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Using the Analyzer
Measurement Techniques
Optimizing the Sensitivity Setting
Sensitivity is the difference (in dB) between the top graticule of the display
and the lowest amplitude feature of the trace to be measured. The WDCA will
use switch gain stages and digital filtering to reduce the rms noise floor 7.8 dB
below the stated sensitivity (a linear factor of 6).
If the range between sensitivity and the top graticule of the display requires
more than one gain stage, the WDCA will take one sweep per gain stage,
resulting in multiple sweeps per trace update. Each gain stage covers about 40
dB range. While it is possible to stretch a gain range downward with digital fil-
tering, it is generally faster to switch gain stages.
A signal that just touches the top display graticule line is the largest signal to
be measured. This is referred to the top of screen. The normalization data is
used to convert the top of screen value to dBm, and that value is used to select
the gain range. Don't confuse the reference level with the largest signal to be
measured: the reference line is normally one division below top of screen. You
can put the reference line at the top of screen by going into Amplitude >
Amplitude Setup and setting the Reference Level Position to 10 divi-
sions. This has the advantage that Amplitude > Peak To REF LEVEL will
put the top of the trace in its optimum position.
Like the top of screen, the instrument uses the normalization data to convert
sensitivity back to an absolute signal level in dBm. The WDCA has a specified
receiver noise floor of –85 dBm, so sensitivities that convert to powers below
–85 dBm result in very long sweep times. The instrument will not allow you to
enter a sensitivity that converts to a power below –91 dBm.
Increase in Sweep Time
As you increase the sensitivity, you will notice that the sweep time increases:
• If another gain range is required, the instrument will take multiple sweeps
for each trace update. You may see the sweep light blink several times. If the
trace update bar is visible, it will show several full sweeps before the visible
trace updates. In practice, most measurement conditions will be covered by
2 sweeps, with 3 sweeps being the most ever used.
• If the new gain range has high enough gain, the sweep will slow down to ac-
commodate the amplifier.
• Digital filtering will be used if the requested sensitivity is high enough, fur-
ther slowing the sweep down.
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