Method and device for reducing beam current modulation caused by mechanical vibrations in a TWT

A method and a device for reducing the modulation of the beam current in a travelling wave tube caused by mechanical vibrations, in which the modulation of the beam current is measured and compared with a reference value. The difference signal generated by the comparison affects the grid bias of the travelling wave tube in such a way that the modulation of the beam current is opposed.

BACKGROUND 
The present invention relates to a method and a device that reduces the 
influence of the mechanical environment on the electrical properties of 
travelling wave tubes. 
The Travelling Wave Tube--TWT--is a wideband amplification tube for signals 
within the microwave range. It is used, for example, in certain types of 
radio links and in radar stations. In transmission-coherent doppler radar 
systems it is usual to utilize grid pulsed TWTs as transmission pulse 
amplifiers. In these systems the transmitted spectral purity of the signal 
is of great importance for, among other things, the ability of the system 
to suppress clutter signals. A TWT is however influenced by the mechanical 
vibration environment and in particularly exposed environments, as in 
aeroplanes or target tracking missiles, influence of vibrations can 
seriously impair the performance characteristics of the system. 
The source of the problems is when the TWT is subjected to mechanical 
vibrations, movements between the electrodes of the electron gun will 
occur. These movements result in the current beam through the tube 
receiving a modulation at the vibration frequency. For frequencies where 
there exist mechanical oscillatory resonances in the electron gun, the 
current modulation becomes considerable. 
The current modulation caused by the vibration leads to the microwave 
signal amplified in the TWT obtaining an amplitude and phase modulation 
that is proportional to the current modulation. The microwave signal will 
therefore obtain side bands located at a distance equal to the vibratory 
frequency on both sides of the carrier frequency. In radar systems this 
gives rise to deteriorated clutter suppresion for target speeds that, from 
a doppler point of view, correspond to the vibration frequencies. The 
usual way to get around the problems of current modulation is to isolate 
the tube, or the complete transmitter, or even the complete radar station, 
from vibrations. Such measures are however complicated and voluminous and 
do not always give a satisfactory result. In certain applications, such as 
target tracking missiles, it is, from a space point of view, hardly 
possible with such measures. 
In certain applications it can be possible to avoid these difficulties by 
replacing the TWTs with other components that are less sensitive to 
vibrations with regard to spectral purity. Injection locked magnetrons or 
cross field amplifiers can, by way of example, be used. These components, 
on the other hand, have inferior high frequency characteristics with 
respect to bandwidth, pulse formation and noise and due to this, other 
system performance characteristics deteriorate as well. 
SUMMARY 
An object of the present invention is to reduce the negative influence on 
the electrical performance characteristics of a TWT, that occur due to the 
modulation of the current beam by the mechanical vibrations that the tube 
is subjected to, but without using space-demanding mechanical devices for 
vibratory isolation. 
Said object is achieved by means of a method and a device, by means of 
which the modulation of the current beam is measured and compared with a 
reference value. The difference signal that is formed during the 
comparison, influences the grid bias of the TWT and accordingly the beam 
current, so that its modulation is counteracted. 
The modulation of the beam current is measured by measuring the cathode 
current or collector current of the tube, measuring the modulation of the 
high frequency signal that has been amplified in the tube etc. 
By making use of electrical signals for measurement of modulation and for 
control of the TWT the invention eliminates the need for space-demanding 
mechanical devices for vibratory isolation and achieves a solution that 
can also be applied in devices with limited available space.

