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Measurement Applications

Resolving Closely Spaced Signals

Signal resolution is determined by the intermediate frequency (IF) filter bandwidth. The

hand held spectrum analyzer, as do conventional spectrum analyzers, traces the shape of its

IF filter as it tunes past a signal. Thus, if two equal-amplitude signals are close enough in

frequency, the filter shapes for the two signals can fall on top of one another and appear as

a single response. Conversely, if two signals are not equal in amplitude but are still close to-

gether, the smaller signal can be hidden under the response of the larger one.

Measurement of Two Signals Having Equal Amplitudes

To resolve two signals of equal amplitude, the resolution bandwidth must be less than or

equal to the frequency separation of the two signals. For example, to resolve two signals of

equal amplitude with a frequency separation of 30 kHz, a resolution bandwidth of 30 kHz

or less should be used. However, most spectrum analyzer IF filter bandwidths are not exact,

varying by as much as ± 20%. This should be taken into consideration whenever testing for

closely spaced signals. Thus, you may want to select the resolution bandwidth within the

lower end of the specification (i.e., –20%, or 24 kHz) to ensure accurate measurements on

two signals spaced to within 30 kHz. In this case the next smallest resolution bandwidth

would be 10 kHz.

Example

Connect two signal sources to the spectrum analyzer input through a signal combiner/split-

ter and set the frequency of one source to 900.0 MHz and the other source to 900.030 MHz.

Set both sources to the same amplitude, preferably –20 dBm. On the Site Master:

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 300 kHz and the center frequency to 900 MHz.

Step 5. Set the resolution bandwidth to 30 kHz and the video bandwidth to 1 kHz.

It should be difficult to discern the presence of two signals. Figure 5-9. shows the results of

the example using a signal combiner/splitter with a loss of 6 dB.

Step 6. Change the resolution bandwidth to 10 kHz.

Figure 5-10 shows the results after changing the RBW using a signal combiner/splitter with

a loss of 6 dB.

5-17

Chapter 5 Spectrum Analyzer Measurements

SgLabs

www.sglabs.it

tel. +39 0755149360

SgLabs

www.sglabs.it

tel. +39 0755149360

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