SECTION III. NOISE MEASURING TECHNIQUES
LEARNING EVENT 12: METHODS OF MEASUREMENT
An ideal receiver would be one with no noise other than that generated by thermal agitation. The degree in
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.
LEARNING EVENT 13: NOISE GENERATOR METHOD
1. 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.
2. 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.
3. 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 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.
LEARNING EVENT 14: SIGNAL GENERATOR METHOD
1. 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 used is the frequency range between the half-power
points of the response curve. For an accurate measurement, the sine wave generator output impedance should
be the same impedance as the normal signal source for the receiver.
SS0031
3-10
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