Instructions

Page 59
8 Scan Mode
8.1 Scan Mode Explanation
Scan mode is a facility which enables spans to be analyzed with a higher resolution than in
normal sweep mode. In scan mode the analyzer creates a file containing data that would
normally be discarded. This high resolution file can be analyzed using the display of the PSA,
on a PC using PSA-Manager, or exported as a CSV file for use with other programs.
Scan mode is only available when option U02 ( and Updatable Firmware v2.xx or above) is
installed.
8.1.1 Limitation of Normal Sweep Mode
The normal sweep mode of the analyzer displays the level at 271 points across the span. The
resolution is defined by the RBW filter, and to adequately resolve a signal frequency, at least
three display points per RBW are needed. Auto RBW set the RBW value to the narrowest
available that will maintain three points per filter width across the screen.
When the RBW is manually set to be narrower than this, the analyzer will continue to sample at
least three points per filter width, but combines these points before displaying them. The level
shown is a single value calculated from all the levels in the block of frequencies represented by
each display point.
Setting an RBW narrower than span/90 provides only limited improvement in resolution and
none at all beyond span/270.
8.1.2 Benefits of Scan Mode
It can be useful to observe and record frequencies and levels to high resolution over a wide
span. This is outside of the capability of a conventional swept spectrum analyzer because of
the limited number of collection and display points available.
For a wide sweep at a narrow RBW the analyzer is collecting more data points than it can
display. Normally these points are discarded with only a single level (as defined by the
detector) being retained for each display point.
In Scan Mode, all of these data points are retained and are saved in a file. This process is
referred to as a “scan” rather than a sweep. The number of points saved is a function of the
span and the RBW, and is equal to (3 x Span/RBW) +1. For example, for a span of 165MHz at
a 30kHz RBW, the number of points saved would be 16,501.
The absolute maximum number of points that can be stored within a scan is 210,000 whilst the
minimum is 541.
For a conventional sweep of a 200MHz span, the appropriate RBW would be 3MHz providing
limited resolution. However in scan mode the RBW could be decreased as far as 3kHz, an
improvement in resolution of 1,000 times.
8.1.3 Displaying Scan Mode Files
Because the scan mode file contains more points than the 271 of the display, a different
method has be used to show the contents.
No information is displayed while the scan is taking place, and only a progress line is shown.
Once the scan is completed, an overview of its contents is displayed. The scan can then be
analyzed in detail using zoom and pan controls.
Note that the scan file is saved within volatile memory and the contents will be lost if the
instrument is turned off, or if scan mode is exited. To retain the scan file data it will be
necessary to save it to a permanent file.