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Measurement of Two Signals Having Unequal Amplitudes

Typically, in real world environments, closely spaced signals do not have equal amplitudes.

Often, the difference between closely spaced signals can be as much as –60 dB. To resolve

two signals of unequal amplitude, the resolution bandwidth must be less than or equal to the

frequency separation of the two signals (the same as resolving two equal amplitude signals).

However, in this case the largest resolution bandwidth that will resolve the two unequal sig-

nals is determined primarily by the shape factor of the IF filter, rather than by the 3 dB

bandwidth. Shape factor is defined as the ratio of the 60 dB bandwidth to the 3 dB band-

width of the IF filter.

Therefore, to resolve two signals of unequal amplitude, the half-bandwidth of a filter at the

point equal to the amplitude separation of the two signals must be less than the frequency

separation of the two signals. If you do not know the specific shape factor of the IF Filter,

perform this measurement as if the signals had equal amplitudes paying close attention to

potential signals having unequal power levels that are closely spaced. This will take some

adjusting among the various resolution and video bandwidth and span functions.

Example

Connect two signal sources to the spectrum analyzer input with a combiner/splitter. Set the

frequency of one source to 1900.0 MHz and the other source to 1900.1 MHz. Set one

source to + 20 dBm, and the other to – 15 dBm.

Step 1. Set attenuator coupling to AUTO.

Step 2. Set the Reference Level to –10 dBm.

Step 3. Set the scale to 10 dB/div.

Step 4. Set the span to 1 MHz and the center frequency to 1900 MHz.

Step 5. Set the resolution bandwidth to 10 kHz and the video bandwidth to 100 Hz.

The two signals should be observed on the spectrum analyzer, each with differ-

ent amplitudes and spaced 100 kHz apart.

Figure 5-11 shows the results of the example using a signal combiner/splitter

with a loss of 6 dB. Marker 2 is set as a delta marker, and shows the difference

in frequency and amplitude between the two peaks.

5-19

Chapter 5 Spectrum Analyzer Measurements

Figure 5-11. Measurement of Two Signals Having Unequal Amplitudes

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Anritsu S113C Guia do Utilizador Página 91