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

shownasa roomtemperatureMESFET(Metalgate
Schottkybarrierfield-effect transistor) amplifier.Alternatively,parametric
down-converters followedbyparametricamplifiersalloperating
in the20° Kenvironment
mightofferbettergainstabilityandgoodnoiseperformance.
A fixedfrequency localoscillator signalsynthesized
fromthestandardreference frequencies isneededfor
thedownconversion. Fetritephaseshiftersprogrammed
by thecentralcomputerareusedto shiftthephaseof
thislocaloscillatorat eachantennato providebeam
steeringandtrackingcapability. Further discussion of
the phase shifter is given in Appendix J.
Using the receiver system described above, the estimated
system noise temperature would vary from about
40 ° K at 500 MHz to about 24 ° K at 3 GHz. The
receiver noise and waveguide loss decrease with frequency
but the spillover loss and sky temperature
increase at lower frequencies. The minimum noise
temperature is expected to occur around 1.5 GHz and to
be about 20° K.
be undertaken using the Cyclops array, good frequency
stability is essential.
The best frequency standard available today is the
hydrogen maser phase locked to a high quality crystal
oscillator (refs. 13-15). Table 9-1 is a comparison of
several state-of-the-art frequency standards. Figure 9-11
shows the spectr',d purity obtainable from the major
kinds of frequency standards. The ordinate is the rms
frequency deviation divided by the operating frequency,
while the abscissa is the averaging time over which the
measurement of Afrms is made. The hydrogen maser gives
the best performance for all averaging times and, for times
on the order of 100 seconds, which are involved in the
Cyclops signal processing, the hydrogen maser is some
30 times better than a rubidium maser and some 500
times better than cesium. Hydrogen masers are not
commercially available at present, but several have been
built by Varian Associates, by the Smithsonian Astrophysical
Observatory, and by others.
I0 -9 "T • l I
THE LOCAL OSCILLATOR SYSTEM
The three major functional components of the
Cyclops local oscillator system shown in Figure 9-1 are
the primary frequency standard, the distribution system,
and the frequency synthesis and phasing systems located
at each antenna. In principle, one could generate
centrally the local oscillator frequencies needed for
reception of a particular band rather than synthesizing
them at each antenna. However, it does not appear
practical to transmit frequencies over a large frequency
range to the individual antennas and maintain the phase
tolerances required. For this reason only two precise
frequencies are generated centrally and distributed, and
the synthesis is done locally at each antenna.
In a system the size of Cyclops with its thousand or
more antenna elements, the cost of any item associated
with each antenna must be kept low, and the cost of
transmission circuits is quite appreciable. However, the
cost of a single item such as the central frequency
standard is an insignificant part of the total system cost,
and to the extent that increased expense in this item will
improve system performance it is easily justifiable. The
spectral purity of the central frequency standard limits
the minimum bandwidth that can be used in the signal
processing, which in turn is directly related to the
minimum detectable signal strength. Since the detection
of very weak signals is the whole object of building so
large an array, only frequency standards of the highest
possible spectral purity and stability should be considered.
Also, if very long base-line interferometry is to
Af
_-
rms
Figure 9-11.
,o-'°_-_'__ Rb,QUART_ i
i0-II
i0-12
_ _" _ _'_,_, _ ARTZ
10-14
L ,
10-15 _ 1 . _L k • 5
,o-4 ,o-2 ,o-O _oz ,04 Io6
AVERAGING TIME, sec
Fractional frequency fluctuation versus
averaging
time.
Since the reliability of the hydrogen maser, as with any
other device, is not perfect, and since the entire Cyclops
array would be inoperative if the frequency standard
were to fail, at least two hydrogen masers should be
provided. In fact, if two standards disagree, a third is
needed to determine which of the two is in trouble, so it
is recommended that the control center include three
hydrogen
masers.
The standard lrequency distribution system must
distribute the reference frequencies required to synthesize
all local oscillator and pilot frequencies at each
antenna site with an unambiguous phase relationship.
This requires that one of the standard frequencies be
equal to the smallest desired tuning increment, for the
following reason. The usual frequency synthesizer contains
frequency dividers as well as multipliers and
mixers. Any frequency divider that divides a reference
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