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.
b. When using the sine wave generator, the measuring procedure is similar to that for the noise generator
method. First, with no signal output from the signal generator, measure the noise power output of the receiver.
Then turn the signal generator on, set the output signal at the center frequency of the response curve for the
receiver, and adjust the output signal level until the test meter indicates twice the power of the no-signal level.
With the reading from the meter1 the noise figure can now be calculated.
4-15.
ENSI METHOD
a. The equivalent-noise-sideband-input (ensi) method of noise level measurement determines the
equivalent input voltage of all random noise that appears in the output of the receiver being tested. This test is
sometimes used in preference to other methods of measuring noise level because, over a limited frequency range,
it is not appreciably affected by changes in the input signal.
b. The receiver volume control should be set to avoid overloading the audio amplifiers, and the tone
control should be set for maximum high-frequency response. The signal generator is set at the center frequency
of the receiver response curve, and adjusted for an unmodulated carrier signal output. A voltmeter is connected in
a manner similar to that used for the noise generator method, and used to measure the output power. The signal
and noise output power can be measured together. The signal output power can then be calculated by subtracting
the noise output power from the combined power output. With this figure, the noise level can be calculated.
Section IV. PARAMETER CONTROL
4-16.
IMPROVEMENT FACTOR
a. For the receiver to deliver to the baseband output a recovered baseband wave of the best possible
quality (highest signal-to-noise ratio), the receiver's amplifier and demodulator stages must be designed not only
for the type of modulation, but also for the exact parameters of the chosen type of modulation. The primary
function of the receiver is to amplify and frequency-translate. If the type of modulation is amplitude modulation
(AM), the receiver must have a sufficiently accurate automatic gain control (AGC) to avoid overload,
nonlinearity, and limiting. Any nonlinear amplifying of an AM wave will distort the demodulated baseband
wave. On the other hand, angle-modulated receivers usually limit the amplitude intentionally to provide better
immunity against noise. Amplitude limiting has no effect on the frequency or phase deviations in an angle-
modulated wave.
distortion in the demodulated baseband signal. This means that for single sideband, the receiver's bandwidth need
be only as
344 L4
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