Noise compensation details are described by assuming the following model test setup.

When the switch connects to the DUT path, the noise power measured by the signal analyzer, PMEAS, is

PMEAS = PDUT + PANALYZER    (1)

where PDUT denotes the average noise power of the DUT, and PANALYZER denotes the noise power of the signal analyzer and the switch.

When the switch is connected to a 50 Ω termination path, the average noise power measured by the signal analyzer is

PCAL = kTB + PANALYZER    (2)

where k = 1.38 × 10-23 m2kg s-2K-1 is the Boltzmann constant, T = 290 K is the the approximate room/reference temperature, and B is the bandwidth, in Hz, over which power measurements are done.

Equations (1) and (2) assume that the analyzer's noise power (PANALYZER) is uncorrelated with the DUT noise power (PDUT) and the noise added by the 50 Ω termination.

When you set the Noise Compensation Type property to Analyzer Only, the DUT noise power is

PDUT ← kTB + max{PMEAS – PCAL,0.0630957 × PCAL}    (3A)

When you set the Noise Compensation Type property to Analyzer and Termination, the portion of the DUT noise power that is in excess of the thermal noise floor, kTB, is

PDUT ← max{PMEAS – PCAL,0.0630957 × PCAL}     (3B)

Note  
  1. Noise powers PMEAS and PCAL are random variables and therefore it is possible that PMEAS – PCAL may turn out to be less than or equal to zero. To avoid returning absurd results, powers reported after noise compensation are never allowed to fall to values less than 0.0630957 × PCAL, that is, 12 dB less than PCAL on a logarithmic scale.
  2. When you set the Noise Compensation Type property to Analyzer and Termination, the reported DUT power can be lower than the thermal noise floor of kT or –174 dBm/Hz. The reported power should be interpreted to be in excess of the thermal noise floor of –174 dB/Hz.

Measurement Guidelines

The general flow of spectral measurements with noise compensation is as illustrated:

For the PXIe-5668 with PXIe-5698 Preamplifier Module, when you perform spectral measurements with noise compensation, RFmx throws an exception, as illustrated:

For the PXIe-5644, 5645, 5646, and 5820 devices, when you perform spectral measurements with noise compensation, RFmx throws an exception, as illustrated:

Recommended Settings

  1. Noise added by the signal analyzer reduces with reduction in RF Attenuation. Keep the instrument’s RF Attenuation as low as possible.
  2. Enabling RF Preamplifier also reduces the noise figure of the signal analyzer. Whenever possible, enable the RF Preamplifier.

Noise Compensation Guidelines

When performing noise compensation in a measurement, it is crucial to understand the different signal analyzer parameters that affect the noise calibration. In order to achieve the noise compensated measurement result, same parameter values for noise calibration and noise compensation must be maintained. The properties that you use for calibration changes depending on the hardware used. The following table lists the RFmx properties for calibration and their corresponding hardware.

 Hardware  RFmx Properties
PXIe-5830/PXIe-5831/PXIe-5832  Center Frequency, Selected Port, LO Source, Downconverter Frequency/Downconverter Frequency Offset, Reference Level, Reference Level Headroom, Preamp Enabled, Cleaner Spectrum, Optimize Path for Signal Bandwidth, External Attenuation, Thermal Correction Headroom Range, Gain from S-Parameter, and Averaging Count
PXIe-5603, PXIe-5605, PXIe-5665, PXIe-5668  Center Frequency, Downconverter Frequency/Downconverter Frequency Offset, Reference Level, Preamp Enabled, Cleaner Spectrum, and Averaging Count
PXIe-5601, PXIe-5663E  Center Frequency, Downconverter Frequency/Downconverter Frequency Offset, Reference Level, and Averaging Count

Noise Calibration Database

The noise calibration database is a storage that RFmx maintains to store all the noise calibration data. This database is maintained individually for each device. Based on the device used, the noise calibration data is stored to their respective databases.

Validate Noise Calibration Database

RFmx provides a utility to check whether noise data is present in the database for the given configuration. This utility references an initialized RFmx session and checks the noise calibration database using the specified configuration. This utility does not require a Measurement Initiate or Commit to be called to obtain the status. It returns True if the noise data is present, else it returns False based on either of the following conditions:

  1. Calibration data is not present for the specified configuration
  2. Difference between the current device temperature and the device temperature at which calibration was performed is beyond the [–5°C, +5°C] range

This utility does not return error if the noise data is not present, but it returns error for the following conditions:

  1. Using this utility on a session initialized as AnalysisOnly mode.
  2. Using this utility on a session initialized with a device that does not support noise compensation

Clear Noise Calibration Database

RFmx provides a utility to clear all the noise calibration database entries. Calling the Clear Noise Calibration Database utility clears all the noise calibration data associated with the device, including the data in the current session. This utility depends on the Noise Calibration Mode property that you configure on the RFmx session. For the same device, RFmx maintains two different databases based on the Noise Calibration Mode property (Manual/Auto). You must set the Noise Calibration Mode property before calling the Clear Noise Calibration database utility to select the database to be cleared.

While performing self-calibration on a device, clear the already existing noise database. This ensures that the previous noise data before the self-calibration is not considered. This utility does not return any error if there is no existing calibration database to be cleared.

Manual Noise Calibration Using C and .NET APIs

Noise calibration, in the context of noise compensation for spectral measurements such as ACP and others, involves noise statistics estimation of the signal analyzer. While performing noise calibration, RFmx estimates and saves all relevant noise statistics to a file on the disk. As long as the RFmx session is kept alive, the noise calibration data, including traces, are cached in the program memory. Data is saved to disk only when the RFmx session is closed, and the cached data is purged from the memory.

Noise spectrum traces are a notable exception to the data saved on the disk considering limiting the noise-calibration file on disk from growing to unreasonably big sizes. Therefore, to ensure that spectral measurements trace results are noise-compensated, you must perform noise calibration and measurement in the same process. Under such conditions, RFmx uses noise calibration data cached in the program memory, instead of retrieving it from a file on the disk.

This constraint of performing noise calibration and measurement in the same process is not applicable if traces are irrelevant to your tests, and therefore you set the All Traces Enabled property to False. Disabling traces is helpful in scenarios where the execution speed of tests is critical.