The different parts of system self-calibration are run back-to-back. Each section is described as follows.

Spectral Optimization—The spectral optimization phase of the system self-calibration has two parts: first, analyzing the system power consumption and doing temperature stabilization; then, once the temperature has been stabilized, the VST goes through a series of spectral optimizations. The optimizations flatten the spectral response and minimize LO leakage. While this calibration typically takes 20 to 30 minutes, the RTG reduces this to 3 to 4 minutes by only focusing on the center frequency currently configured for the RTG.

Delay Calibration —There is a non-zero delay through RTG’s acquisition, DSP processing, and generation. This delay must be measured to allow for accurate target delays. Measuring delay is performed by connecting the RTG’s RF output back to the RF input. This can be done by moving cables or changing a switch position if it is included in the test fixturing. With the output and input connected in loopback, the RTG generates a signal that passes through the system and its return is collected. The acquired signal is analyzed to determine minimum target delay.

Minimum Attenuation—Using the same setup as delay calibration, the attenuation calibration adjusts the VST’s output power level throughout its usable range. For each output power level it passes a signal through the system and measures losses. It builds a table that contains the attenuation (or gain) of the system for each RF signal generator output power level. Output power level changes are not synchronized and are not changed once targets are being generated (in active mode). However, this calibration allows a user to select an amount of attenuation that will always be applied to the targets through the Target Common Attenuation setting. This provides a way to use analog attenuation to account for any common attenuation in the system. The Target Common Attenuation setting must be set after calibration but before starting target generation.

Offset Frequency—The RTG has access to the entire instantaneous bandwidth (IBW) of the device (2 GHz for PXIe-5842, 1 GHz for all others). Radars with bandwidths less than the system IBW can be flexible in aligning the center frequency of the radar with respect to the center frequency of the RTG. For static radars, NI recommends that you offset the RTG and radar frequencies to minimize any interference from residual system LO. Flexible frequency radars can utilize the IBW for frequency hopping scenarios. Applying proper amplitude and doppler to the generated targets requires taking under consideration the offset in the radar’s frequency and the RTG system frequency. There are two modes in which the frequency compensation can be made:

• Static scenarios—Update the offset frequency value either in the UI or through the API. This frequency will be used to apply corrections to all configurations.
• Dynamic scenarios—Configure the RTG with an enable frequency correction option. When set to TRUE, the RTG will measure the frequency of the radar pulse and apply the appropriate corrections. When set to FALSE, the RTG will use the value entered in the offset frequency parameter to apply the appropriate corrections. The only caveat is that if the desired target delay is less than the amount of time required to measure the frequency and calculate the compensation, the user-entered offset frequency will used in place of the measured frequency. Refer to Attenuation for more information about on-the-fly (OTF) correction.

Agile Attenuator Calibration (if in your system)—The agile attenuator provides a built-in loopback connection required for system self-calibration, which removes the need for additional switching or manual cable changes.

External Attenuation and Delay—The RTG System Self Calibration procedure can measure delay and attenuation of the system. The procedure requires connecting the generation back to the acquisition. If using the Agile Attenuator (PXIe-5699), this loopback can occur internally. Any parts of the signal path between the radar and the RTG that are not included in this loopback are not included in the system self-calibration. The External Attenuation and the External Delay configuration parameters allow a user to inform RTG of the expected losses and delays in this path.