said capsule. 
FIG. 4 shows a modification in which the main cavity 2 is tuned by a 
varactor 12 with a superposed filter 15 mounted on the column 7, while 
FIG. 5 shows a further modification in which the main cavity 2 is tuned by 
a dielectric bar 13 and an iris 14. Experimental results obtained with 
this oscillator arrangement according to the invention have shown that 
with loads which strongly reflect at the harmonics (including the case of 
an oscillator with an iris), the output power level can be varied by 
harmonic tuning alone, by about 5 dB over the entire wide mechanical 
tuning band of the oscillator (8.2-14 GHz in the case of the waveguide 
oscillator WR-90 in the configuration with the short-circuiting piston). 
In addition, both in the case of varactor tuning and tuning by the voltage 
applied across the diode, the harmonic effect alone produces a drastic 
improvement in the linearity of the tuning (in the case of varactor 
tuning, it improved from a percentage deviation of 8% to one of 0.5%) with 
a simultaneous strong reduction in the signal amplitude variations. The 
high linearity of the modulation curves thus obtainable, which is much 
better than that which can be obtained by simply eliminating the harmonic 
components of the signal by absorption, shows the possibility of utilising 
harmonic tuning in order to minimise the effects due to intrinsic 
non-linearities in the modulation mechanism (it should be noted in this 
respect that the capacity of a varactor is proportional to the square root 
of the applied voltage). It has also been observed that harmonic tuning 
gives a variation of about 6 dB in the temperature coefficient df/dT of 
the oscillator operating frequency. When operating with mechanical 
frequency compensation--utilising materials having coefficients of thermal 
expansion which are different from each other--the facility for 
continuously varying this temperature coefficient enables complicated 
calibration procedures to be avoided and to extend the operation of a 
temperature compensated structure to a wider frequency range, and to 
devices having a less stringent statistical quality distribution. 
It is important to note that, as confirmation of the theoretical 
assumption, the power level, the electronic tuning characteristics and the 
temperature stability of the oscillator frequency can be optimised 
simultaneously. 
Finally, the same type of harmonic tuning gives substantial reduction 
(sometimes exceeding 10 dB) in the F.M. noise of the oscillator. However, 
this occurs normally for auxiliary cavities having a length different from 
that which optimises all the other parameters.