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3 Short-Range Comms & IoT Mode

3.12 Monitor Spectrum Measurement

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

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

instrument at different attenuation settings, both with the preamp on and off, in a

measurement that is affected by both instrument noise and instrument 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.

Short Range Comms & IoT Mode User's &Programmer's Reference 1665

Keysight x-series Instruction Manual

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