A phase modulator capable of large linear phase deviation, low loss, and wide band operation with good thermal stability was developed for deep space spacecraft transponder (DST) applications at X-band (8.415 GHz) and Ka-band (32 GHz) downlinks. The design uses a two-stage circulator-coupled reflection phase shifter with constant gamma hyperabrupt varactors and an efficient modulator driver circuit to obtain a phase deviation of +/-2.5 rad with better than 8 percent linearity. The measured insertion loss is 6.6 dB +/- 0.35 dB at 8415 MHz. Measured carrier and relative sideband amplitudes resulting from phase modulation by sine wave and square modulating functions agree well with the predicted results

Mueller, R. O.

Mysoor, N. R.

English

NASA Technical Reports Server

TDA Progress Report42-112 February 15, 1993
f-/0
N93-24670
Performance Results of a 300-Degree Linear Phase
Modulator for Spaceborne Communications
Applications
N. R. Mysoor and R. O. Mueller
SpacecraftTelecommunicationsEquipmentSection
A phase modulator capable of large linear phase deviation, low loss, and wide-
band operation with good thermal stability has been developed for deep space space-
craft transponder (DST) applications at X-band (8.415 GHz) and Ka-band (32 GHz)
downlinks. The design uses a two-stage circulator-coupled reflection phase shifter
with constant gamma hyperabrupt varactors and an efficient modulator driver cir-
cuit to obtain a phase deviation of +2.5 rad with better than 8 percent linearity.
The measured insertion loss is 6.6 dB 4-0.35 dB at 8415 MHz. Measured carrier
and relative sideband amplitudes resuIting from phase modulation by sine wave and
square modulating functions agree well with the predicted results.
I. Introduction
The telecommunication transponders [1] for Cassini and
other future deep space missions require an X-band linear
phase modulator. The design is to provide 4-2.5 rad of
peak phase deviation to accommodate downlink modula-
tion of telemetry and ranging signals. The tolerance on the
phase deviation linearity is 8 percent. The phase modula-
tor design specifications [2] for the deep space transponder
applications are listed in Table 1. The modulator design
concepts, development, analysis, and measured results are
presented in this article. The description of the bread-
board phase modulator is presented in Section II, the test
data and analysis are presented in Section III, and the
conclusions are presented in Section IV.
II. Description of the Phase Modulator
The X-band phase modulator is designed as a cascaded
two-stage circulator-coupled reflection phase shifter [2,3,4]
with constant gamma hyperabrupt varactors. The gallium
arsenide (GaAs) varactor diode is well suited for the phase
modulator application as it can provide rapid phase change
with the applied voltage. The diode capacity parame-
ter, gamma (F), is the slope of the capacitance-voltage
curve as plotted on log-log paper. The gamma of a typical
hyperabrupt varactor varies widely with applied voltage,
which makes these varactors unsuitable for linear phase-
modulator applications. Recent device processing develop-
ments [4] have enabled the construction of varactors with
gammas that remain constant over a limited voltage bias
107
https://ntrs.nasa.gov/search.jsp?R=19930015481 2020-03-17T07:09:53+00:00Z
range. The GaAs varactors 1 used in this investigation have
a constant gamma equal to 1.5. The varactor diode is usu-
ally modeled as a junction capacitance in series with the
resistance of the GaAs substrate and epitaxial layers. The
junction capacitance of varactor diodes is modeled as
co ,1)
where Cj(V) is the junction capacitance at reverse bias
voltage, V. The value Co is the junction capacitance at
V = 0, and • is the built-in potential, which is equal to
1.3 V for GaAs.
The series resistance (Rs) of varactor diodes can be
determined from the measured diode's quality factor, Q.
The Q factor is a measure of loss in the varactor diode.
