Results of recent measurements of the long-term phase, amplitude and group delay stability of the 400-kW S-band (2.115-GHz) transmitter are reported. Various control parameters which are responsible for many of the observed instabilities are identified. Further tests to identify the parameters responsible for the remaining instabilities are suggested

Bhanji, A. M.

Hoppe, D. J.

English

NASA Technical Reports Server

TDA Progress Report 42-87 July- September 1986 
Long-Term Amplitude and Phase Stability of the 400-kW 
2.1 15-GHz Transmitter 
D. J. Hoppe and A. M. Bhanji 
Radio Frequency and Microwave Subsystems Section 
Results of recent measurements of the long-term phase, amplitude and group delay 
stability of the 400-kW S-band (2.115-GHz) transmitter are reported. Various control 
parameters which are responsible for many of the observed instabilities are identgied. 
Further tests to identify the parameters responsible for the remaining instabilities are 
suggested. 
1. Introduction 
determine the stability of the 400-kW S-band (2.1 15-GHz) 
transmitter over a typical track time are described. Of particu- 
lar importance are the amplitude. phase, and group delay sta- 
bility of the transmitter system. In addition to showing that 
was controlled by an HP Series 200 computer. and used to 
drive the 400-kW transmitter. Measurements of the transfer 
The Of measurements which were taken to function through the transmitter and several other transmitter 
parameters were monitored through an Hp data acquisition 
unit and the existing monitoring system. Data was acquired 
at regular intervals and stored on a disk for later processing. 
. 
the transmitter meets the specifications which have been ’ 
placed on it, the tests also show which of the various control 
parameters of the transmitter (e.g., beam voltage, temperature) 
are responsible for the observed instabilities. A brief descrip- 
tion of the experiment is given. and plots of some of the more 
important results are presented. The results are interpreted. 
and some suggestions for improving the transmitter’s stability 
are discussed. Further tests to verify these improvements are 
described. 
II. Experimental Setup 
The interconnection of the various test equipment used to 
monitor the transmitter parameters during the simulated track 
period is depicted in Fig. 1 .  An HP 8510 network analyzer 
The transmitter was operated continuously for approxi- 
mately a 4-h period with no operator intervention except in 
the case of an automatic emergency shutdown (e.g., crowbar). 
Data was gathered at approximately 30-s intervals throughout 
the 4-h period. The gain ana phase through the transmitter 
system were measured by the network analyzer at 2.114, 
2.1 15. and 2.1 16 GHz for each data collection time. The phase 
and amplitude data taken by the network analyzer was 
obtained by averaging 5 12 measurements at each frequency of 
interest. In addition, the existing transmitter monitoring sys- 
tem was interfaced to  the HP computer through an HP data 
collection system. This allowed for the monitoring of a num- 
ber of auxiliary transmitter parameters throughout the experi- 
ment. In particular, klystron beam voltage. beam current. 
filament current, filament voltage. vacuum current. magnet 
81 
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current. body current. and collector current were monitored 
throughout the simulated track. The inlet temperature of the 
coolant used to cool the klystron and water load was also 
measured using a separate thermometer. Transmitter output 
power at 2.115 GHz. reflected power at 2.1 15 GHz. and drive 
power to the klystron at 2.115 GHz were also recorded. Using 
the phase data collected at 2.114 and 2.116 GHz, the group 
delay at 2.115 GHz was calculated. The transmitter was 
operated in a saturated mode with a nominal output power 
of 400 kW. 
111. Results 
Figures 2 through 5, 6(a), and 7(a) summarize the long- 
term behavior of some of the transmitter parameters. Fig- 
ure 6(a) shows transmitter output power as a function of time 
over the 250-min experiment time. The sudden drop in output 
power at approximately sample number 150 was caused by 
an automatic firing of the crowbar system. The transmitter 
was then brought up to  full power as soon as possible, and the 
experiment was resumed. The starting value for the output 
power was 397.2 kW (saturated). The maximum power observed 
during the experiment was about 15 kW above the starting 
value; the minimum was about 6 kW below the initial output 
power. These values are well within the specified limits for the 
