Specifications

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Average detection
Although modern digital modulation schemes have noise-like characteristics,
sample detection does not always provide us with the information we need.
For instance, when taking a channel power measurement on a W-CDMA
signal, integration of the rms values is required. This measurement involves
summing power across a range of analyzer frequency buckets. Sample
detection does not provide this.
While spectrum analyzers typically collect amplitude data many times in
each bucket, sample detection keeps only one of those values and throws
away the rest. On the other hand, an averaging detector uses all the data
values collected within the time (and frequency) interval of a bucket. Once
we have digitized the data, and knowing the circumstances under which
they were digitized, we can manipulate the data in a variety of ways to
achieve the desired results.
Some spectrum analyzers refer to the averaging detector as an rms detector
when it averages power (based on the root mean square of voltage). Agilent
PSA and ESA Series analyzers have an average detector that can average
the power, voltage, or log of the signal by including a separate control to
select the averaging type:
Power (rms) averaging averages rms levels, by taking the square root of the
sum of the squares of the voltage data measured during the bucket interval,
divided by the characteristic input impedance of the spectrum analyzer,
normally 50 ohms. Power averaging calculates the true average power, and
is best for measuring the power of complex signals.
Voltage averaging averages the linear voltage data of the envelope signal
measured during the bucket interval. It is often used in EMI testing for
measuring narrowband signals (this will be discussed further in the next
section). Voltage averaging is also useful for observing rise and fall behavior
of AM or pulse-modulated signals such as radar and TDMA transmitters.
Log-power (video) averaging averages the logarithmic amplitude values (dB)
of the envelope signal measured during the bucket interval. Log power
averaging is best for observing sinusoidal signals, especially those near
noise.
11
Thus, using the average detector with the averaging type set to power provides
true average power based upon rms voltage, while the average detector with
the averaging type set to voltage acts as a general-purpose average detector.
The average detector with the averaging type set to log has no other equivalent.
Average detection is an improvement over using sample detection for the
determination of power. Sample detection requires multiple sweeps to collect
enough data points to give us accurate average power information. Average
detection changes channel power measurements from being a summation
over a range of buckets into integration over the time interval representing
a range of frequencies in a swept analyzer. In a fast Fourier transfer (FFT)
analyzer
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, the summation used for channel power measurements changes
from being a summation over display buckets to being a summation over
FFT bins. In both swept and FFT cases, the integration captures all the power
information available, rather than just that which is sampled by the sample
detector. As a result, the average detector has a lower variance result for the
same measurement time. In swept analysis, it also allows the convenience of
reducing variance simply by extending the sweep time.
11. See Chapter 5, Sensitivity and Noise.
12. Refer to Chapter 3 for more information on the FFT
analyzers. They perform math computations on
many buckets simultaneously, which improves the
measurement speed.