d. The Y factor may be expressed in terms of a number or a ratio, or in decibels. It is usually expressed
in terms of decibels. To express the Y factor in decibels, use the formula
Section III. NOISE MEASURING TECHNIQUES
4-12.
METHODS OF MEASUREMENT
An ideal receiver would be one with no noise other than that generated by thermal agitation. The degree
to which a receiver approaches this ideal is indicated by the noise figure. There are several methods that can be
used to obtain the measurements necessary to determine the noise figure.
4-13.
NOISE GENERATOR METHOD
a. A noise generator is designed to produce a random noise signal that covers a frequency range in
excess of the receiver bandwidth. The dc input reading of the generator can be converted to obtain the true noise
power. The noise generator method of determining the noise figure has the advantage over other methods because
no knowledge of either the gain or the response characteristics of the amplifier is necessary, since the amount of
noise from the noise generator is amplified and governed by the effective bandwidth. The noise generator method
of measurement consists of comparing the noise actually present in the receiver with the calibrated output of the
noise generator. The measurements are taken with an ac voltmeter, a db meter, or a milliwatt meter.
b. For an accurate measurement, the noise generator output impedance is adjusted to the same
impedance as the normal signal source for the equipment under test. This is the impedance at the transmission
line termination from an antenna or antenna multicoupler. The shortest possible leads should be used between the
noise generator and the receiver.
c. The indicator (an ac voltmeter, db meter, or milliwatt meter) may be connected across either the
detector load or the receiver output. If an ac voltmeter is used as an indicator, the noise generator should be
adjusted for an output voltage 1.4 times the no-input voltage indication; if a db meter is used, the noise generator
should be adjusted for a 3-db increase over the no-input meter indication; if a milliwatt meter is used, the noise
generator should be adjusted for twice the no-input reading. The noise figure is then indicated on the output level
control of the noise generator.
4-14.
SIGNAL GENERATOR METHOD
a.
Sine wave signal generators are usually available in maintenance shops more often
than noise
generators. However, the signal generator method is not as practical or accurate as the noise
generator
method for field measurements.
When using the signal generator method, you must
take into
account the bandwidth and response curve of the receiver.
Generally the bandwidth
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