Solution
It is not required to use the Cal/DUT Input/Output Loss settings to run an RFmx NF measurement, but it is highly recommended to achieve the most accurate results.
When choosing between External Preamp Gain and DUT Output Loss for the RX path, the correct option depends on the physical setup and calibration strategy:
Use External Preamp Gain when:
- The RX path (including any preamp) is present during both calibration and measurement.
- Calibration is performed at the start of the RX path, where the DUT connects to the RX path.
- The entire RX path is treated as a single gain block, which is practical when measuring individual preamp gain and cable losses is inconvenient.
This method is mathematically valid and simplifies the configuration.
Use DUT Output Loss only when:
- The RX path consists entirely of passive lossy elements.
- These elements are excluded during calibration and included during measurement.
- There are no active components (e.g., preamps) in the path.
DUT Output Loss is calculated based on the physical temperature of the lossy elements (default: 297K). Small temperature variations near room temperature have minimal impact on NF accuracy.
Additional Notes:
- Do not mix active and passive components in DUT Output Loss.
- Use either External Preamp Gain or DUT Output Loss, but not both for the same RX path.
- A de-embedding file can replace Cal/DUT Input Loss settings only if it is properly segmented and accurately represents each part of the signal path.
Calibration Notes:
During NF calibration, the TX and RX paths must be properly characterized. In setups with a preamp, Calibration Loss applies only to the TX path, while the RX path gain is handled via External Preamp Gain.
For many RF instruments, DUT Input Loss often equals Cal Loss, especially when a loopback cable is used.
Calibration Methods:
Power Meter Method
This method involves using a power meter to measure path loss directly:
- TX Path: Disconnect the DUT and connect a power meter to the end of the TX path. Generate a signal from the VSG and measure the loss.
- RX Path: Insert a THRU at the end of the TX path, connect the RX path, and place the power meter at the RX end. This setup allows characterization of RX path loss using TX calibration offsets.
- Variation: The power servo method uses a coupler and power meter to adjust SG power in real time, but most users still prefer full system calibration for accuracy.
VNA Method
This method uses a Vector Network Analyzer to characterize each path:
- Connect each TX or RX path to a VNA and generate an S2P file.
- If the path is complex, measure it in segments (e.g., static and switched sections) and cascade the S2P files to represent the full path.
SMU Configuration:
If the NF setup uses an external noise source (not the PXIe-5842 SNS), an SMU is required to power it:
- Only PXIe-4138 and PXIe-4139 are supported for this purpose.
- The SMU must be configured before calibration or measurement.
- If the SMU is used to power a DUT (e.g., amplifier), ensure the DUT is powered before calibration begins.