The expression for Q as a function of the bias voltage,
used in the simulation, was obtained by curve fitting the
manufacturer's (Microwave Associates') data at 50 MHz:
Q(V) = 1538 - 115.6[V[-4-118,21Vl2+ 21.961v1z
- 1.683[V] 4 (2)
The diode series resistance is then given by
Rs(V) = [27r × 50 x 106 x Q x Cj(V)] -1 (3)
A phase modulator consisting of a two-section reflection
phase shifter [2] and a driver circuit was breadboarded and
tested. The phase modulator photograph and schematic
are illustrated in Figs. 1 and 2, respectively. The module is
made out of aluminum alloy 6061, which is tin-lead plated
to ensure ease of soldering and good electrical conductivity.
The size of the module is 81 × 61 × 14.3 mm. The two-
section phase shifter layout is etched on a 0.508-mm-thick
RT/Du'roid 6002 soft substrate of a dielectric constant
equal to 2.94. The constant 1.5 gamma varactors 2 are
mounted with a contact strip at the end of an optimized
30-ohm, 2.794-mm-wide transformer. The diode capaci-
tance at -4 V is equal to 0.65 pF. The diode package par-
asitic capacitance and inductance are equal to 0.23 pF and
0.20 nil, respectively. TRAK Microwave 6.35-mm micro-
puck circulators [79"9001] and isolators [89"9001] are used
1The varactors were manufactured by Microwave Associates,
MA-46411-126.
2 Ibid.
in this breadboard. The driver circuit is etched on 0.787-
ram-thick RT/Duroid 5880, which has a dielectric constant
of 2.2. The phase modulator driver circuit schematic is
shown in Fig. 3. The functions of the phase modulator
drive circuit are to sum and amplify the modulation in-
put signals and to provide composite drive voltage to the
varactor diodes. The modulation input signals include
the spacecraft telemetry, ranging, and differential one-way
ranging (DOR) signals. The modulation frequency range
is from 1 kHz to 20 MHz. The selected wideband op-amp
for this application is Comlinear CLC 505.
III. Experimental Results
The measured values of phase deviation and inser-
tion loss for the phase modulator are compared with the
predicted results in Fig. 4. The predicted results were
obtained by the computer microwave computer-assisted-
design (CAD) simulation of an extensive circuit model of
the circuit schematic shown in Fig. 2. The measurements
were conducted using a test RF signal level of +9 dBm.
The insertion loss for this unit is 6.35 -I-0.25 dB, the phase
deviation is 4-150 deg, with a linearity better than -I-8 per-
cent of a best-fitted straight line (BSL). The varactor bias
voltage range is 5 -t-3.5 V. The model accurately predicted
a linear phase slope of 42 deg/V. The predicted insertion
loss, including circulator and isolator losses, is 7.0 -1-0.4 dB,
which is about 0.65 dB higher than the measured values.
The discrepancy in the insertion loss is most likely due to
the uncertainty in the measured insertion loss of tile circu-
lators and isolators. The insertion loss of these devices was
measured in a separate test fixture. Circulators measured
in this test fixture had a port-to-port one-way insertion
loss of 1 4-0.1 dB. The test fixture is not enclosed and,
therefore, has radiation losses which the module does not
have. Also, circulators were not epoxied in the test fixture
as they are in the module; this will result in higher losses
since there is not a solid ground contact. The measured
results of the phase modulator are compared with its de-
sign specifications in Table 1. The phase modulator meets
or exceeds all specifications.
Figure 5 shows the predicted and measured gain versus
frequency characteristics of the phase modulator driver cir-
cuit at -35 deg C, +25 deg C, and +85 deg C. A separate
breadboard was used to test the driver circuit. The mea-
sured response is fiat and drops only 0.4 dB, from 1 kHz to
20 MHz. The measured 3-dB bandwidths at -35 deg C,
+25 deg C, and +85 deg C are 92 MHz, 84 MHz, and
75 MHz, respectively. The measured results are actually
better than the Simulation Program with Integrated Cir-
cuit Emphasis (SPICE) predicted circuit response using a
108
conservative circuit model for the op-amp integrated cir-
cuit. The model correctly predicted a larger bandwidth at
lower temperatures. The de power consumption with an
8-V peak-to-peak output swing is 112 mW; when no signal
is applied, the power consumption is 32 mW.
