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

COAXIAL
LINE
DIRECTIONALCOUPLER
FREQUENCIES
MHZ
INCOMING OUTGOING RECOVERY
I 250
I
I 6.25
275
225
6.875
5.625
kO00
25
__
_-_iI BRANCH LINES _
I
I
REPEATER
:__
MAIN
TRUNK
LINES
M = MODULATOR
UNIT
\
ISTA_
A = ANTENNA STATION UNIT
T : TERMINATOR UNIT
OUT
Figure 9-21. Standard frequency distribution system.
Figure 9-20. Terminator unit.
unit. The repeaters thus subdivide the system into a
series of individually phase compensated links.
It is proposed that I-5/8 in. diameter cable be used to
distribute the standard frequencies. At 250 MHz the loss
in this cable is about 12 dB/km. In a l O-km diameter
array, one repeater will be needed per main tunnel, and
one for each of the longer side tunnels that branch prior
to the main tunnel repeater. Thus there will be no more
than two repeaters between any antenna and the central
station. This does not mean that the dispersion errors,
calculated earlier, must be tripled, since these were
computed for the total cable length. Reflection errors,
on the other hand, may add.
It is not necessary for the modulator, antenna station
and terminator units to have zero phase shift, though
this could be achieved. All we ask of the distribution
system is that it have phase stability. Fixed phase errors
can and will be calibrated out by phasing the elements
on a radio source. Since the same standard frequencies
are used to synthesize the pump frequencies for up-conversion
and the later down conversion to IF, any phase
errors in the standard frequencies tend to cancel out in
these two processes, leaving only the phase error at the
received frequency. Thus a phase stability of 10° at !0
GHz implies a stability of i.5 ° at 1.5 GHz. The
tolerances we have assumed are probably tighter than
necessary for reception up to 3 GHz.
While the Cyclops array contains many more antennas
than the proposed VLA system, the overall
dimensions of the array are smaller. The VLA report
concluded that local oscillator phasing was feasible for
the VLA, and we conclude that the same is true for
Cyclops.
COST
ESTIMATES
Almost all of the receiver system can be considered in
terms of a cost per antenna element. All the receiver
electronics and cryogenics represent a cost per antenna
as do the distribution systems. The cost of the IF
distribution system is included in the next chapler. The
only part of the receiver system tha! represents a Fixed
cost per array is the central frequency standard and lhe
associated electronics. The cost of this is estimated al
$200,000. For a thousand-element array this is equiv-
103