Automatic tuning of cavity klystron using sampled RF output

A processor-controlled drive motor system for tuning a cavity klystron monitors the output (amplitude-vs-frequency) of the klystron and compares that monitored performance output with an intended amplitude-vs-frequency profile. Differences between the two characteristics are employed by the processor to generate a set of tuning cavity control signals through which respective stepping motors for displacing each cavity tuning slug are driven. The processor iteratively adjusts the cavity tuner control signals in accordance with a prescribed kylstron tuning program until the monitored amplitude response is within a prescribed tolerance of a preestablished characteristic stored in memory.

FIELD OF THE INVENTION 
The present invention relates to microwave (e.g. satellite) communication 
systems and is particularly directed to an arrangement for automatically 
tuning a cavity klystron to achieve a prescribed amplitude response. 
BACKGROUND OF THE INVENTION 
Klystrons are commonly employed as a basic signal source in microwave (e.g. 
satellite link) communication systems. As such, they are required to 
exhibit a prescribed output characteristic or amplitude response (e.g. 
flatness) over an operating bandwidth centered about a selected center 
frequency. Unfortunately, the tuning mechanism through which operation of 
the klystron is controlled is an extremely sensitive mechanism that does 
not offer the repeatability desired of signal control devices. 
Specifically, a klystron cavity tuner typically consists of a plurality of 
copper cavities and associated tuning slugs which are displaced back and 
forth in their respected cavities to establish the operational 
characteristics of the klystron. Usually, each slug is wrapped with a 
tungsten wire to assure a tight fit in its cavity. As tungsten is a 
considerably harder metal than copper, repeated movement of the tuning 
slug will wear down the wall of the cavity, thereby changing its 
dimensional tolerances and, consequently, its intended operational 
characteristics. 
Because of the mutual interdependence of the tuning of the respective 
cavities, a klystron cannot be tuned by simply adjusting each tuning slug 
in an arbitrary order to a preestablished setting. Instead, control of the 
amplitude response of a klystron must be carried out by repeated back and 
forth adjustment of each tuning slug, through the use of a respective 
vernier (micrometer) adjustment knob for each slug, the rotational setting 
of which is graduated according to a prescribed tuning (number) chart. In 
a typical terminal environment, the klystron is housed in a protective 
equipment cabinet, access to the tuning elements of which is accomplished 
by way of a panel door. When tuning the klystron, the terminal operator 
rotates a roller chart to view the number settings to which the slug 
tuning knobs must be set, unlocks the knobs from their current positions, 
and then proceeds to tune the klystron, adjusting the knobs in a 
prescribed sequence and in accordance with the strict number settings of 
the tuning chart. If a setting is exceeded, even only slightly, the tuning 
adjustment must be backed off considerably and the procedure reinitiated 
which eliminates mechanical backlash. It may be appreciated, therefore, 
that errors in operator accuracy involving conventional mechanical 
adjustment mechanisms can add excessive tolerances to an already critical 
procedure. In fact, it has been found the amplitude response of a klystron 
tuned by two different operators under the same conditions will seldom be 
the same for both operators. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the effective non-repeatability 
of tuning cavity klystrons with conventional hand manipulated mechanical 
elements following a time consuming and considerably inexact 
"by-the-numbers" procedure is obviated by a processor-controlled drive 
motor system which monitors the output (amplitude-vs-frequency) of the 
klystron and compares that monitored performance output with an intended 
amplitude-vs-frequency profile. Differences between the two 
characteristics are employed by the processor to generate a set of tuning 
cavity control signals through which respective stepping motors for 
displacing each cavity tuning slug are driven. The processor iteratively 
adjusts the cavity tuner control signals in accordance with a prescribed 
klystron tuning program until the monitored amplitude response is within a 
prescribed tolerance of a preestablished characteristic stored in memory. 
Because the tuning of the klystron is based upon monitoring its 
performance, rather than according to a "by-the-number" chart sequence, 
considerably improved accuracy of klystron operation over the 
operator-controlled approach is afforded. Moreover, because operator 
intervention is removed, human error is eliminated. In effect, the present 
invention assures repeatability of performance, over successive 
adjustments.