transmitter, which are +135 kW and -106 kW (k1.3 dB). 
The phase stability of the transmitter system, from the out- 
put port of the HP 8510 network analyzer to  the input port, is 
depicted in Fig. 7(a). Maximum phase changes of +12 deg and 
-6 deg were observed, if the extraneous data taken during the 
time the transmitter was off is neglected. Once again, these 
values are well within the allowable k550 deg specified for the 
transmitter. 
Figure 2 shows the change in group delay through the 
transmitter system as a function of time. As was explained 
earlier, the delay was determined by using phase data at 
2.1 14 GHz and 2.1 16 GHz to approximate the slope of the 
phase curve at 2.1 15 GHz. Some smoothing has been applied 
to the data before plotting. Peak deviations of +0.8 ns and 
-1.5 ns observed, once again well within the 13.3-ns speci- 
fication for the system. 
The final plots show the three control parameters which 
are believed to  be most important in determining the phase 
and amplitude stability of the transmitter. Klystron beam 
voltage is shown in Fig. 3.  No downward drift from the 
initial value of 57.6 kV was observed, but an increase of about 
750 V can be seen. This corresponds to a regulation of about 
1.3 percent. Subsequently, an inspection of the beam voltage 
regulation circuitry revealed an incorrect resistor value, which 
caused the reference voltage to behave abnormally. As will be 
seen later, much of the observed instability in gain and phase 
can be attributed to this parameter. 
The inlet coolant temperature was also monitored and is 
depicted in Fig. 4. The sharp decrease in temperature near 
sample 150 is associated with the klystron trip-off, and an 
exponential rise in temperature after the transmitter was 
restarted can be clearly seen. 
The final control parameter that is of interest is the drive 
power to the klystron, which is provided by a 10-W, Class C 
amplifier. The initial drive power of 5.91 W had a peakincrease 
of about 80 mW, and a decrease of about 95 mW, as can be 
seen in Fig. 5. It will be shown in the next section that since 
the klystron was operated in a saturated mode, these small 
drive level changes do not significantly affect the amplitude 
stability of the transmitter, and the phase changes induced 
through AM-to-PM conversion in the saturated klystron 
appear to be masked by other effects. 
IV. Discussion 
In addition to verifying that the transmitter system meets 
its stability requirements, it is also of interest to  determine 
which control parameters are responsible for the observed 
instabilities, and to what extent. The effects of the various 
control parameters (i.e., beam voltage, temperature, drive 
power, etc.) may be summarized by two simple equations: 
. .  
t = time 
to = starting time of the experiment 
Po,,(t) -Pout(to) = change in output power from initial 
value 
0 ( t )  - 6 (fo) = change in phase from initial value 
82 
V,(t) - V B ( t 3 )  = change in beam voltage from initial 
value 
T ( t )  - T ( t o )  = change in coolant temperature from 
initial value 
Pdrive ( t )  - Pdrive ( to)  = change in klystron drive power from 
initial value 
The factors K ,  - K ,  are the pushing factors for power out, 
and K4 - K 6  are the pushing factors for output phase. These 
are the numbers we wish to  determine. 
The results for output power are shown in Fig. 6 .  Figure 
6(a) shows the raw output power data. It is well known that 
the beam voltage is an important parameter in determining 
klystron output power. A preliminary value for K ,  in kW/kV 
was determined from published tube data, and further refined 
by trial and error. The final value obtained was about 20 kW/ 
kV, and the results for the change in power out with this beam 
voltage effect removed are shown in Fig. 6(b). 
An exponential effect is clearly visible after the transmitter 
was restarted. This effect is related to the coolant tempera- 
ture depicted in Fig. 4. The temperature effects were removed 
by trial-and-error fitting of the curves, and a value of 
-0.9 kW/"C was determined for K z .  The power out data with 
both the beam voltage and coolant effects removed is shown in 
Fig. 6(c). 
Upon comparing Fig. 6(c) and Fig. 5 ,  it does not appear 
that the remaining output power instability is related to  drive 
power. Filament voltage and current are also known to  have an 
effect on output power, but, like drive power, no correlation 
between the filament data and the remaining output power 
instabilities could be seen. The remaining 4-kW (1 percent) 
