A maser amplifier was incorporated into a low noise radiometer designed to measure large-scale anisotropy in the 3 deg K microwave background radiation. To minimize emission by atmospheric water vapor and oxygen, the radiometer is flown in a small balloon to an altitude to 25 km. Three successful flights were made - two from Palestine, Texas and one from Sao Jose dos Campos, Brazil. Good sky coverage is important to the experiment. Data from the northern hemisphere flights has been edited and calibrated

Fixsen, D. J.

Wilkinson, D. T.

English

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^^^5a -- 696	 .
ound Radiation
is
^y
Dale J. Fixsen
and
David T. Wilkinson
Physics Department
Princeton University
Princeton, New Jersey
08 54 4
AUG 1982
RECEIVED
Prepared for
The Jet Propulsion Laboratory
California Iiistitute of Technology
June, 1982
(NASA-CR-169212) BASE& RAUICKETER FC&
CUSKIC BACKGROUND bADIATICN ANISOTROPY
MEASUREMENTS Final Report (Fiinceton ODiv -,
N. J.) 13 p HC A02/MF A01 	 CSCL 2UE
N82-30538
Uncla s
G3/36 28597
W,
-I-
SUMMARY
A maser amplifier, built at JPL, has been incorporated into a low-
noise radiometer designed to measure large-scale anisotropy in the 3°K
microwave background radia-ion. To minimize emission by atmospheric water
vapor and oxygen, the radiometer is flown in a small balloon to an altitude of
25 km. Three successful flights have been made - two from Palestine, Texas
and one from Sao Jose dos Campos, Brazil. Good sky coverage is important to
the experiment.
Data from the northern hemisphere flights has been edited and calibra-
ted, and a copy sent to S. Gulkis at JPL. Analysis of the data is currently
under way at Princeton University.
I. Radiometric System
The maser-based radiometer system is shown in Figure 1. Below, the
various important components of this system are discussed, with special em-
phasis on the maser.
1. Maser
The 4-stage reflected wave maser used in this experiment was designed
by Robert Claus (JPL) and Craig Moore (NRAO). One stage of the maser con-
lists of ruby-filled waveguide (see Fig. 2) with a three-port circulator
at one end and a reflector at the other end. This arrangement enables the
wave to pass through the ruby twice. To avoid oscillation the gain of a
single stage must be limited to about 10. Four stages are required to pro-
d-ice sufficient gain to minimize the noise of the following receiver. The
four stages are "stacked" to place t l•-- four rubies in the magnetic field of
a superconducting magnet. There are additional circulators between the
stages with loads on the third port to absorb waves going backwards, since
-2-
unavoidable reflections would allow backward waves to contribute noise. At
the circulators the microwaves propagate in the direction indicated by the
arrows. The microwaves propagate down the ruby and are reflected off the
end and propagate br_ek up with a gain of about 9 dB. The electronic gain
through the 4 rubies is 36 dB, but the losses in the walls and circulators
reduce this to about 30 dB of net gain.
To obtain a 400 MHz bandwidth	 the magnetic field is tapered along
the length of the ruby. (8800 to 8950 gauss in 15 cm), so different parts of
the ruby amplify different frequencies. The field taper is obtained by
making the magnetic pole pieces closer together at the bottom than at the top.
By changing the taper, gain can be traded for bandwidth and vice-versa.
The pump frequency required is 2f+3 GHz for maser frequencies between
10 and 35 GHz, and the bandwidth must be twice the maser bandwidth. Thus the
pump must be a broad-band source or sweep across the band in a time short
compared with the spin relaxation time, which is 30 ms. The pump frequency
was swept across the band at 10 kHz.
Providing enough pump power to saturate the pumped transitions over
a 1 GHz band proved to be a problem. The relaxation time implies a power
consumption of about 15 mW. The pump source, an electronically tunable
Gunn diode (51.4 to 52.4 GHz), produces about 70 mW. This is reduced to
50 mW by losses in the wave guide to the maser. The short wavelength pump
power enters the bottom of the maser (see Fig. 2) through waveguide which
is too small to pass the maser frequency. There is a discontinuity at the
interface between the two sizes of wave guide . ,hich has a frequency
dependent reflection. It was determined experimentally that about 1/6 of
the remaining power is absorbed by the ruby. The rz3t of the power continues
through the ruby and is absorbed by the loads at the top of the maser.
The low pump power absorption reduces the maser gain and makes the
gain a strong function of pump power (1 dB increase in pump power results in
2 dB increase in gain). The output power of the pump oscillator depends on
frequency and	 temperature. This is the dominating instability in the maser.
There are several problems which result in gain ripple (gain variation
as a function of frequency).	 One of the major problems is internal reflec-
tions. The ruby has a dielectric constant of about 10. The wave guide is
reduced to 1/3 normal size (in both dimensions) to limit the 24.6 GHz waves to
a single mode. A stepped transition in conjunction with a 17 mm taper on a
ruby limit the power reflection to about 5%. The circulators have a reflec-
tion of about 1%. These two reflections limit the gain of the one stage
of the maser to about 10 since if the gain times the reflection exceeds unity
the maser is unstable.
End effects of the magnet also introduce gain ripple. The magnet pole
pieces have superconductors on the sides to keep the field uniform across the
face. However at the ends of the magnet, the field spreads and lowers the
magnetic field. This condition was corrected by inserting a tapered iron shim
between the maser and the pole face.
There are several ways the ripple might be reduced. A longer magnet
would solve the end-effects problem. Making the ruby without a taper (or
using some inactive dielectric for the taper) would make the shimming problem
somewhat more tractable. A more powerful pump would reduce some ripple
