Impairment Calibration
- Updated2024-08-02
- 3 minute(s) read
Impairment Calibration
I/Q impairments, such as I offset, Q offset, I/Q gain imbalance and I/Q phase skew, degrade the spurious performance of the I/Q modulator. The offsets contribute to carrier suppression. Gain imbalance and phase skew contribute to the creation of an image for the single-sideband RF signal output that is mirrored across the carrier. For a continuous waveform at a frequency +fs as the baseband signal generated at a carrier frequency of fc, the expected LO leakage and the image is shown in the following plot.
The image power relative to the desired RF output (ISR), in dBc, is given by the following formula:
where ε represents I/Q gain imbalance
φ represents I/Q skew in radians
The carrier to desired output (CSR), in dBc, is given by the following formula when the output power is 0 dBm:
where α represents volts offset on the I channel
β represents volts offset on the Q channel
Impairment calibration minimizes the effects of the I/Q impairments. Impairment calibration is a characterization procedure performed at NI during the manufacturing process or during an annual external calibration. The procedure involves using a spectrum analyzer to measure the modulation impairments of the PXIe-5673/5673E Vector Signal Generator. Once these impairments are measured, they are stored on the PXIe-5673/5673E where they can then be used to correct each generated signal. The result of this correction is improved carrier and image suppression.
Impairment calibration is automatically applied to every signal generated on the PXIe-5673/5673E. Impairments vary over frequency, and multiple sets of impairments for different frequencies are stored on the PXIe-5673/5673E. Interpolation is automatically performed when the generated signal frequency does not match a frequency point at which impairments were stored. Impairments are also corrected for temperature drift. Every time a session is initialized, the onboard temperature is read and used to correct the stored impairments according to the temperature coefficients stored alongside the impairments.
Correcting for Temperature Drift
To correct for temperature drift without having to re-initialize a session, call the niRFSG Perform Thermal Correction VI or the niRFSG_PerformThermalCorrection function.
Applying Additional Impairment Correction
You can also apply additional impairment correction with the following properties or attributes:
| Property | Attribute |
|---|---|
| I Offset | NIRFSG_ATTR_IQ_I_OFFSET |
| Q Offset | NIRFSG_ATTR_IQ_Q_OFFSET |
| Gain Imbalance | NIRFSG_ATTR_IQ_GAIN_IMBALANCE |
| IQ Skew (Degrees) | NIRFSG_ATTR_IQ_SKEW |
Related Topics
Improving Carrier and Image Rejection Performance
NIRFSG_ATTR_IQ_I_OFFSET Attribute
NIRFSG_ATTR_IQ_Q_OFFSET Attribute