DETAILED DESCRIPTION 
With reference to FIG. 1 the invention will now be described in the form of 
an exemplary embodiment. 
Reference numeral 1 in the drawing denotes a Travelling Wave Tube (TWT). 
The tube is provided with a collector connection 2, a cathode connection 
3, an anode or body connection 4 and a grid connection 5. The helix 
structure (or other equivalent structure) 6 of the tube is also shown 
symbolically. The TWT can also be provided with more collectors, but as 
this has no significance for the present invention, only one collector is 
depicted in the drawing. 
Those voltage sources necessary for the operation of the tube are 
constituted by a collector voltage source 7, connected between the 
collector connection 2 and, via the block 9, the cathode connection 3. If 
the TWT has more collectors, then each one of these are connected to a 
voltage source. A cathode voltage source 8 is connected between the body 
connection 4 of the tube and, via the block 9, the cathode connection 3. 
The body connection of the tube is in most cases "grounded". Further there 
is a grid voltage source 10 which, via block 9 and block 11, is connected 
between the cathode connection 3 and the grid connection 5 respectively. 
The block 11 is constituted by a device that influences (modulates) the 
grid voltage in dependence upon a control signal from a control device 
(not shown), connected to the point 12. The control signal can, by way of 
an example, be a pulse train whereby the current through the TWT (and 
therefore also its output signal) can be pulse modulated. In certain 
applications even the collector is earthed, whereby the cathode and 
collector voltage sources can be replaced by a common voltage source. 
The function of the travelling wave tube is well known for a man skilled in 
the art and will therefore only be briefly touched upon. In the tube, a 
well focused electron beam that travels through a wire spiral, a so called 
helix, is generated by an electrode system (electron gun). The fundamental 
property of the travelling wave tube is, by means of an interaction 
between the electron beam through the helix and a signal ("wave") that 
propagates alongside it, to transfer the kinetic energy of the electrons 
to the wave that is thereby amplified. It should be pointed out that the 
helix can be replaced by a series of connected cavities. This is mostly 
common in tubes intended for high power. 
As was mentioned in the beginning, the electron beam can be modulated by 
means of external influence, for example mechanical vibrations. This 
modulation will influence the wave, due to an interaction between the 
electron beam and the wave. The mechanical vibrations will thus cause an 
undesired modulation of the amplified signal. 
Since the current--the beam current--which the electron beam represents, 
constitutes a part of the current through the tube, the modulation of the 
electron beam can be measured as modulation of the current--cathode 
current 13--which passes through the tube. The cathode current is 
comprised mainly of two components: the body or helix current 15 through 
cathode voltage source 8 and the collector current 14 through collector 
voltage source 7. 
As is evident from FIG. 1 the cathode current 13 passes the block 9. This 
block comprises devices for measuring the cathode current and can be 
constituted by a current transformer, a serial resistance over which a 
voltage is measured or some other type of device that can indicate the 
current strength. Alternatively, which can be deduced from that which has 
been written previously, the body/helix current 15 and the collector 
current 14 can be measured and thereafter added. The added values 
correspond to the cathode current 13. If the TWT is provided with more 
collectors then the different collector currents have to be added of 
course. 
As the collector current, in travelling wave tubes which work well, is 
several times larger than the body current, it is sufficient in many 
applications to only measure the collector current to determine the 
modulation of the beam current. 
In the cases where the travelling wave tube is used in pulsed operation the 
beam current is sampled during the pulse, for instance with a sample and 
hold circuit. By comparing the measured/sampled value of the cathode 
current with a reference value, a difference signal is formed. This 
difference signal is transferred to the block 11 for control (modulation) 
of the grid voltage. The beam current will thereby be affected and, by a 
suitable choice of "sign" or "phase" of the difference signal, the 
modulation of the beam current can be counteracted. The feedback of the 
cathode current to the grid bias implies a control loop that strives to 
bring the modulation of the beam current towards zero. 
It is thus possible with the described device to sense the modulation of 
the beam current caused by the mechanical vibrations and, by means of 
control of the grid bias of the tube, to reduce this modulation. 
As an alternative to measuring the modulation by means of the cathode 
current, even the high-frequency, amplified signal can be measured. In 
this case as shown in FIG. 2, a part of the signal is coupled, with a 
directional coupler or corresponding other device 16, to a quadrature 
detector 17 where the high frequency signal is brought back to the base 
band. The modulation signal caused by the vibrations can then be filtered 
out in a band-pass or low-pass filter 18. After comparison 19 with a 
reference value 20, the so-called difference signal be transferred to the 
block 11 for, as an analogy to what has been described earlier, control of 
the grid bias. Further it should be mentioned that instead of controlling 
the grid bias it is possible to control a device for amplitude and phase 
modulation of the input signal to the tube. It should however be mentioned 
that these embodiments of the present invention assume that the travelling 
wave tube works with a sufficiently low input signal so that the tube is 
not saturated. 
The invention is not limited to the described embodiments, but may be 
varied within the scope of the appended claims.