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100 Chapter 4
Making Measurements
Making the Spectrum (Frequency Domain) Measurement
NOTE Parameters under the Advanced key seldom need to be changed. Any
changes from the default advanced values may result in invalid
measurement data.
Make sure the
Spectrum (Freq Domain) measurement is selected under
the
Measure menu. Press the Meas Setup key to access a menu which
allows you to modify the averaging and trigger source for this
measurement (as described in the “Measurement Setup” section). In
addition, the following parameters can be modified:
•
Span - Allows you to modify the frequency span. The range is
10.000 Hz to 10.000 MHz with 1 Hz resolution, depending on the
Res
BW
setting. Changing the span causes the resolution bandwidth to
change automatically, and will affect data acquisition time.
•
Res BW - Allows you to set the resolution bandwidth for the FFT, and
to toggle its mode between
Auto and Man (manual). If set to Auto, the
resolution bandwidth is set to
Span/50 (2% of the span). If set to Man,
you can enter a value ranging from 100.0 mHz to 3.00000 MHz. A
narrower bandwidth will result in a longer data acquisition time.
•
Advanced - Allows you to access the menu to change the following
parameters. The FFT advanced features should be used only if you
are familiar with their operation. Changes from the default values
may result in invalid data.
—
Pre-ADC BPF - Allows you to toggle the pre-ADC bandpass filter
function between
On and Off. The pre-ADC bandpass filter is
useful for rejecting nearby signals, so that sensitivity within the
span range can be improved by increasing the ADC range gain.
—
Pre-FFT Fltr - Allows you to toggle the pre-FFT filter between Flat
(flat top) and
Gaussian. The pre-FFT filter defaults to a flat top
filter which has better amplitude accuracy. The Gaussian filter
has better pulse response.
—
Pre-FFT BW - Allows you to toggle the pre-FFT bandwidth function
between
Auto and Man (manual). The pre-FFT bandwidth filter
can be set between 1 Hz and 10 MHz. If set to
Auto, this pre-FFT
bandwidth is nominally 50% wider than the span. This
bandwidth determines the ADC sampling rate.
—
FFT Window - Allows you to access the following selection menu.
Unless you are familiar with FFT windows, use the flat top filter.
Decimation 0; Auto
IF Flatness On
Table 4-1 Spectrum (Frequency Domain) Measurement Defaults
Measurement Parameter Factory Default Condition