DETAILED DESCRIPTION 
Before describing, in detail, the particular improved klystron cavity 
tuning scheme in accordance with the present invention, it should be 
observed that the present invention resides primarily in a novel 
structural combination of conventional signal processing and motor drive 
circuits and not in the particular detailed configurations thereof. 
Accordingly, the structure, control and arrangement of such conventional 
circuits have been illustrated in the drawings by readily understandable 
block representations and schematic diagram, which show only those 
specific details that are pertinent to the present invention, so as not to 
obscure the disclosure with structural details which will be readily 
apparent to those skilled in the art having the benefit of the description 
herein. In addition, various portions of the signal processing circuitry 
(including data processor) have been appropriately consolidated and 
simplified in order to emphasize those portions that are most pertinent to 
the present invention. Thus, the block diagram illustrations of the 
drawings do not necessarily represent the mechanical structural 
arrangement of the exemplary system, but are primarily intended to 
illustrate the major structural components of the system in a conventional 
functional grouping, whereby the present invention may be more readily 
understood. 
Referring now to FIG. 1, there is shown a schematic block diagram of a 
cavity klystron tuning system in accordance with the present invention. In 
the exemplary embodiment, a klystron power amplifier 11, such as a 
VKX-7780F-1, five cavity klystron, manufactured by Varian, has its output 
coupled to an RF output microwave link 12. By way of a coupler 13, the RF 
output of the klystron 11 is coupled over link 14 to a digitally 
controlled spectrum analyzer 15 whereby the output of the klystron 11 may 
be monitored. For this purpose, spectrum analyzer 15 may comprise a 
Hewlett Packard 8566 RF spectrum analyzer which has an output (IEEE-488) 
data bus 17 for supplying all of the information pertaining to the RF 
signal on input link 14 being monitored. A pictorial illustration of an 
exemplary amplitude versus frequency of the output of the klystron as 
monitored by spectrum analyzer 15 is represented in FIG. 1 by the enlarged 
portion of the display screen 16 adjacent analyzer 15. The amplitude 
response data from the spectrum analyzer is coupled over data bus 17 to a 
processor 21 which operates, in effect, as a smart motor controller for 
operating an assembly 26 of tuning motors for the five cavities of the 
klystron. Processor 21 includes customary I/O buffer circuitry, central 
processing unit and associated memory in which the control program for 
operating the drive motors in accordance with the amplitude response as 
monitored by the spectrum analyzer 15 is stored. Output signals for 
controlling assembly 26 of drive motors for the cavities of the klystron 
are coupled over link 22 to a set of motor drive amplifiers 23. The 
outputs of amplifiers 23 are coupled over link 24 to stepping motor 
assembly 26 which contains (five) respective stepping motors for 
controlling the displacement of a set of drive rods or shafts 27 for the 
cavity tuning slugs of the klystron. In order to monitor the displacement 
of each of the drive shafts in response to the action of the stepping 
motors, a set of shaft position encoders 25 is provided. The output of 
each encoder 25 is coupled over link 27 to supply the processor 21 with an 
indication of the position of each shaft, and thereby the location of each 
tuning stub within the klystron cavity. 
In operation, a tuning program, to be discussed below, for controlling 
stepping motors of the tuning motor assembly 26 and thereby the 
displacement of the tuning slug shafts 27 of the klystron, is loaded into 
memory of processor 21. 
