Agilent Spectrum Analysis Basics - Agilent Technologies

Frequency accuracy
So far, we have focused almost exclusively on amplitude measurements.
What about frequency measurements? Again, we can classify two broad
categories, absolute and relative frequency measurements. Absolute
measurements are used to measure the frequencies of specific signals.
For example, we might want to measure a radio broadcast signal to verify
that it is operating at its assigned frequency. Absolute measurements are also
used to analyze undesired signals, such as when doing a spur search. Relative
measurements, on the other hand, are useful to know how far apart spectral
components are, or what the modulation frequency is.
Up until the late 1970s, absolute frequency uncertainty was measured in
megahertz because the first LO was a high-frequency oscillator operating
above the RF range of the analyzer, and there was no attempt to tie the LO to
a more accurate reference oscillator. Today’s LOs are synthesized to provide
better accuracy. Absolute frequency uncertainty is often described under
the frequency readout accuracy specification and refers to center frequency,
start, stop, and marker frequencies.
With the introduction of the Agilent 8568A in 1977, counter-like frequency
accuracy became available in a general-purpose spectrum analyzer and
ovenized oscillators were used to reduce drift. Over the years, crystal
reference oscillators with various forms of indirect synthesis have been
added to analyzers in all cost ranges. The broadest definition of indirect
synthesis is that the frequency of the oscillator in question is in some way
determined by a reference oscillator. This includes techniques such as phase
lock, frequency discrimination, and counter lock.
What we really care about is the effect these changes have had on frequency
accuracy (and drift). A typical readout accuracy might be stated as follows:
±[(freq readout x freq ref error) + A% of span + B% of RBW + C Hz]
Note that we cannot determine an exact frequency error unless we know
something about the frequency reference. In most cases we are given an
annual aging rate, such as ±1 x 10 -7 per year, though sometimes aging is
given over a shorter period (for example, ±5 x 10 -10 per day). In addition,
we need to know when the oscillator was last adjusted and how close it was
set to its nominal frequency (usually 10 MHz). Other factors that we often
overlook when we think about frequency accuracy include how long the
reference oscillator has been operating. Many oscillators take 24 to 72 hours
to reach their specified drift rate. To minimize this effect, some spectrum
analyzers continue to provide power to the reference oscillator as long as the
instrument is plugged into the AC power line. In this case, the instrument is
not really turned “off,” but more properly is on “standby.” We also need to
consider the temperature stability, as it can be worse than the drift rate.
In short, there are a number of factors to consider before we can determine
frequency uncertainty.
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