This article describes the techniques used and experimental results obtained in improving transmitter stability by control of the klystron body temperature. Related work in the measurement of klystron phase control parameters (pushing factors) is also discussed. The contribution of waveguide temperature excursions to uplink phase stability is presented. Suggestions are made as to the direction of future work in this area

Perez, R. M.

English

NASA Technical Reports Server

TDA Progress Report 42-t09 May 15, 1992
/j_. =:oD
N92-29373
Improvements in X-Band Transmitter Phase Stability
Through Klystron Body Temperature Regulation
R. M. Perez
Radio Frequency and Microwave Subsystems Section
This article describes the techniques used and experimental results obtained
in improving transmitter stability by control of the klystron body temperature.
Related work in the measurement of klystron phase control parameters (pushing
factors) is also discussed. The contribution of waveguide temperature excursions
to uplink phase stability is presented. Suggestions are made as to the direction of
future work in this area.
I. Introduction
Long-term spacecraft tracking-link phase stability is an
important parameter in the detection of gravity waves
by Doppler methods. This link phase stability require-
ment dictated reducing the phase stability contribution of
the DSN 20-kW X-band (7.167-GHz) uplink microwave
transmitters to an Allan deviation of 5 x I0 -15 for a
1000-second averaging period. This requirement was met
in the DSN 20-kW X-band transmitters 1 by closely reg-
ulating the klystron cathode voltage. It was desired to
decrease this figure by an order of magnitude to 1 x 10 -I6
to allow future operation with improved phase stability.
To achieve this goal a two-part approach was followed,
which consisted of identifying and measuring the magni-
tude of the klystron phase-pushing factors and control-
1 R. Perez, "DSS 15 20 kW X-band Transmitter Test Results Re-
gaxding Stability Requirements," Interoffice Memorandum 3337-
89-098 (internal document), Jet Propulsion Laboratory, Pasadena,
California, October 3, 1989.
ling those found to be significant contributors. Most prior
transmitter phase stability improvement work has been
concentrated on improving the klystron cathode (beam)
voltage regulation, since the large effect of this parameter
on the phase of the transmitter output signal is well estab-
lished [1]. Efforts in this area have succeeded in regulating
the beam voltage to about 0.01 percent over an 8-hour
period and to nearly 0.003 percent over 1000 seconds, re-
sulting in a beam voltage-induced phase shift on the order
of 0.1 degree over eight hours of operation. In view of this,
it was decided to concentrate on controlling other causes
of phase instability.
II. Measurement of Phase-Pushing Factors
To establish the magnitude of the phase-pushing fac-
tors of the Varian 876P klystron used in the DSN 20-kW
X-band transmitters, a measurement system (see Fig. 1)
was configured. This system allows the independent vari-
ation of klystron operating parameters and the measure-
114
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ment of subsequent changes in output signal phase. An
IIP 8510A network analyzer was used as the phase detec-
tor in conjunction with a coaxial switch, which enabled
sampling of the phase of the klystron input and output
signals and of a reference coaxial line. In this manner
a differential phase measurement was made by using the
phase of the reference line signal and the klystron input
signal to normalize the phase of the klystron output sig-
nal. This resulted in the removal of any output signal
phase fluctuations due to input signal or instrumentation
instabilities from the phase-pushing factor data. Measure-
ment of klystron operating parameters and temperatures
was performed coherently with each phase measurement to
permit later analysis. It should be noted that this config-
uration enabled control of the klystron body coolant tem-
perature independent of the remaining cooling circuits and
permitted the direct measurement of the klystron body
temperature phase-pushing factor for the first time.
The magnitude of output signal phase fluctuation due
to stimulus change was then measured for the following
klystron parameters: beam voltage, drive RF level, body
