Response and Software Equalization
- Updated2023-02-20
- 3 minute(s) read
Response and Software Equalization
Digital equalization is the process of adjusting the frequency content of the signal so that the effective gain is independent of the frequency. In terms of frequency response, a flat response or constant gain versus frequency is desirable for highest modulation quality in the generated signal.
The PXI-5610 upconverter module introduces small variations in the gain or phase response of the NI RF signal generator. These variations are introduced by the anti-imaging filters used in performing image-free upconversion.
Plots of the typical magnitude response of the PXI-5610 upconverter module with the digital equalization disabled (upper graph) and enabled (lower graph) are shown in the following figures. Use NI-RFSG to enable or disable software equalization and to query the enabled or disabled state of the software equalizer.
As can be seen from the typical response, there is ±2 dB max gain variation between the center of the band and the outer edges of the passband. This variation results in degradation of the modulation quality for wide-bandwidth signals generated using high symbol rates, which are greater than approximately 500 kS/s.
You can reduce the variation in the frequency response by equalizing the digital data stream to compensate for the passband response of the PXI-5610 upconverter. You can control the equalization using Digital Equalization Enabled property or the NIRFSG_ATTR_DIGITAL_EQUALIZATION_ENABLED attribute.
When digital equalization is enabled, software equalization improves the modulation quality for wider instantaneous bandwidth (greater than 500 kHz) signal generation.
| Symbol Rate | Recommended Equalization | Comments |
|---|---|---|
| ≤500 kS/s, Filter Alpha < 0.25 | disabled (default for PXI-5670/5671) | There is little performance gain from turning on equalization because the occupied bandwidth is 500 kHz or less. Disabling equalization is recommended for faster throughput. |
| >500 KS/s | enabled (default for PXIe-5672) | The modulation quality (error vector magnitude) is improved by enabling equalization for wide loop bandwidth (greater than approximately 500 kHz) signals. |
For faster test times with equalization enabled, save a digital waveform that has been pre-equalized. Saving allows the digital filtering operation and the associated computation time to be done once, as opposed to every time the same waveform pattern is generated. NI provides low-level tools to extract the equalization filter coefficients stored in the onboard nonvolatile memory. Contact NI Technical Support if you need to access these tools.
Other Considerations
The factory-determined equalizer filter coefficients are valid for 25 °C ± 10 °C. For operation outside this temperature range, NI recommends that the equalizer filter be determined under the operating environment.
It is helpful to calibrate and compensate the entire signal chain on a system basis for some advanced applications, such as the following:
- Custom equalizer filtering operations that take into account not only the passband response of the PXI-5610 upconverter but an external signal conditioning device such as a filter or amplifier.
- Determining equalizer coefficients in the field for ambient temperature outside the 25 °C ± 10 °C range.
In cases like these, NI recommends disabling the software equalization and applying the equalization filter in your application program before passing the data to NI-RFSG. NI application development environments, including LabVIEW and LabWindows™/CVI™, have built-in tools for digital filtering operations to assist with this task.
Configuring Software Equalization Using NI-RFSG
- To enable software equalization, set the Digital Equalization Enabled property to Enable or the NIRFSG_ATTR_DIGITAL_EQUALIZATION_ENABLED attribute to NIRFSG_VAL_ENABLE.
- To disable software equalization, set the Digital Equalization Enabled property to Disable or the NIRFSG_ATTR_DIGITAL_EQUALIZATION_ENABLED attribute to NIRFSG_VAL_DISABLE.