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

systemas for the IF systemitself.For a thousand
element 10-km-diameter array this would be about 5000
to 7000 miles of cable. Clearly, the use of IF cable for
all the delay would be cumbersome and costly.
Nevertheless, it might be worth considering making
all the IF cables in the IF distribution system the same
length as the longest cable. This would add about 50%
more IF cable and, using the equalization scheme
proposed, would require no more repeaters. The additional
cost would average about $5000 per line for the
lO-km array, which may not be much greater than the
cost of an electronic delay unit. The extra IF cable could
be accommodated by running it out a side tunnel as far
as necessary and then back to the antenna involved. We
would then have an array already phased and stabilized
for looking at the zenith and would need only to add the
variable delay to tilt the delay plane.
Acoustic surface wave delay lines appear to be the
best broadband delay devices available at this time. In
these devices, a thin film interdigital electrode structure
is used to launch a Rayliegh or Love type surface wave
on a piezoelectric crystal substrate such as quartz or
lithium niobate. A similar electrode structure detects the
passage of the wave at the other end or at an
intermediate point. The surface waves propagate with
essentially no dispersion and with very little loss
(_ l dB//Jsec). The loss, bandwidth, and dispersion problems
occur in the transducers (electrode structures) used
to couple tile electrical signals into and out of the crystal.
A good survey of acoustic surface wave techniques is
given by R.N. White in the August 1970 Proceedings of
the IEEE.
Lithium niobate lines appear most attractive at this
time. Some of the properties obtainable with this
material are listed in Table 10-3.
TABLE 10-3
be lengthened
by an amount
a
t_ = v(r_ - c) (1 --a ) (21)
thereby filling in the basic delay cone in Figure 10-10 to
a new cone (shown dotted) having its apex a/c below the
delay r_. The total delay that now must be added by the
delay system to achieve full steerability is 2a/c and is the
same for each IF line. Although full use will never be
made of the variability of central delay units, several
advantages including interchangeability accrue from having
all units
alike.
It is proposed that the delay system associated with
each line consist of individual delay units connected in
tandem, each unit having a delay
r k = ¢o + 2k nsec (22)
when switched in, and a delay ro when switched out.
For a lO-km array, k ranges from -2 to +14. The delay
required in any line (in units of 250 picoseconds) is then
expressed as a 17-digit binary number and the delay
units are switched in or out if the digit they represent is
a 0 or 1 respectively.
t
r
COMPUTER
CONTROL
Velocity 3.45X 10s cm/sec
Temperature coefficient 85 ppm/°C
Insertion loss 10- 35 dB
Fractional bandwidth 75%
Center frequency 0.1 to 1 GHz
Delay precision