Sinusoidal and square modulating waveforms were ap-
plied to the phase modulator, and their resulting spectra
[5,6] were monitored on a calibrated spectrum analyzer.
All measurements were performed at 25 deg C and with a
carrier frequency of 8415 MHz. Figure 6 shows the result-
ing spectrum when a sinusoidal modulating wave was ap-
plied to the phase modulator. The modulating frequency
used is 1 MHz, and the peak modulation index is 2.4 rad.
A comparison of measured and predicted carrier, first,
second, and third sideband levels for the case of sinusoidal
modulation is shown in Fig. 7. A modulation frequency of
10 MHz was used in the measurements. The peak phase-
modulation index, fl, ranges from 0.2 to 2.4 tad in 0.2-rad
steps. The predicted relative carrier and sideband levels [6]
were computed by evaluating the appropriate Bessel func-
tions of the first kind of order n, J,_(fl). The subscript n is
an integer. It represents the carrier for n equal to zero and
sidebands for n not equal to zero. The relative carrier level
for a given modulation index is determined by evaluating
20 log[]J0(fl)[]. Similarly, the relative sideband levels may
be determined by evaluating 20 Iog[[J,_(fl)[] for [n[ > 0.
Figure 7 shows excellent agreement between theory and
measurement for sinusoidal phase-modulating waves. Neg-
ligible amplitude modulation distortion was observed in
this case.
Figure 8 shows the spectrum that results when a square
modulating wave at a frequency of 100 kIIz and a peak
modulation index of 1.571 rad is applied to the phase mod-
ulator. A comparison of measured and predicted carrier,
first, third, and fifth sideband levels for the case of square-
wave modulation is shown in Fig. 9. A modulation fre-
quency of 100 kHz was used in the measurements. The
relative carrier level in decibels is determined [6] by evalu-
ating 20 log[[ cos(fl)]], where fl is the peak modulation in-
dex. The relative sideband levels may be determined [6] by
evaluating 20 log[](2/nTr)sin(fl)[] , where n is tile number
of the sideband. Excellent agreement between predicted
and measured square-wave modulation results is demon-
strated in Fig. 9.
The data illustrated in Figs. 7 and 9 were obtained by
the judicious use of a spectrum analyzer. For small mod-
ulation indices, setting the first sideband level is the most
accurate way [5] to obtain a particular value of a modula-
tion index. For example, if a modulation index of 0.2 rad
(sinusoidal modulation) is needed, then the modulation
power level should be adjusted until the first sideband level
falls 20.0 dB below the unmodulated carrier level. The er-
ror in the measurement is determined by the accuracy of
the spectrum analyzer. If the above reading is accurate to
within 1 dB, then the modulation index would be accurate
to within 0.03 rad. For larger modulation indices, the first
sideband will give large errors. In this case, the carrier
level should be used to set the modulation index. The de-
cision of whether to use the carrier or the first sideband
level can, in general, be made by choosing the one that has
the steepest slope [5] near the desired modulation index.
IV. Conclusions
A two-section analog X-band reflection phase modu-
lator with an efficient driver circuit was developed. The
phase modulator performance is accurately predicted by
the theoretical modeling and simulations. By using hy-
perabrupt junction GaAs varactor diodes of a constant
gamma of 1.5, a full ±2.5-tad deviation phase modulator
was realized by cascading two phase shifter circuits. An
efficient phase modulator driver circuit was also developed
and tested. The measured gain-frequency characteristics
of the modulator driver circuit over the design temper-
ature range agreed well with the predicted results. The
modulator driver 3-dB bandwidth and dc power consump-
tion at 25 deg C are 84 MHz and 112 roW, respectively.
At 8415 MHz, the measured phase shift and insertion loss
of the phase modulator are equal to ±2.5 rad, with better
than ±8 percent linearity, and 6.35 ±0.25 dB, respectively.
Measured carrier and sideband spectra showed excellent
agreement with theory with negligible amplitude modula-
tion distortion for both sinusoidal and square phase mod-
ulating waves.
Acknowledgments
The authors gratefully acknowledge the valuable support of A. W. Kermode for
comments on design and analysis and J. H. Meysenburg for breadboard fabrication
and testing.
