Agilent Technologies X Series Manual page 380

AMPTD Y Scale
analyzer settings such that the above conditions hold true only some of the time. A user making tests of
this nature should consider opting for the , which will never throw the bypass switch, at
Standard Path
the expense of some degraded noise performance.
The low noise path is useful for situations where the signal level is so low that the analyzer performance
is dominated by noise even with 0 dB attenuation, but still high enough that the preamp option would
have excessive third-order intermodulation or compression. The preamp, if purchased and used, gives
better noise floor than does the "Low Noise Path." However, its compression threshold and third-order
intercept are much poorer than that of the non-preamp path. There are some applications, typically for
signals around 30 dBm, for which the third-order dynamic range of the standard path is good enough, but
the noise floor is not low enough even with 0 dB input attenuation. When the third-order dynamic range
of the preamp path is too little and the noise floor of the standard path is too high, the Low Noise Path
can provide the best dynamic range
The graph below illustrates the concept. It shows, in red, the performance of an analyzer at different
attenuation settings, both with the preamp on and off, in a measurement that is affected by both analyzer
noise and analyzer TOI. The green shows the best available dynamic range, offset by 0.5 dB for clarity.
The blue shows how the best available dynamic range improves for moderate signal levels with the low
noise path switched in. In this illustration, the preamp improves the noise floor by 15 dB while degrading
the third-order intercept by 30 dB, and the low noise path reduces loss by 8 dB. The attenuator step size
is 2 dB.
There are other times where selecting the low noise path improves performance, too.
Compression-limited measurements such as finding the nulls in a pulsed-RF spectrum can profit from
the low noise path in a way similar to the TOI-limited measurement illustrated. Accuracy can be
improved when the low noise path allows the optimum attenuation to increase from a small amount like
0, 2 or 4 dB to a larger amount, giving better return loss at the analyzer input. Harmonic measurements,
such as second and third harmonic levels, are much improved using the low noise path because of the
superiority of that path for harmonic (though not intermodulation) distortion performance.
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