B+K precision 2165A, Instruction Manual, Page: 20 / 28

20
Introduction to Spectrum Analysis
Notice that the spurious-free measurement range can be
extended by reducing the level at the input mixer. The
only limitation, then, is sensitivity. To ensure a maximum
dynamic range on the CRT display, check to see that the
following requirements are satisfied.
1.
The largest input signal does not exceed the optimum
input level of the analyzer (typically –27 dBm with 0
dB input attenuation).
2.
The peak of the largest input signal rests at the top of
the CRT display (reference level).
Frequency Response
The frequency response of an analyzer is the amplitude
linearity of the analyzer over its frequency range. If a
spectrum analyzer is to display equal amplitudes for input
signals of equal amplitude, independent of frequency,
then the conversion (power) loss of the input mixer must
not depend on frequency. If the voltage from the LO is
too large compared to the input signal voltage then the
conversion loss of the input mixer is frequency dependent
and the frequency response of the system is nonlinear.
For accurate amplitude measurements, a spectrum
analyzer should be as flat as possible over its frequency
range.
Flatness is usually the limiting factor in amplitude
accuracy since it’s extremely difficult to calibrate out.
And, since the primary function of the spectrum analyzer
is to compare signal levels at different frequencies, a lack
of flatness can seriously limit its usefulness.
Tracking Generators
The tracking generator (Models 2620A and 2630 only) is
a special signal source whose RF output frequency tracks
(follows) some other signal beyond the tracking generator
itself. In conjunction with the spectrum analyzer, the
tracking generator produces a signal whose frequency
precisely tracks the spectrum analyzer tuning. The
tracking generator frequency precisely tracks the
spectrum analyzer tuning since both are effectively tuned
by the same VTO. This precision tracking exists in all
analyzer scan modes. Thus, in full scan, the tracking
generator output is a start-stop sweep, in zero scan the
output is simply a CW signal.
The tracking generator signal is generated by synthesizing
and mixing two oscillators. One oscillator is part of the
tracking generator itself, the other oscillator is the
spectrum analyzer lst LO. The spectrum analyzer/tracking
generator system is used in two configurations: open-loop
and closed-loop. In the open-loop configuration, unknown
external signals are connected to the spectrum analyzer
input and the tracking generator output is connected to a
counter. This configuration is used for making selective
and sensitive precise measurement of frequency, by tuning
to the signal and switching to zero scan.
In the closed-loop configuration, the tracking generator
signal is fed into the device under test and the output of
the device under test is connected to the analyzer input.
In this configuration, the spectrum- analyzer/tracking-
generator becomes a self-contained, complete (source,
detector, and display) swept frequency measurement
system. An internal leveling loop in the tracking generator
ensures a leveled output over the entire frequency range.
The specific swept measurements that can be made with
this system are frequency response (amplitude vs.
frequency), magnitude only reflection coefficient, and
return loss. From return loss or reflection coefficient, the
SWR can be calculated. Swept phase and group delay
measurements cannot be made with this system; however,
it does make some unique contributions not made by other
swept systems, such as a sweeper/network analyzer, a
sweeper/spectrum analyzer, or a sweeper/detector
oscilloscope.
Precision tracking means at every instant of time the
generator fundamental frequency is in the center of the
analyzer passband, and all generator harmonics, whether
they are generated in the analyzer or are produced in the
tracking generator itself, are outside the analyzer
passband. Thus only the tracking generator fundamental
frequency is displayed on the analyzers CRT. Second and
third order harmonics and intermodulation products are
clearly out of the analyzer tuning and, therefore, they are
not seen. Thus, while these distortion products may exist
in the measurement set-up, they are completely eliminated
from the CRT display.
The 1 dB gain compression level is a point of
convenience, but it is nonetheless considered the upper
limit of the dynamic range. The lower limit, on the other
hand, is dictated by the analyzer sensitivity which, as we
know, is bandwidth dependent. The narrowest usable
bandwidth in turn is limited by the tracking generator
residual FM and any tracking drift between the analyzer
tuning and the tracking generator signal.