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

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Figure J-2. Step approximation to ramp.
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QUANTIZATION LEVEL, n
Sidebands
due to approximation.
the coil current is reversed-and that the device exhibits
hysteresis. For a positive frequency offset, the phase
shifter would be driven into saturation with a large
negative current to ensure a stable phase condition, and
then to a predetermined preset point. This would take
place in approximately a microsecond.
The phase would then be advanced at the appropriate
rate by increasing the positive current until a total
excursion of 21r had occurred. The cycle would then be
repeated by resetting the phase shifter as indicated by
the dotted lines in Figure J-4. For negative frequency
offsets, the stable reset condition is achieved by saturating
the phase shifter with a positive current.
For accurate phase control it is desirable to be able to
set the initial phase and then specify the rate of change
(the frequency offset). Microwave phase shifters can be
set to an accuracy of about 2° . This is adequate for
Cyclops. Rate control may be accomplished by frequently
updating the phase shift desired, with this phase shift
stored in a digital numeric register as suggested in
Chapter 10, or by a true rate system (Figure J-5) in
which the input and output signals at the phase shifter
are sampled by directional couplers and then mixed. The
difference frequency may then be compared with the
commanded frequency offset, and if the two are not the
same, an error signal may be generated that can be used
to modify the sweep rate of the phase shifter so, as to
drive the error to zero.
types cost roughly the same so that the' ferrite phase
shifter is to be preferred on the basis of improved
reliability. A typical phase shift versus control current
curve of a ferrite phase shifter is shown in Figure J-4.
Note that the phase shifter is nonreciprocal-that is,
different phase shifts are obtained when the polarity of
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Figure J-5. Rate controlled phase shifter.
Figure .1-4.
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Ferrite phase shifter transfer characteristic.
REFERENCE
1. Klein and Dubrowsky, The Digilator, a New Broadband
Microwave Frequency Translator, IEEE
Trans. on Microwave Theory and Techniques,
MTT-15, 3, March 1967, pp. 172-179.