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3 Short-Range Comms & IoT Mode

3.6 Power Stat CCDF Measurement

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.

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 instrument 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.

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

Keysight x-series Instruction Manual

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