3 WLAN Mode
3.4 SEM Measurement
just the signal power. This is the operation when the detector is Average, and the
Average Type is set to Power.
In other cases, operation is often not quite as good but still highly effective. With
peak detection, the noise floor is estimated based on the RBW and the duration of
the bucket using the same equations used in the noise marker function. The voltage
of the noise is subtracted from the voltage of the observed signal-plus-noise
measurement to compute the estimated signal voltage. The peak detector is one
example of processing that varies with detector to give good estimates of the signal
level without the analyzer noise.
For best operation, the average detector and the power scale are recommended, as
already stated. Peak detection for pulsed-RF can still give excellent effectiveness.
FFT analysis does not work well, and does not do NFE well, with pulsed-RF signals,
so this combination is not recommended. Negative peak detection is not very useful,
either. Sample detection works well but is never better than the average detector
because it does not smooth as well. The Normal detector is a combination of peak
and negative peak behaviors and works about as well as these.
For best operation, extreme smoothing is desirable, as already stated. Using narrow
VBWs works well but using very long bucket durations and the average detector
works best. Reducing the number of trace points makes the buckets longer.
For best operation, the power scale (Average Type = Power) is optimum. When
making CW measurements in the presence of noise without NFE, averaging on the
decibel scale has the advantage of reducing the effect of noise. When using NFE,
the NFE does an even better job than using the log scale ever could. Using NFE with
the log scale is not synergistic, though; NFE with the power scale works a little
better than NFE with log averaging type.
The results from NFE with internal preamp can often be lower than the theoretical
noise in a signal source at room temperature, a noise density of -174 dBm/Hz. This
is expected and useful behavior, because NFE is designed to report the amount of
input signal that is in excess of the thermal noise, not the amount that includes the
thermal noise. This can be a useful behavior because thermal noise often interferes
with what you want to measure, instead of being part of what you want to measure.
Note that NFE is not adequately accurate to always be able to read below kTB.
Adaptive NFE provides an alternative to fully-on and fully-off NFE. Fully-on NFE
can, notably in cases with little or no averaging of the spectrum, result in a display
that is distractingly unfamiliar in the variability in response to low level signals.
Fully-off NFE fails to achieve the potential improvement in dynamic range and
associated accuracy of measurement of low-level signals. Adaptive NFE controls the
degree of potential improvement in the noise floor to give more improvement for
those instrument settings that can make good use of the potential improvement —
those settings with high degrees of variance reduction through some variant of
averaging. When the potential improvement is small, the display acts like the NFE-
WLAN Mode User's &Programmer's Reference 640
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