2. When using the sine wave generator, the measuring procedure is similar to that for the noise generator
method. First, with no sign 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 meter, the noise figure can now be calculated.
LEARNING EVENT 15: ENSI METHOD
1. 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.
2. The receiver volume control should be set to avoid overloading the audio amplifiers and the tone control
should be set for maximum HF 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 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
LEARNING EVENT 16: IMPROVEMENT FACTOR
1. For the receiver to deliver to the baseband output a recovered baseband wave of the best possible quality
(highest S/N), 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 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.
3-11
SS0031
Previous Page
