Agilent Spectrum Analysis Basics - Agilent Technologies

From the graph, we see that a –10 dBm signal at the mixer produces a
second-harmonic distortion component of –45 dBc. Now we tune the analyzer
to the 6 GHz second harmonic. If the preselector has 70 dB rejection, the
fundamental at the mixer has dropped to –80 dBm. Figure 7-11 indicates
that for a signal of –80 dBm at the mixer, the internally generated distortion
is –115 dBc, meaning 115 dB below the new fundamental level of –80 dBm.
This puts the absolute level of the harmonic at –195 dBm. So the difference
between the fundamental we tuned to and the internally generated second
harmonic we tuned to is 185 dB! Clearly, for harmonic distortion, dynamic
range is limited on the low-level (harmonic) end only by the noise floor
(sensitivity) of the analyzer.
–45
–50
–60
Internal distortion (dBc)
–70
–80
–90
–100
–110
–115
–120
–90 –80 –70 –60 –50 –40 –30 –20 –10 0
Mixed level (dBm)
Figure 7-11. Second-order distortion graph
What about the upper, high-level end? When measuring the oscillator
fundamental, we must limit power at the mixer to get an accurate reading
of the level. We can use either internal or external attenuation to limit
the level of the fundamental at the mixer to something less than the 1 dB
compression point. However, since the preselector highly attenuates the
fundamental when we are tuned to the second harmonic, we can remove
some attenuation if we need better sensitivity to measure the harmonic.
A fundamental level of +20 dBm at the preselector should not affect our
ability to measure the harmonic.
Any improvement in dynamic range for third-order intermodulation
measurements depends upon separation of the test tones versus preselector
bandwidth. As we noted, typical preselector bandwidth is about 35 MHz at
the low end and 80 MHz at the high end. As a conservative figure, we might
use 18 dB per octave of bandwidth roll off of a typical YIG preselector filter
beyond the 3 dB point. So to determine the improvement in dynamic range,
we must determine to what extent each of the fundamental tones is
attenuated and how that affects internally generated distortion. From
the expressions in Chapter 6 for third-order intermodulation, we have:
(k 4 /8)V LO V 1 2 V 2 cos[ω LO – (2ω 1 – ω 2 )]t
and
(k 4 /8)V LO V 1 V 2 2 cos[ω LO – (2ω 2 – ω 1 )]t
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