caused by the frequency-dependent output of the pump (and the reflections)
as well as stabilize the maser. Small coils could be placed on the face of
the magnet allowing dynamic tuning of the maser.
The gain is a very strong function of temperature. The gain depends
on the exponential of the inversion, which in turn has an exponential
T-4-
dependence on the temperature. To maintain a constant temperature the maser
is cooled by boiling liquid helium. The boiling point of liquid helium is a
function of the pressure, so during the flight the pressure is maintained,
at 1 atm, by a simple diaphragm regulator. 	 In addition, the pressure and
temperature are monitored throughout the flight. The pressure remained
constant to 1% over the flight (the sensor limits the measurement accuracy),
a 1% decrease in the pressure would lead to a 10% increase in the gain.
An external magnetic field was applied to the dec,ar to determine the
effects the earth's magnetic field would have on the maser. The gain of the
maser changed about 2%/gauss when a field was applied to the pump or to the
dewar containing the maser. After shielding the pump and coating the surface
of the maser shield with lead, a superconductor at 4 K, there remained
an effect of about .2%/gauss. To stop the field lines from entering the top
of the can a magnetic shunt, consisting of two parallel bars (each 4 mm x
10 mm x 120 mm) of cold rolled steel, was placed across the top of the shield.
The remaining residual effect was measured at about .05%/gauss.
In summary, the maser has an effective gain of 1000 and an effective
bandwidth of 300 MHz. The predicted noise was 6 K which agree • 3 with the
measured value of 6 + 2 K. The amplification becomes non-linear (1 dB
compression point) when the input is 6 x 10 8 K , giving the maser 80 dB 	
3
of dynamic range.
1
2. Horns
The circular corrugated horns are 43 em long and with a 16 cm aperture.
Grooves are cut at 3/8 wave intervals, one quarter wave deep, in the aluminum
walls of the horns. These corrugations reduce side-lobe response and using
the measured horn patter (see Fig. 3) the total ground radiation the horn
at 4 K so the added antenna t
r
-5-
receives is estiamted at .4 mK. Radiation from the walls of the horns is
estimated at 3 K.
3. CouplerE and Windows
Bethe hole couplers insert a calibrated signal into the system to
provide in-flight alibration. The source is a broad band noise diode which
is stable over a wide temperature range (5% variation 77 K to 300 K). The
42 and 47 dB couplers add 6.2 K and 1.2 K respectively when the diodes are
turned on. Losses in the transitions and couplers add 10 K to the antenna
temperature.
The mica windows have less than .7% reflection and add less than 1 K
when clean and dry. When moistened by condensation they can contribute 20 K.
4. JPL Cooled Switch
The switch is a circulator, similar to the circulators in the maser
except the magnetic field is supplied by an electromagnet rather than permanent
magnets. The circulator admits radiation from one horn or the other, depend-
ing on the current in the electromagnet. On each of the three ports there is
a stepped transition to a reduced height waveguide (.078"). When a magnetic
field is applied to the ferrite cylinder at the center, the incoming wave turns
to the right or left depending on the sign of the magnetic field. Two bits ^af
alumina (dielectric constant 10) at each input improve the match. The measured
reflection is .5% when the coil current is adjusted to give the least cross
coupling between the ports (.5 y). Th,: resistivity of the titanium walls
accounts for almost all of the attenuation (15X) in the switch. The switch is
i
W,	 M
-6-
5. Receiver
The bandpass filter (fl in Fig. 1) passes 23 to 27 GHz. Its trans-
mission at the pump frequency is 10 -9 . Since it follows the maser its
contribution to the system noise is negligible.
The mixer and LO mix the signal down to 10-410 MHz with a 5.4 dB loss.
Only the upper sideband is used. A variable attenuator between the LO and
the mixer was adjusted to minimize the noise of the mixer. The output is on
a 50 coaxial cable. The noise temperature of the mixer is 1400 K (single
sideband) or 3 K when referred to the antenna.
The IF amplifier has 54 -db gain from 10 to 500 MHz. The mixer, L0,
IF amplifier and detector were shielded in a copper and aluminum box. The
box was in thermal contact with 1 liter of freezing water, which kept the
temperature within 1 K of 273 K over the duration of the 1980 December flight.
The 1981 June fligh` started with warm water which did not freeze until near
the end of the flight. The noise temperature of the IF amplifier is about
300 K or 3 K referred to the antenna.
The detector is a tunnel diode. Noise from the diode is equivalent
to a 5052 resistor after detection. This noise and the front-end noise of
the following op-amp addabout .5 mK/vgz-, or the equivalent of 10 K at the
antenna, which must be added in quadrature to the other system noise. The
system is summarized in Table 1.
Following the detector the switched signal is amplified with an
adjustable gain amplifier which was followed by a lockin amplifier and a
low pass filter. Finally the data is digitized and relayed to the ground.
-7-
II. Theoretical and Actual System Performance
The radiometer system performance is summarized in Table 1, where the
calcualted (or measured) noise and the relevant gain (or loss) is given for
each stage. In the laboratory the system noise from the switch to the
detector is measured to be 16 K , in good agreement with 16 K estimated from
Table 1.
The predicted in-flight noise of 33 k can be used to calculate
AT )	 which can be compared to measured values. (T in this c::.e is
rms Theory
the difference in antenna temperatures in the two horns.)
Table 1 Source of Noise in System
Gain Noise Antenna
(dB) (K) Noise
Sky - 3 3
Horns -.09 3 3
Transition -.04 2 2
Waveguide -.2 8 8
Window -.01 - -
Switch -.6 1 1
Maser 28 6 8
Mixer -5.4 1400 3
IFA 54 300 3
Detector
	 10 (added by quadrature)
System	 33 K
The noise equation for this radiometer is
AT
rms
)	 - 2.2 T system
;I
	
