Agilent Spectrum Analysis Basics - Agilent Technologies

In a factory setting, there is often an in-house frequency standard available
that is traceable to a national standard. Most analyzers with internal
reference oscillators allow you to use an external reference. The frequency
reference error in the foregoing expression then becomes the error of the
in-house standard.
When making relative measurements, span accuracy comes into play.
For Agilent analyzers, span accuracy generally means the uncertainty in
the indicated separation of any two spectral components on the display.
For example, suppose span accuracy is 0.5% of span and we have two signals
separated by two divisions in a 1 MHz span (100 kHz per division). The
uncertainty of the signal separation would be 5 kHz. The uncertainty would
be the same if we used delta markers and the delta reading would be 200 kHz.
So we would measure 200 kHz ±5 kHz.
When making measurements in the field, we typically want to turn our
analyzer on, complete our task, and move on as quickly as possible. It is
helpful to know how the reference in our analyzer behaves under short warm
up conditions. For example, the Agilent ESA-E Series of portable spectrum
analyzers will meet published specifications after a five-minute warm up time.
Most analyzers include markers that can be put on a signal to give us
absolute frequency, as well as amplitude. However, the indicated frequency
of the marker is a function of the frequency calibration of the display, the
location of the marker on the display, and the number of display points
selected. Also, to get the best frequency accuracy we must be careful to
place the marker exactly at the peak of the response to a spectral component.
If we place the marker at some other point on the response, we will get a
different frequency reading. For the best accuracy, we may narrow the span
and resolution bandwidth to minimize their effects and to make it easier to
place the marker at the peak of the response.
Many analyzers have marker modes that include internal counter schemes
to eliminate the effects of span and resolution bandwidth on frequency
accuracy. The counter does not count the input signal directly, but instead
counts the IF signal and perhaps one or more of the LOs, and the processor
computes the frequency of the input signal. A minimum signal-to-noise ratio
is required to eliminate noise as a factor in the count. Counting the signal
in the IF also eliminates the need to place the marker at the exact peak of
the signal response on the display. If you are using this marker counter
function, placement anywhere sufficiently out of the noise will do. Marker
count accuracy might be stated as:
±[(marker freq x freq ref error) + counter resolution]
We must still deal with the frequency reference error as previously discussed.
Counter resolution refers to the least significant digit in the counter readout,
a factor here just as with any simple digital counter. Some analyzers allow
the counter mode to be used with delta markers. In that case, the effects of
counter resolution and the fixed frequency would be doubled.
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