drop in power may be related to  changes in the monitoring 
system, ambient temperature, or some other control parameter 
that was not monitored. 
A similar procedure was used to  determine the phase 
pushing factors K,-K,. The measured phase data is shown in 
Fig. 7(a). A beam voliage effect of about 20 deg/kV was then 
removed from this data and is plotted in Fig. 7(b). Once 
again, a temperature effect is clearly visible after the trans- 
mitter is turned back on. Figure 7(c) shows the output phase 
with these effects removed. The value determined for K5 was 
-0.8 deg phase/"C. Once again, the remaining phase instability 
does not seem to be correlated with klystron drive power 
or filament voltage and current, and a value for K6 could not 
be determined. 
Two major effects remain in the phase data after beam 
voltage and coolant temperature effects are removed: a long- 
term drift of about 6 deg. and two phase steps, one early in 
the data (samples 10-15) and one at about samples 225-325. 
These phase steps appear to  be correlated with the steps in 
group delay appearing in Fig. 2. The long-term drift may be 
caused by changes in ambient temperature, or instrument 
drift. The steps in phase and group delay may be attributed 
to changes in drive to the 10-W, ClassC amplifier or in its 
power supply, but do not appear to be related to temperature 
or any klystron parameter. Further tests of the drive system 
are required to  determine the cause of the observed steps in 
phase and group delay. 
Using the phase data that was collected, calculations were 
also performed to approximate the fractional frequency 
stability for the open-loop transmitter system. A summary 
of the equations used is given in the appendix. In particular, 
a value of 4.9 X for Af/f was determined for an averag- 
ing time of 990 s. Figure 8 shows the calculated values of 
Af/f for various averaging times (7) for the open-loop 
transmitter. 
Other control parameters such as filament voltage and 
current and magnet current were also monitored, but do not 
appear to  be nearly as important as beam voltage and coolant 
temperature in determining transmitter stability. 
V. Conclusions 
In conclusion, the long-term stability tests have shown that 
the 400-kW transmitter easily meets its specifications for 
amplitude, phase, and group stability. Two major contributors 
to the observed instabilities have been determined: (1) the 
beam voltage with pushing factors of 20 kW/kV and 20 deg/kV 
at saturation, and (2) the inlet coolant temperature at -0.9 
kW/"C and -0.8 deg/"C. Further work is required to determine 
the contributors to the remaining amplitude, phase, and group 
delay variations. In particular, a more detailed look at the 
system of amplifiers driving the klystron would probably be 
beneficial. If future missions or experiments require greater 
transmitter stability than was measured during this test, 
improvements in beam voltage and coolant temperature 
regulation should be considered. 
83 
Acknowledgments 
The authors acknowledge the assistance of Marlyn Gregg, John Daeges, and Phil Boss 
in performing the measurements and interpreting the data, and Roger Meyer of the 
Communications Systems Research Section for discussions regarding the -fractional 
frequency results. 
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Fig. 6. Change in output power: (a) uncorrected, (b) beam voltage corrected, 
(c) beam voltage and temperature corrected 
88 
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Fig. 7. Change in phase: (a) uncorrected, (b) beam voltage corrected, (c) beam 
voltage and temperature corrected 
89 
Fig. 8. Fractional frequency stability 
90 
Appendix 
Calculation of Fractional Frequency Stability From Measured Phase Data 
For a general signal with some phase instability, we have 
v = A cos [Oaf + @(t) ]  
The instantaneous phase in radians is 
e ( t )  = mot + @(ti 
and the instantaneous frequency is 
The instantaneous fractional frequency fluctuation is 
Next, we find the average value of u,(t) over an interval of 
seconds: 
Equation (A-8) may be evaluated for discrete values of 7, 
namely, integer multiples of the sample time (30 s). 
Equation (A-8) is evaluated for each interval of seconds in 
the data set, and the Allan variance of these evaluations of 
Eq. (A-8) is found. This process is repeated for each value of 
7, and the results are usually plotted on a log-log scale as 
log (u) vs log (7). 
91 


Long-term amplitude and phase stability of the 400-kW 2.115-GHz transmitter

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