with the single input, the phase detector output
different manufacturers have different methods
is zero (the cosine of the angle is zero).
of determining the signal quadrant. Manu-
The point where the two signals are in phase
facturers also differ on whether the final reading
is a leading or a lagging phase shift. This means
or 180 degrees out of phase is the point of
maximum deflection on the meter. The difference
that you should be familiar with the type of phase
between the in-phase and 180 degrees out-of-phase
angle voltmeter you are using. You can't assume
points is in the direction of needle swing, not the
that the method used to determine phase angle
distance it swings. Upon approaching the point
on one type of meter can be used on another.
of maximum deflection, the rate of change of the
meter reading decreases. This happens because the
DIFFERENTIAL VOLTMETER
cosine has a smaller rate of change near 0 degree.
This makes it difficult to read the exact point of
The differential voltmeter is a reliable piece
maximum deflection.
of precision test equipment that provides
Most commercial voltmeters determine the
extremely accurate volt age measurements. Its
point at which the signals are 90 degrees out of
general function is to compare an unknown
phase as quadrature. The cosine has a maximum
voltage with a known internal reference voltage
rate of change as it approaches 90 degrees, thus
and show the difference in their values. The Model
it gives a better indication on the meter. When
893A, differential voltmeter (fig. 2-9) is commonly
the voltmeter is setup for this point, there must
used by naval personnel.
be some way of converting the phase shifter
You can use the differential voltmeter as a
reading. You have to convert the reading to show
conventional transistorized electronic voltmeter
the correct amount of phase shift, rather than 90
(TVM) or as a differential null voltmeter. You can
degrees more or less than the actual amount.
also use it to measure variations of a voltage near
Some confusion exists in this area because
some known value (null detector), high resistance
Figure 2-9.-Differential voltmeter.