In order to properly tune each of the cavities to achieve the desired 
response, the correction program stored in memory of processor 21 is 
prepared using the instructions provided by the klystron manufacturer for 
each of the cavities of the klystron. The manufacturer will supply a data 
sheet indicating how displacement of the cavity tuner will effect the 
overall response produced by the tube. As an example, for the 
above-referenced klystron type VKX-7780F-1, manufactured by Varian, the 
following cavity response conditions are defined: 
Cavity No. 1: displacement of the tuning stub to increase the frequency of 
the cavity will cause the output response of the klystron to tilt to the 
high end of the band; conversely, a decrease in frequency for the cavity 
will tilt the response of the klystron towards the lower end of the band; 
Cavity No. 2: this cavity is initially tuned to broaden the response when 
going from high efficiency tuning to broad-band tuning. Once broad-band 
tuning is obtained, this cavity is employed to both broaden the response 
(primarily at the lower frequency end of the band) and to make adjustments 
for power output and gain; 
Cavity No. 3: this cavity is initially used to broaden the response when 
going from the high efficiency tuning to broad-band tuning. When 
broad-banded, this cavity will affect the high end of the band. When tuned 
higher in frequency, the bandwidth at the high frequency end will increase 
with a slight reduction in power level at the high end of the band. This 
cavity is normally adjusted in conjunction with cavity No. 4. To 
compensate for the reduction in power output slightly when cavity No. 3 is 
increased in frequency, cavity No. 4 should be moved slightly lower in 
frequency; 
Cavity No. 4: cavity No. 4 is initially used to obtain power when going 
from a synchronous tuning condition to a high frequency tuning condition. 
Once the tube has been broad-banded, the cavity will affect primarily the 
power level at the high frequency end of the response with a lesser effect 
on the bandwidth at the high end of the band; 
Cavity No. 5: cavity No. 5 has effectively the same impact on the response 
output as cavity No. 1, except that it has a greater effect on the high 
frequency end of the response. 
Given such a description of the functional effect of each cavity tuner for 
the particular klystron of interest, a control program is prepared to map 
its amplitude response into a sequence of control operations for each of 
the cavity tuners. In so doing, the control program stored in processor 21 
continuously compares the output response of the klystron 11 as monitored 
by spectrum analyzer 15 with the intended characteristic contained in the 
program and uses differences between the two, namely the difference 
between sought-after and actual amplitude response, to drive the stepping 
motors for the respective cavity tuners. 
As an illustration, consider the set of response curves shown in FIGS. 2-5 
for the above-mentioned VKX-7780F-1 type klystron, which differ from a 
sought-after flat response symmetrically centered about a center frequency 
e.g. Fc=8.0 GHz. 
Tuning of the klystron is initiated by a coarse tuning procedure wherein 
each of the cavity tuning drive motors of assembly 26 is caused to be 
rotated to a predetermined position corresponding to a prescribed 
frequency. As noted above, "coarse-tune" information is supplied from the 
klystron manufacturer, indicating an initial displacement of the tuning 
stubs for the frequency of interest. Using that information, the settings 
of the stepping motor encoders 25 are calibrated to provide the processor 
21 with a reference position from which to start. As an example, 
considering the above-referenced center frequency of 8.0 GHz, the tuning 
shaft encoders for each of the five cavities of the klystron may 
correspond to the values: Cavity No. 1=30; Cavity No. 2=26; Cavity No. 
3=31; Cavity No. 4=21; and Cavity No. 5=16. 
The number of revolutions for each cavity tuner is determined by starting 
the count of the encoders 25 from a full counter-clockwise position (zero) 
or against the klystron mechanical stop for each tuning shaft 27. Whenever 
a klystron is inserted or replaced, the tuning shafts are tuned to zero to 
assure that each encoder's position correctly corresponds to that 
location. As a result, when the center frequency is to be changed, the 
tuners do not have to be returned to zero. Its associated encoder 25 
indicates the relative position and starts, or remembers, the count from 
that point. 
