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

A02 .... + No (25)
(i C 1
where No is the cable loss at Wo in nepers. Figure 9-16 is a
plot of A0t and A02 vs. w_/co0. The second system
(A0z) is clearly superior, since the first required wl tobe
extremely small (or very near coo/2) to avoid large phase
errors from dispersion. If we choose the second system
with co0 = 1 GHz and cJt = 25 MHz, the frequencies on
the line will be 225 MHz, 250 MHz, and 275 MHz. These
are easily picked up by a single coupler and easily
separated by filters. The dispersion phase error is then
0.00125 radians/neper or about 0.72 ° for the 14 nepers
of a 5 km run of 1-5/8 in. diameter cable. This
represents 7.2 ° error at l0 GHz, but since the error will
not vary by more than -+5%, and since fixed errors are
calibrated out of the system, the performance should be
excellent.
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treybJ
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32
Figure 9-16.
.I .2 .3 .4 .5
co
I/oJO
Phase error due
to cable dispersion.
the order of 3% (30 dB down, VSWR = 1.06) we can
expect rms phase errors of about 4XlO -2 radian-that
is, about 0.2 ° at 1 GHz or 2° at 10 GHz. These phase
errors may be subject to considerable variation but are
tolerable in absolute magnitude.
For the reasons given above, the phase compensation
method shown in Figure 9-15 is recommended for the
I-GHz standard frequency distribution system. There
appears to be no reason not to use the same system for
distributing the 25-MHz standard frequency. If all
frequencies are scaled down by 40 to 1, the phase errors
from cable dispersion are reduced by 6.3 to 1. Reflection
errors should also reduce by at least the same
amount. Although simple directional couplers designed
for minimum forward loss at 250 MHz will have about
28 dB more forward loss at 6.25 MHz, the line loss
(assuming 30 to 35 dB loss between repeaters) is also
reduced by about the same amount, so the minimum
signal levels obtained from the coupler should be
comparable in both bands. The coupler directivity
should also be comparable. Thus, it appears that we can
use the same cable and couplers for both standard
frequencies without resorting to a carrier system for the
lower frequency.
Figure 9-17 is a block diagram of the central standard
frequency unit. A majority vote of the three hydrogen
masers is taken as tire reference frequency of 1 GHz. This
frequency is successively divided to obtain tire various
frequencies needed at the central station as well as the
lower of the two standard frequencies to be distributed.
The 1 GHz and 25 MHz standard frequencies are used to
drive several modulator units each of which feeds a main
trunk cable in a main tunnel.
The system of Figure 9-15 has the further advantage
that the signals on the line are at one quarter rather than
one half the standard frequency output w0. They are
well removed from the RF bands of the receivers. The
line losses are less, so fewer repeaters are needed.
Furthermore line reflections are easier to control at the
lower
frequency.
Although not shown in Figure 9-15, directional
couplers are needed to pick the signals off the line, not
merely to help in separating the signals, but to reduce
reflection errors. Directional couplers not only introduce
less reflection than bridging amplifiers, they
discriminate against single reflections from downstream
discontinuities. With perfect directivity, only double
reflections can cause trouble. Single reflections must
override the front to back ratio of the coupler. With
coupler direclivities of 40 dB and (single) reflections on
LOCAL
DISTRIBUTION
'
4
ii
250MHz
IGHZ
_-_-7
OTHER STD L _r_J
FREQS.
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Figure 9-17.
;IC ]
Frequency
I GHZ
25 MHz
IP
P
standard.
TO
MODULATOR
UNITS
101