Project Cyclops, A Design... - Department of Earth and Planetary ...

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UP-CONVERTER+PARAMP
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UP-CONVERTER+MASER
TRAVELING WAVE MASER 4
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I 2 5 I0
masers may require a separate cryostat with about 1-W
capacity at 4° K.
A water-cooled helium compressor with output at
500 psi will be needed to run the refrigerators. A small
additional compressor may be needed to operate the
Joule-Thompson circuit in the 4° K units. These
compressors may be located at the base of each antenna.
The supply and return lines carry helium at near-ambient
temperature. These stainless steel lines would require
helical spring sections to negotiate the telescope axes.
Rather than having compressors at each antenna, a
single large unit might serve a group of four to six
antennas. The economics of this have not been studied.
A single central unit is probably undesirable because of
the large piping cost and because a failure of this central
unit would shut down the entire array. Even with
individual units, high reliability and freedom from
maintenance is imperative.
Following the masers the signals are further amplified
before final down conversion to the intermediate frequency.
In Figure 9-10 this further amplification is
Figure 9-9.
FREQUENCY,
GHZ
Up-con verters and maser noise temperature.
500 [0 675 TO 910 TO 1250 T0 1000 TO 2740 10
675 910 1230 1060 2240 3000
MHZ MHZ MHZ MHZ MHZ MHZ
7
7 <
The up-converter can have an instantaneous bandwidth
of 10% and in addition is tunable over a 30%
PHASE
LOCKED
band. Thus only six feed horns and 12 up-converters are
needed to cover the 0.5 to 3 GHz range. When the
up-converters are driven with the appropriate pump
frequencies, the outputs of all bands are in the same high
frequency band and can be selected by waveguide
switches as the input to a single high frequency maser
(for each polarization) as shown in Figure 9-10.
As shown, the six up-converters (three for each
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Jc
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IP
polarization) associated with three bands are clustered in _ t_
a 10 W capacity, 20 ° K closed cycle cryostat, while the
other larger unit six up-converters is used and if the are waveguide in a second runs, cryostat. particularly Ifa _--- _ _
for the higher bands, can be kept short enough, then all i __ i
12 up-converters and the I O-GHz masers can be enclosed o I o. [IO GHz----T._/] I!
in this single unit. Because the IO-GHz masers are 2o K i 4 "__ I
20 ° K ambient, the 4° K cooling load is much less than ..... _ -
relatively small in size, and can be thermally guarded in a L \',,j
MESFET
for an all maser system. _/' AMPLIFIER
In addition to having low RF loss, it is imperative PHASE LOCKED [ p[ PHASe ] "[. -_
LOCAL _----1-i_r1-_-_- _ _'-"-I_ t- _',c' IF OUT
that the waveguides place as small a thermal load as [ OSCILLATOR tL_-H-I_ k/ \
possible (< 0.2 W per input line) on the 20 ° K cooling COMPUTER ....
system. Refrigerators with simultaneous cooling capacity CONTROL
at 20° K and 4° K are not presently available. Thus the Figure 9-10. Receil,erbhwkdiagram(om'polarization).
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