K/Yiz
Theory	 (B.W.)
One factor of /2- comes from the 50% duty cycle of the Dicke switch;
another vr2- comes from the fact that the error refers to the difference
in two uncorrelated temperatures. The noise is increased another 10% by
r-8-
blanking of the signal at transition times of the Dicke switch. For an
effective bandwidth of 300 MHz the equation gives
ATrms)	 - 4.2	 mK/ 3 z .
Theory
	
In using data from balloon flights to find AT )
	 one must
rms Actual
first correct for switch frequency leakage which contaminated the data.
The lockin amplifier output filter (low pass) did not cut off fast Bnough
to completely eliminate residual signal at the Dicke switch frequency
(--2hz in the December 1980 flight). This beat with the 4Hz sampling rate
to give a slow beat of about 15 mK amplitude. Fortunately, the beat fre-
quency was stable enough that the effect could be fit and removed from most
of the data.
At times when maser gain was near optimum (-30 dB) the measured noise
was
AT
rms Actual	
^)	 - 4 to 5	 mK/	 ,
in excellent agreement with the theoretical value.
III. Data Analysis and Limitations
Data analysis is still underway at Princeton, so this report can only
summarize work done so far., and anticipate where limitations in accuracy will
come.
i^
1. Preliminary Work
The data tapes from NSBF had many problems - telemetry dropout,
miss-ng time code, missing position data and occasional scrambling of data
within a record. These problems were fixed at great effort and cost, and
about 10% of the data were irretrievably lost. Then the switch frequency
effect ( see above) was fitted and removed, and the radiometer data calibra-
ted in.o antenna temperature (mK). Finally balloon position and orientation
and sidereal time were used to associate each radiometer reading with two
positions in the sky ( 90° apart). From this data set a sky map can be made,
or a sky model can be fit directly to the data stream. Both are done.
The usual model fit includes celestial dipole and quadrupole distribu-
tions and a model of Galactic radiation at 24.6 GHz. The Galactic radiation
model includes "point" sources (mostly H II regions and supernovae remnants),
	 1
3
free-free emission (mostly from the Galactic plane) and synchrotron radiation
e xtrapolated from long wavelength maps. The free -free emission is estimated
from Ha surveys of the plane and high frequency ( v > 5 GHz) surveys wherever
they exist. The Ha maps suffer from extinction, so are lower limits only,
and the high frequency maps are incomplete. So, the free -free map is not
very good. The synchrotron map must be extrapolated from 0 . 408 GHz to
24.6 GHz, so the spectral index ( - 2.80) must be known to - ±. 05. No one
knows what the appropriate spectral index is, or how it varies over the sky.
At this point we believe that uncertainties in the Galactic radiation
model will be the main limitation on finding coefficients for the celestial
dipole and quadrupole components. Since Galactic radiation is strongest near
the plane, we have tried cutting out data from Jbi < 10° . This will probably
give the most reliable results, but the analysis is still underway.
L_
URI
OF x
Horns
•	 Transistions
00'00f
scree,7	Pump	 Couplers	 r	 f2•	 Detector
IF Amplifier (54 dB)
Local Oscillator
(24.4 GHz)
Switch	 Reciever
Moser
Figure 1.
From	 From
Horn 4
	 Horn 2
Ci rct
M
ORIGHN'AL	 V13
OF POOR Q 4UAU i Y
rn ll:
I
Pump	 Pump
Figure 2.
ao H Pl ans
	
i E Plans
•v
10 - -10
•x
Q
^
^
20 — 	i —20
30 — —.30
40—
	 j —40
I
50—
—50
60 — —60
Q ^
70— —70
80	 60	 40	 20	 0	 20	 40 60	 80
Angle (Degrees)
Figure 3.


Maser radiometer for cosmic background radiation anisotropy measurements

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