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As mentioned above, the user rate sampling modes increase the maximum Main
Time length at the expense of possible aliasing of out-of-band signals into the
frequency span of the measurement. The spectrum ranges subject to the alias
conversion are called alias exposure zones. To determine if any out-of-band
signals exist in the exposure zones, run the Alias Exposure Zone Checker macro.
6.1 Setting up the Alias Exposure Zone Checker
Follow this procedure for setting up the Alias Exposure Zone Checker:
1. Start the VSA application.
2. Click on Utilities\Macros\Recall. Navigate to the Examples\Macros
subdirectory in the directory where you installed the VSA application (for
example, C:\Program Files\Agilent\89600 VSA\Examples\Macros). Select
RecallSetupAfterAliasCheck and AliasChecker fi les, and click OK.
3. Right click on the top of the VSA application. You will see a popup showing
the various toolbars. Select the Macros toolbar. This will make the alias
checker icons appear on your VSA application toolbar.
6.2 Using the Alias Exposure Zone Checker
The Alias Exposure Zone Checker calculates the frequencies of the alias
exposure zones for any combination of center frequency, span, and digitizer
sample rate. It then measures the power in each zone and compares it to the
power of the desired signal in the span of the measurement setup at the time
the checker was started. An unwanted signal in an exposure zone is a source of
interference if it is larger than the desired signal’s amplitude minus 40 dB. This
–40 dBc threshold is approximately the same level as the quantization errors in
the digitizer’s 10-bit digitizer, and represents the limit of the digitizer’s dynamic
range. The –40 dBc level is suffi cient for 1 to 2 percent EVM measurements.
Launch the checker macro by using the tool path Utilities/Macros/Alias
Checker or click its icon (shown below) on the bottom of the tool bar.
NOTE: The Alias Exposure Zone Checker cannot be used with the U1066A-001
(DC440) HF Input.
When the macro stops, a full-span spectrum trace (Figure 7) will appear in Trace
A (green). It shows the desired signal plus any unwanted signals. Superimposed
is an orange Trace C showing boxes for each exposure zone frequency range.
Any unwanted signal falling inside the zone box is a source of alias interference.
If the height of any box is higher than the orange reference line, the power in
that zone is greater than the –40 dBc threshold of interference. Click Yes in the
checker dialog box to return to the original measurement setup. Click No for
further examination of the out-of-band signals. You can return to the original
measurement setup by clicking the Recall Setup After Alias Check macro icon.
6 Out-of-Band Alias
Exposure Zones