Keysight x-series User Manual

3 Spectrum Analyzer Mode
3.7 Burst Power Measurement
Low Noise Path Enable
Low Noise Path Enable provides a lower noise floor under some circumstances,
particularly when operating in the 21–26.5GHz region. With the Low Noise Path
enabled, the low band/high band switch and microwave preamp are bypassed
whenever all the following are true:
–
The instrument is not in the Low Band, meaning:
–
the start frequency is above 3.5GHz and
–
the stop frequency is above 3.6GHz
–
The internal preamp is not installed, or if installed, is set to Off or Low Band
Note that this means that, when any part of a sweep is done in Low Band, the Low
Noise Path is not used, whether or not the Low Noise Path Enable is selected in the
user interface. Also, if the preamp is turned on, the Low Noise Path is not used,
whether or not the Low Noise Path Enable is selected in the user interface. The only
time the Low Noise Path is used is when Low Noise Path Enable is selected, the
sweep is completely in High Band (> 3.6 GHz) and no preamp is in use.
For measurements that use IQ acquisition, the low noise path is used when Center
Frequency is in High Band (> 3.6GHz) and no preamp is in use. In other words, the
rules above are modified to use only the center frequency to qualify which path to
switch in. This is not the case for FFTs in the Swept SA measurement; they use the
same rules as swept measurements.
Note that the Low Noise Path, while giving improved DANL, has the disadvantage of
decreased TOI performance and decreased gain compression performance relative
to the standard path.
Note also that the bypass switch is a mechanical switch and has finite life, so if the
Low Noise Path Enable is selected, it is possible to cause frequent cycling of this
switch by frequently changing instrument 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 Standard Path, which will never throw the bypass switch, at the
expense of some degraded noise performance.
The low noise path is useful for situations where the signal level is so low that the
instrument 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
Spectrum Analyzer Mode User's &Programmer's Reference 1283

Keysight x-series Specifications

General

Analysis Bandwidth	Up to 1 GHz (depending on model)
Real-Time Bandwidth	Up to 1 GHz (depending on model)
Form Factor	Benchtop
Connectivity	LAN, USB, GPIB
Resolution Bandwidth	1 Hz to 10 MHz (depending on model)