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

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

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

Integrated Publishing, Inc. |