Anritsu spectrum master ms2711d User Manual page 118

Example 1: 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. IF filter bandwidths can vary by ± 5%. 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., –5%, or 28.5 kHz) to
ensure accurate measurements on two signals spaced to within 30 kHz. In this case the next
smaller 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 MS2711D Spectrum
Master:
Step 1. Set the input attenuator to 20 dB.
Step 2. Set the Ref 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 Auto.
Even with averaging on, it could be difficult to discern the presence of two signals. Figure
4-3 shows the results of the example using a signal combiner/splitter with a loss of 6 dB.
Figure 4-3. Measurement of Two Signals Having Equal Amplitudes, RBW = 30 kHz
Chapter 4 Measurement Fundamentals
4-7