Having initially set the klystron tuner positions at the coarse locations 
provided by the manufacturer, klystron 11 is turned on to provide an 
initial or coarse output characteristic over line 14 to spectrum analyzer 
15. In accordance with the program stored in processor 21, with the 
klystron now being coarse-tuned, the next step is to obtain the maximum 
output power from the klystron 11. This is achieved by step tuning each 
cavity. The tuning procedure stored in the memory of processor 21 begins 
with cavity No. 1, coupling a signal over link 24 to its associated drive 
motor 26 causing the motor to step in a prescribed direction. If the RF 
output power over link 12 increases from the klystron, processor 21 causes 
the drive motor to be stepped further in the same direction. On the other 
hand, if the output power had decreased, the motor is driven two steps in 
the opposite direction to cause a power increase. Once the output power of 
the klystron has increased 1 dB for the cavity of interest, processor 21 
proceeds to the stepping motor for the next cavity, namely cavity No. 2 
and carries out the same prodedure that it carried out for cavity No. 1. 
This process is repeated for all five cavities and then begins again at 
cavity No. 1, repeating the above procedure to increase the output power 
by an additional increment of 1 dB for each cavity. This iterative advance 
of the stepping motors 26 is carried out until maximum power, as monitored 
by spectrum analyzer 15 and processor 21, is achieved. Maximum power is 
recognized when the last step for the stepping motor for each cavity of 
interest causes a decrease in the output power. At this point, processor 
21 steps the motor back to its previous position prior to the detected 
decrease in klystron output power. 
Once maximum RF output power from klystron 11 has been established using 
the above sequence, spectrum analyzer 15 would detect an amplitude 
response curve on either side of the center frequency Fc=8.0 GHz. That 
characteristic is digitized and supplied to processor 21 over bus 17. The 
resultant pattern is compared in processor 21 with a desired 
characteristic, as stored in memory, and processor 21 next proceeds to 
adjust the cavity tuners (via stepping motor assembly 26) until the output 
characteristic), as monitored by spectrum analyzer 15 falls within a 
prescribed tolerance or threshold of the characteristic stored in memory 
of processor 21. 
As examples of this operation, let it be assumed that the desired output 
amplitude response of klystron 11 is a flat response substantially equally 
distributed about some center frequency (e.g. Fc=8.0 GHz). FIG. 2 
illustrates the condition in which there is a "glich" or "wrinkle" at the 
high end of the amplitude response. In this circumstance, the program 
stored in processor 21 causes the stepping motor for cavity No. 4 to be 
rotated in a direction which would slightly increase the frequency to 
flatten out the upper portion of the curve. 
FIG. 3 shows an exemplary klystron response in which there is a hole or 
depression in the central part of the response at a small signal level. In 
this circumstance, the processor causes the stepping motor for cavity No. 
1 to be rotated in a direction to increase the frequency, while that for 
cavity No. 2 is displaced to lower the frequency for that cavity. 
The response in FIG. 4 illustrates an acceptable and flat response at the 
lower end of the bandwidth but an insufficiently large response at the 
high end of the bandwidth. In this circumstance, processor 21 drives the 
motor to displace the tuning rod for cavity No. 3 to a position causing a 
higher frequency for cavity No. 3 and a lower frequency for cavity No. 4. 
FIG. 5 illustrates a response curve having a 40 MHz bandwidth but not 
equally centered on each side of the center frequency. In this case, 
processor 21 drives the stepping motors for all of the cavities to 
slightly increase the frequency until the curve shifts. Depending upon the 
resultant characteristic monitored by spectrum analyzer 15, further 
displacement of the drive shafts of the output of the stepping motors is 
conducted until the response curve is flattened and centered about the 
center frequency. 
As will be appreciated from the foregoing description, the 
processor-controlled drive motor system of the present invention provides 
a mechanism for automatically and precisely tuning a cavity klystron that 
does not suffer from the cumbersome and inexact procedure conventionally 
employed by a terminal operator. By monitoring the amplitude vs frequency 
of the klystron as it is being tuned, the system of the present invention 
is able to adapt its iterative control procedure to rapidly bring the 
output characteristic to within a prescribed tolerance. 
While I have shown and described an embodiment in accordance with the 
present invention, it is understood that the same is not limited thereto 
but is susceptible of numerous changes and modifications as known to a 
person skilled in the art, and I therefore do not wish to be limited to 
the details shown and described herein but intend to cover all such 
changes and modifications as are obvious to one of ordinary skill in the 
art.