Radar System Engineering

SEC. 13.4] ANGLE-DATA TRANSMITTERS 487
elude potentiometers, variable transformers, variable condensers, and
generators.
Potentiometers.-Potentiometers are principally used to provide
voltages whose only frequency components are those resulting from the
scanning. Because even the best potentiometers have a certain amount
of brush ‘(jitter” which can be removed only by filtering, they are not
very satisfactory for resolving range sweeps. A wide variety of linear
potentiometers differing in accuracy, size, ruggedness, and so on have
been developed specifically for radar use (see Vol. 17).1 For many purposes
they provide the simplest method of data transmission and often
they are more effective than any other device. A difficulty arises,
however, in those cases where it is necessary to shift the sector under
view. This can be done within limits by adding a fixed voltage to the
circuit, but over any extended angle it is necessary to introduce the shift
mechanically by using a differential gear or by rotating the body of
the potentiometer. This requires either that the potentiometer be near
the operator or that a remote mechanical control be provided. Since the
second method is costly, some other data-transmission system is usually
chosen in preference.
Several varieties of potentiometer have been made with sine or with
sine and cosine characteristics, for use as resolvers (Vol. 17). These have
been designed with great care and are fairly good at low turning rates
and low signal frequencies. As data transmitters they are occasionally
used for purposes of pre-time-base resolution on slowly scanning systems,
or as a basis of information for computers.
Variable Transformers (Resolvers, Synchros, etc.).—Figure 13.6a illustrates
the principle of certain variable transformers called “resolvers,”
‘‘ synchros, “ “ selsyns, “ “ autosyns, ” and so on, which are widely used
as position-data transmitters. An iron-cored coil (rotor) of special shape
is mounted on a freely turning axis inside a slotted-iron framework much
like a motor stator. Two or three stator coils are symmetrically wound
into the slots in such a way that the coupling of each with the rotor is
proportional to the sine of the rotor angle measured with respect to a
position of zero coupling. The device thus fulfills the requirements of a
resolver for a-c signals. An important aspect of this process is that the
polarity of the output signal at a given phase of the input signal reverses
as the synchro passes through a null position for that particular secondary
winding.
If the stators of an excited resolver, called the “ transmitter,” are
loaded with the stators of a second, called the” repeater” or the” receiver,”
(Fig. 13.6b) the latter will experience currents producing a changing
I ilfany of thesepotentiometersare useful as control elementsfor purposesother
thm scanner-datatransmission.