temperature, filament current, and focusing electromag-
netic current. The results of these measurements may be
found in Table 1.
III. Measurement of Phase Stability
After performing the pushing-factor measurements
above, it was found that the klystron body temperature
fluctuations showed the largest phase pushing of the pa-
rameters described above, given the typical operational
variations encountered for each parameter. To stabilize
the temperature of the klystron body, a refrigerated chiller
(Neslab HX-75) capable of regulating the coolant temper-
ature to 0.1 deg C was purchased. A system was then
assembled (see Fig. 2) to perform long-term transmitter
phase stability measurements (see [2] for a description of
the stability measurement system-operating principles).
Phase stability measurements were then performed un-
der two conditions: cooling the klystron body with the
system heat exchanger and independent body cooling with
the Neslab IIX-75 refrigerated chiller. The results of these
measurements may be found in Fig. 3 in the form of Allan
deviations.
IV. Discussion of Results
The measurement of the klystron pushing factors re-
vealed some significant points:
(2)
'(6)
(1) The klystron output signal phase is most strongly
affected by fluctuations in the beam voltage, with a
value of 45 deg/kV; however, given the tight regu-
lation of tile beam power supply in the DSN 20-kW
X-band transmitters, the beam voltage phase-push-
ing contribution becomes secondary to other factors.
No phase pushing was detected due to changes in
the klystron-focusing electromagnetic current or fil-
ament current. This should not be taken to mean
that phase pushing by these parameters is not pos-
sible; rather, no phase change was observed due to
a +10 percent change in focusing current and fila-
ment current, and a phase measurement resolution
on the order of 0.1 degree. Wider excursions in these
parameters carry a risk of exceeding the operating
ratings of the klystron with ensuing damage. Nor-
mal drift of these parameters in operation is on the
order of 1 to 2 percent; thus, it is unlikely that fil-
ament or focusing-current instabilities significantly
affect overall transmitter stability.
(3) The temperature of the klystron body is a strong
phase-pushing factor (see Fig. 4), with a value of
0.9 deg/deg C. It can be seen that given typical heat
exchanger temperature fluctuations in the 5-deg-C
range, this becomes the dominant klystron output
phase-pushing factor in the presence of a tightly reg-
ulated beam voltage.
(4) A relationship was noticed between the output sig-
nal phase and the temperature of the transmitter
output waveguide coolant. An analysis of the phase
shift due to thermal expansion of the waveguide was
performed (see Appendix), which confirmed this hy-
pothesis. The expected phase pushing by wave-
guide temperature fluctuations agrees closely with
the measured values (see Fig. 5).
(5) Cooling the klystron body independently with the
Neslab HX-75 chiller resulted in a marked improve-
ment in transmitter phase stability, particularly in
the l0 s- to 104-second region critical to gravity wave
detection [3]. At a 1000-second averaging time, the
Allan deviation changed from _ = 3.25 x 10 -16 with-
out body temperature control to a = 5.88 × 10 -17
with body temperature control (see Fig. 3).
A simulation was performed to investigate the ex-
tent to which the klystron pushing factors thus far
identified account for the overall transmitter phase
stability performance. The point-to-point variation
in each parameter (beam voltage, body temperature,
and waveguide temperature) from the test time se-
ries record was multiplied by its respective phase-
pushing factor to obtain the phase step produced by
115
changes in each parameter. These phase changes
were added algebraically to produce a composite
phase file, the Allan variance of which was com-
puted. It is postulated that the difference between
the Allan variance of the synthetic phase file and
the Allan variance measured for the whole trans-
mitter is due to phase-pushing factors other than
the variations in klystron beam voltage, body tem-
perature, and waveguide temperature. A compari-
son of the results may be seen in Fig. 6. Note that
the performance of the simulated transmitter (using
only beam voltage and body and waveguide temper-
atures) is significantly superior to that of the actual