109
References
[1] N. R. Mysoor, J, D. Perret, and A. W. Kermode, "An X-Band Spacecraft
Transponder for Deep Space Applications--Design Concepts and Breadboard
Performance," IEEE Trans. Microwave Theory Tech., vol. MTT-40, no. 6,
pp. 1192-1198, June 1992.
[2] N. R. Mysoor and R. O. Mueller, "Design and AnMysis of Low-Loss Linear
Analog Phase Modulator for Deep Space Spacecraft X-band Transponder (DST)
Application," The Telecommunications and Data Acquisition Progress Report _2-
105, vol. January-March 1991, Jet Propulsion Laboratory, Pasadena, California,
pp. 136-145, May 15, 1991.
[3] R. Garver, "360 deg Varactor Linear Phase Modulator," [EEE Trans. Microwave
Theory Tech., vol. MTT-17, pp. 137-147, March 1969.
[4] E. C. Niehenke, V. V. DiMarco, and R. Friedberg, "Linear Analog Hyperabrupt
Varactor Diode Phase Shifters," IEEE 1985 International Microwave Theory and
Techniques Symposium, Digest of papers, pp. 657-660, June 1985.
[5] E. A. Whitman, "Phase Modulation Measurement Techniques for Improved Ac-
curacy," Microwave Journal, pp. 113-116, June 1978.
[6] F. Stocklin, Relative Sideband Amplitudes vs. Modulation Index for Common
Functions Using Frequency and Phase Modulation, Goddard Space Flight Cen-
ter, distributed by National Technical Information Service, U. S. Department of
Commerce, Springfield, Virginia, November 1973.
11o
Table 1. Measured results and design specifications for the
phase modulator.
Parameter ...... Specification Measurement (25 deg C)
Insertion loss 10 dB, max
Amplitude modulation 4-0.5 dB, max
Input return loss 14 dB, rain
Output return loss 14 dB, rain
RF input level +10 dBm, max
(for linear operation)
Linearity 4-5 percent
(best straight line)
to 4-2 rad
Modulation sensitivity 2 4-0.2 rad/V
Modulation input 4-2.5 V, max
signal level at 4-2.5 rad
Op-amp gain
RF modulation bandwidth
-0.25 dB bandwidth " M-_z-,-_-r_n
-3 dB bandwidth ................... 82 _z,_ .......nun
DC power (at 20 MHz
and +2.5 rad)
+5 V 25 mA, max
-5 V 25 mA, max
Modulation voltage range
(to varactors)
6.6 dB, max
+0.15/-0.35 dB
25 dB (8415 4-50 MHz)
33 dB (8415 4-50 MHz)
+10 dBm, max
+4.8/-3.1 percent
2,1 rad/V
:t:1.28 V, max
2.8 v/v
28 MHz
160 MHz
7.0 mA
6.9 mA
-1.5 to -8.5 V
111
Fig. 1. DST X-band phase modulator and driver circuit assembly.
112
MODULATED
880 F 1 OUTPUT OUTPUT
ISOLATOR
L LENGTH
W WIDTH
MLIN MICROSTRIP LINE
TLIN TRANSMISSION LINE
DIMENSIONS IN MILLIMETERS
ANGLES IN DEGREES
MODULATOR
2nd SECTION
CIRCULATOR
MLIN:
W = 1.40
(~ 50 ohms)
CAP:
UNMODULATED
INPUT 880 F 1 INPUT
ISOLATOR
-r _ 3.9 pF (MODEL INCLUDES RIBBON
STUB: _L INTERCONNECT)
ANG = 20 • MLIN'
L = 5.56 / W = 1'.73, L = 2.79
(42 ohms)
' ' W-= 0.25 L MLIN:
L = 5.56 W = 1.73, L= 1.17
(109ohms) [ (42ohms)
MLIN:
W _ 2.79, L = 4.88
(30 ohms)
_ STUB: W = 0.81, L = 2.44
3 TLIN: MODEL FOR RIBBON TO VARACTOR
| Z = 150 ohms
la E =9deg
MA 46411-126
-1_ GaAs HYPERABRUPT VARACTOR
_' Cj = 0.65 pF, T"= 1.50
Fig. 2. DST X-band phase modulator schematic.
