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

arenowsentoutover the cable. At the antenna station
the incoming frequency and the two outgoing frequencies
are added to obtain col. In principle, the order
of addition is immaterial, but greater freedom from
spurious products will obtain if the two inputs to
any mixer differ considerably in frequency. Thus we
can add col to (coo/2) + 5 and then add the sum to
(coo 2) - 5 - col as shown, but we should not add
col to (coo/E) - 5 - col and then add the sum,
(OOo/2) - 5 to (oo0/2) + 5, since 5 will be small, or zero.
However, we can subtract these two inputs in a mixer
as shown at the right of Figure 9-14 to obtain the
frequency 26. If the remote oscillator is made voltage
controllable, we can now apply the output of the
rightmost mixer, after appropriate low-pass filtering, as
the control signal to this oscillator and thus phase lock
the entire system, which sets 5 = 0. In the absence of
cable dispersion and reflections, there will be zero phase
error.
If, for example,
(,Oo/2 = 500 GHz and cot = 200 GHz,
then (coo/2) - cot = 300 GHz and the three frequencies
on the cable are widely separated; no narrow filters are
needed to isolate them. The problem is that they are too
widely separated. The directional couplers must cover a
wide band and dispersion in the cable produces a
significant error.
A second alternative, which avoids these difficulties,
is shown in Figure 9-15. The remote oscillator now has
the frequency (COo/4) + 5, while the standard frequency is
(600/2). The difference product, coo/4 - 5, is then modulated
with co, in a balanced mixer, and' both the upper
and lower sidebands are returned over the cable. At the
antenna station units these frequencies are added to
twice the incoming frequency, either by using a doubler
as shown, or a third mixer so that (ooo/2) + 5 is added
twice. The fundamental phase error in this system is 45r,
or twice as great as before, but since we are setting/5 = 0
by phase locking the remote oscillator, this difference is
immaterial.
With the phase lock in operation the incoming
frequency is coo/4 and the outgoing frequencies are
(coo/4) -+ co,- We can now choose col large enough to
avoid highly selective filters, yet small enough to avoid
cable dispersion problems. Since the two outgoing
frequencies are symmetrically disposed about the incoming
frequency, cable dispersion causes only a second
order effect.
In coaxials (and open wire lines) the conductor losses
cause the attenuation of the line to increase as the
square root of frequency. This loss causes an added
phase, over and above that associated with the propagation
time of a TEM wave, which amounts to one radian
POINT
(i)
LSB
FREQUENCY
toO
-¥*_
w 0
toO
® T
@
@
too
-a--8
too
to,
_--_+_,
coo
k23 -T -8 -to I
too
(Z) q- - _ - ",
®
@
Figure 9-15.
_0
to O
+8 -co I
too
PHASE
Oo
0
-Oo+(T+
81
tOO
-Oo+O,+(_+8)z
-Oo- O, + (-_
g)T
+_)r
-Oo +e, + (28-to,)
-80 -8 I + (2_ +(.Oi)
280
80-81 +(28+to I
48"r
--280 + 48"r
4(80+_T )
Standard frequency delay compensation
(Cyclops proposal II).
per neper of attentuation. Since this phase term varies as
co 1/2 rather than directly as co, it does not represent a
constant delay. The effect of this cable dispersion is
analyzed in Appendix K, where it is shown that with
6 = 0, the system of Figure 9-14 will have a phase error
A01 = -
while that of Figure 9-1 5 will have an error
No (24)
T
100