Compression Measurement Setup

The following figure shows the typical compression measurement setup for the PXIe-5668. Note that the attenuator is internal to the PXIe-5606 downconverter. You can use a variety of different techniques to perform compression measurements for your system. NI recommends using the best equipment setup for your specific application.


1378

  1. Signal Source
  2. DUT
  3. Attenuator
  4. Vector Signal Analyzer

Choosing an Optimal PXIe-5668 Attenuation Setting for Compression Measurements

To test the compression limits for a DUT, ensure that the PXIe-5668 itself is not contributing any substantial error by being driven to near its own compression limits. NI recommends setting a measurement instrument 10 dB to 20 dB below the compression specifications.

Complete the following steps to help ensure that the measurement system does not substantially contribute to DUT compression testing.

Note Use the NI-RFSA Soft Front Panel (SFP) in spectrum mode as you complete the following steps.
Tip Estimate the DUT gain and compression point and apply these values to the reference level setting and signal source power level before completing the following steps. This process allows you to complete the steps faster.
  1. Power on the system in the previous diagram, including the signal source. Allow the equipment to warm up as specified in the device specifications (typically 30 minutes).
  2. Tune the PXIe-5668 to the signal source frequency. A 10 dB per division vertical scale is a good first setting.
    Note During compression measurements, the frequency span of the PXIe-5668 can be substantially narrower than the maximum frequency span of the PXIe-5668. This situation occurs because the input for compression measurements is typically a discrete tone.
  3. Disable the PXIe-5668 preamplifier, if present, to reduce internal distortion.
  4. Adjust the PXIe-5668 reference level to obtain a signal power that is between the reference level setting and the second graticule down.
  5. Increase the signal source power until one of the following conditions is met:
    • The power of the tone displayed in the PXIe-5668 fails to track dB-for-dB the input level changes. For example, if you change the DUT input power by 1 dB and the PXIe-5668 measured power changes by only 0.5 dB, a reasonable conclusion is that compression is being approached, which is indicated by the dB-for-dB tracking failure.
    • The power of the tone exceeds the PXIe-5668 reference level.
  6. If the measured power level exceeds the PXIe-5668 reference level, increase the reference level by at least 10 dB. Repeat this step as necessary.
  7. Set the PXIe-5668 reference level such that the displayed power level is approximately 8 dB to 12 dB below the reference level. This level is the recommended setting and is nominally 20 dB below the digitizer full-scale rating. Lower reference level (higher gain) settings may be used if instrument linearity is assured.
    To assure linearity, temporarily increase the internal step attenuator by 10 dB. The displayed power of the PXIe-5668 should decrease by the amount of increased attenuation, indicating that the PXIe-5668 is operating within its linear range.
At this point, under normal circumstances, the PXIe-5668 is sufficiently below its own compression point so as not to affect any DUT compression tests. As you proceed with DUT compression testing, you may need to further fine-tune the PXIe-5668.