113
DIFFERENTIAL
ONE-WAY 360
RANGING
360
RANGING
TELEMETRY 1
TELEMETRY 2
COMLINEAR
CLC 505
1 K_:'
30
60
-5 VDC
0.2#F _:100 K 9
•_ _ TO VARACTOR 1
)-5 VDC
100 K
._o TO VARACTOR 2
4O0 K
Fig. 3. DST phase modulator driver circuit schematic.
-r-
CO
UJ
<E
"1-Q.
t I I 1 I t I |180 TWO-SECTION, r" = 1.5 _ 4.7z_ MEASURED PHASE SHIFT
- O PREDICTED PHASE SHIFT
- <} MEASURED INSERTION LOSS f-
- [] PREDICTED INSERTION LOSS f -
90 -- TEMPERATURE 25°C J
-9C
-18C t I 1 I I ] 8.3
1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5
REVERSE BIAS VOLTAGE, V
Fig. 4. Measured and predicted results for the phase modulator.
m
c_
03
O
_J
Z
o
6.5
rr
UJ
(t')
Z
114
en
"10
z"
<E
L_
' ' ''""1 ........ 1 ' ' ...... I ........ t ' ' ......
................................................ °,o°°°°0 .....................
t
T= -35°C
T= -35°C
T= 25°C
T= 25°C
T= 85°C
T = 85°C
MEASURED
............... PREDICTED
...... MEASURED
.... PREDICTED
..... MEASURED
m I PREDICTED
6 I I I Illlll I I t ItlllJ I I 1 ]llllf I I I Illlll I t J
103 104 105 106 107 108
FREQUENCY, Hz
Fig. 5. Measured and predlcted gain versus frequency for the
phase modulator driver circuit at temperatures of -35, 25, and
85 deg C.
0
-1 o
CO
o -20
LM
> -3O
ILl
.-I
._ -40
<
-50
LU
_ 4o
-70
--80
I I
I I t
8411
I
I,
I I I I
8413
I
8415 8417
FREQUENCY, MHz
' 'I
8419
Fig. 6. Measured phase-modulated spectrum for the case of e
slnusoldal modulating wave at a frequency of 1 MHz and a peak
modulation index of 2.4 rad.
10
-10
d
> -20
UJ
_J
_J
<_
E -30
o
uJ
o_
-40
-5O
-6O
10
I ' I ' ' ' ' ' I
CARRIER
THIRD /
SIDEBAN_/_ O MEASURED , _
I _/ _ PREDICT,/_ ] _ _ _ _ _ EID ,,
20 40 100 200
PEAK MODULATION INDEX, deg
Fig. 7. Measured and predicted carrier, first, second, and third
sideband levels versus peak modulation index for the case of a
sinusoidal modulating wave at a frequency of 10 MHz.
en
W
..J
<E
LLI
0 1 I I
-10 -
-20
-30
-40
-50
--60- j_-70 -
-8O I I I
8414.6
I
8414.8
I I
I t I I
8415.0 8415.2
FREQUENCY, MHz
I I I
FF
8415.4
Fig. 8. Measured phase-modulated spectrum for the case of a
square modulation wave at a frequency of 100 kHz and a peak
modulation Index of 1.571 rad.
115
10 t '
CARRIER
0
-10 FIRST
SIDEBAND
m
o
..i
-20
U/
_J
I- -30
O FIFTH
IJ.l
n SIDEBAND
GO
-40
O MEASURED
-50 _ PREDICTED
THIRD
SIDEBAND
-60 1
10 20 40 100 200
PEAK MODULATION INDEX, (:leg
Fig. 9. Measured and predicted carrier, first, third, and fifth side-
band levels versus peak modulation Index for the case of a square
modulating wave at a frequency of 100 kHz.
! ....
116


Performance results of a 300-degree linear phase modulator for spaceborne communications applications

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