Agilent Spectrum Analysis Basics - Agilent Technologies

Typical performance describes additional product performance information
that is not covered by the product warranty. It is performance beyond
specification that 80% of the units exhibit with a 95% confidence level over
the temperature range 20 to 30 °C. Typical performance does not include
measurement uncertainty. During manufacture, all instruments are tested
for typical performance parameters.
Nominal values indicate expected performance, or describe product
performance that is useful in the application of the product, but is not
covered by the product warranty. Nominal parameters generally are not
tested during the manufacturing process.
The digital IF section
As described in the previous chapter, a digital IF architecture eliminates
or minimizes many of the uncertainties experienced in analog spectrum
analyzers. These include:
Reference level accuracy (IF gain uncertainty)
Spectrum analyzers with an all digital IF, such as the Agilent PSA Series,
do not have IF gain that changes with reference level. Therefore, there is no
IF gain uncertainty.
Display scale fidelity
A digital IF architecture does not include a log amplifier. Instead, the log
function is performed mathematically, and traditional log fidelity uncertainty
does not exist. However, other factors, such as RF compression (especially for
input signals above –20 dBm), ADC range gain alignment accuracy, and ADC
linearity (or quantization error) contribute to display scale uncertainty. The
quantization error can be improved by the addition of noise which smoothes
the average of the ADC transfer function. This added noise is called dither.
While the dither improves linearity, it does slightly degrade the displayed
average noise level. In the PSA Series, it is generally recommended that
dither be used when the measured signal has a signal-to-noise ratio of greater
than or equal to 10 dB. When the signal-to-noise ratio is under 10 dB, the
degradations to accuracy of any single measurement (in other words, without
averaging) that come from a higher noise floor are worse than the linearity
problems solved by adding dither, so dither is best turned off.
RBW switching uncertainty
The digital IF in the PSA Series includes an analog prefilter set to 2.5 times
the desired resolution bandwidth. This prefilter has some uncertainty in
bandwidth, gain, and center frequency as a function of the RBW setting. The
rest of the RBW filtering is done digitally in an ASIC in the digital IF section.
Though the digital filters are not perfect, they are very repeatable, and some
compensation is applied to minimize the error. This results in a tremendous
overall improvement to the RBW switching uncertainty compared to analog
implementations.
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