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AMPTD Y Scale
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.
µW Preselector Bypass
This key toggles the preselector bypass switch for band 1 and higher. When the microwave presel is on,
the signal path is preselected. When the microwave preselector is off, the signal path is not preselected.
The preselected path is the normal path for the analyzer.
The preselector is a tunable bandpass filter which prevents signals away from the frequency of interest
from combining in the mixer to generate in-band spurious signals (images). The consequences of using a
preselector filter are its limited bandwidth, the amplitude and phase ripple in its passband, and any
amplitude and phase instability due to center frequency drift.
Option MPB or pre-selector bypass provides an unpreselected input mixer path for certain X-Series
signal analyzers with frequency ranges above 3.6 GHz. This signal path allows a wider bandwidth and
less amplitude variability, which is an advantage when doing modulation analysis and broadband signal
analysis. The disadvantage is that, without the preselector, image signals will be displayed. Another