transmitter, particularly for averaging times greater
than 10 seconds. This strongly suggests that future
gains in long-term transmitter phase stability are to
be made by concentrating on components other than
the klystron (the klystron filament and focus cur-
rents have no measured impact on output phase, as
mentioned above). For example, examination of the
phase stability of the RF control components and
driver amplifier regarding temperature and possibly
control voltages is likely to be fruitful. The ampli-
tude stability of these components should also be ex-
amined, due to amplitude modulation-phase modu-
lation (AM-PM) conversion in a saturated klystron.
V. Conclusion
The phase-pushing factors of the Varian 876P klystron
used in the DSN 20-kW X-band transmitters have been
measured. Tight temperature regulation of the klystron
body temperature has resulted in appreciably improved
transmitter phase stability, surpassing the goal of _ =
1 x 10 -16 to 5.88 x 10 -IT for the 1000-second averaging
time set for the task. The effect of waveguide temperature
changes on the phase of the transmitter output signal has
been identified and verified analytically. Some promising
areas for future work in transmitter stability improvement
have been suggested.
References
[1] R. Kolby, "Evaluation of the VA876P Klystron for the 20-kW X-Band Uplink
Transmitter," DSN Progress Report _2-5_, vol. September-October, Jet Propul-
sion Laboratory, Pasadena, California, pp. 41-50, September 1979.
[2] B. Conroy and D. Le, "Simultaneous Measurement of Allan Variance and Phase
Noise Spectrum at Fractions of a Millihertz," Review of Scientific Instruments,
vol. 61, no. 6, pp. 1720-1723, June 1990.
[3] S. Nelson and J. Armstrong, "Gravitational _Vave Searches Using the DSN,"
TDA Progress Report _2-9_, vol. April-June 1988, Jet Propulsion Laboratory,
Pasadena, California, pp. 75-85, August 15, 1988.
116
Table 1. Phase-pushing factors for the Varlan 876P klyatron.
Parameter Phase-pushing factor
Beam voltage, deg/V 0.045
Drive level, deg/dB 2.3
Body temperature, deg/deg C 0.9
Filament current None measured
Focusing current None measured
117
COMPUTER
CONTROLLER
IEEE-488_V_ j_
HP 8510A
NETWORK ANALYZER
REFERENCE TEST
HP 59306A
RELAY
ACTUATOR
DRIVER
AMPLIFIER
KLYSTRON
Fig. 1. Phase-pushing factor measurement system.
SIGNAL
GENERATOR
HP 83660A
u_T
ST POINT
xl
AMPLIFIER
t t
LOW-PASS I I HP 3852A
FILTER _1 DATA0.5 Hz COLLECTION
I COMPUTERCONTROLLER
Fig. 2. Phase stability measurement system.
PHASE
SHIFTER
118
-13
-14 [
_-15
g
_-16
W
o
z -175
J
-18
I I I I
WITHOUT
- - _ __7_._ STABLE -
- _ _ W,TH-
-_C_. . _ ._,oSTABLE
_ _c'__ _ BODY
 .Lo,O
-19 I I I I
0 1 2 3 4 5
LOG _, sec
Fig. 3. Transmitter phase stability Improvement.
<
T
O_
ELi
>
W
r,r
2 I I i I I t
-- MEASURED
--- CALCULATED
1
i" f'_
i J
t i / •
01 ' ,', s
-1 , /,
-2
r
•-31 I 1 ] I I I
0 10 20 30 40 50 60
TIME, rain
Fig. 5. Phase shift due to wavegulde lempereture flucluations.
7O
15
10
-o
ui
cO
<_
I
o_ 5
IJJ
>
UJ
n-- 0
-5
-20
I I I i
I I I I
-15 -10 -5 0
RELATIVE BODY TEMPERATURE, deg C
Fig. 4. Klystron body temperature pushing of phase.
-131 t l I 1
-14(_
_-16 D -
NOISE
18 - "_X _ FLOOR _
I
SYNTHESIZED _
-19 I I I 1
0 1 2 3 4
LOG _, see
Fig. 6. Comparison of measured and synthesized
Allan variances.
119
Appendix
Phase Shift of a Signal Due to Waveguide Temperature Changes
The guide wavelength of a signal in a rectangular wave-
guide and TEI,o mode is
)ti = )to (A-l)
- k_--2
where
which results in a new guide wavelength:
)tO
)t.t' = (A-4)
i )to 21 - (2A _---2AA)
Likewise, the length of the waveguide section changes by
)t0 = free space wavelength
A = waveguide width
The phase shift for a waveguide of length l is
Al = IpAT (A-5)
The expanded waveguide has a phase shift of
¢i = 360_i deg (A-2) 3601 + AI¢I = _ deg (A-6)
Given a thermal expansion coefficient for the waveguide
material and a temperature change of AT, the resulting
waveguide width change is
AA = ApAT (A-3)
where
The phase shift induced by the temperature change is then
A¢ = ¢! - ¢i (A-7)
By substituting Eqs. (A-2) and (A-4) and simplifying,
p = thermal expansion coefficient in ppm/C
AT = temperature change in deg C A¢ = 360 (/+ A/ I))t] _ deg
(A-8)
120


Improvements in X-band transmitter phase stability through klystron body temperature regulation

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