Interpolation

Overview

In some use cases, you may need to make approximate measurements at various frequencies and power levels. Interpolation enables you to rapidly change frequencies of interest without changing calibration files and without performing a new calibration with the STS RF Multiport Calibration Module.

By default, interpolation is turned off in the NI-RFPM driver. Do not enable interpolation in a production environment. In a production environment, always perform the correct frequency and power calibration to ensure measurement accuracy.

Configuration

To enable or disable interpolation, use the following properties or attributes:

LabVIEW Property Name: Interpolation Enabled

C Attribute Name: NIRFPM_ATTR_INTERPOLATION_ENABLED

LabVIEW Property Name: Maximum Frequency Span for Interpolation

C Attribute Name: NIRFPM_ATTR_MAX_FREQ_SPAN_FOR_INTERPOLATION The maximum frequency span for interpolation that is supported is 100 MHz. Negative and zero spans are not supported.

Theory of Operation

When interpolation is enabled and the span between calibration points is less than or equal to the maximum span specified, the NI-RFPM driver performs linear interpolation for specified frequency values that are not contained in the calibration file. This feature supports only scalar transmit and receive offsets; there is no interpolation support for vector error coefficients used in S-parameter measurements.

Interpolation applies to the data returned by the following functions:

Fetch Receiver Scalar Offset: niRFPM_FetchReceiverOffsetByParameters Fetch Source Scalar Offset: niRFPM_FetchSourceOffsetWithSettingsAndSettlingTime

Receiver interpolation is performed when interpolation is enabled and when the requested reference level matches a calibrated reference level. If reference level coercion is enabled, different reference levels can be used for interpolation, as long as the reference levels are within the coercible range. If power level coercion is enabled with NIRFPM_ATTR_POWER_LEVEL_COERCION_METHOD, then interpolation for the source can interpolate across different power levels the similar to the receiver.