Attenuation

The core function of RTG system self-calibration and attenuation is to correct for parasitic attenuation—both internal and external—to maintain a default 0 dB input-to-output loop gain. Users specify attenuation per target, however that attenuation is optimized for dynamic range using a combination of digital and analog attenuation options. Some attenuations in the path are user controllable, some are common to all targets, some are fixed and set by the RTG System Self-Calibration, and some are externally controlled by the end user.

Dynamic Range Optimization—The RTG maximizes analog attenuation for better dynamic range and minimizes digital attenuation to preserve signal-to-noise ratio (SNR).

Attenuation Sources—Attenuation sources include user-controlled, RTG-controlled, and fixed factors. These are summarized in Sources of Attenuation.

Loop Gain—By default, the relative power from input to output of the VST with the RTG IP active, is 0 dB, with at least one target active and 0 dB digital target attenuation and common attenuation.

Sources of Attenuation

The RTG attenuation strategy ensures accurate signal capture and regeneration with optimized dynamic range. Users must carefully configure external attenuation and consider common attenuation to achieve the best results. The optional PXIe-5699 module further enhances performance by enabling a dynamic common attenuation to maximize analog attenuation while minimizing the impacts of digital attenuation.

The RTG system has several sources of attenuation.

Large Power Reduction—A DUT can output a large signal that must be attenuated to prevent overloading the RTG input. However, this attenuation cannot be present during RTG System Self-Calibration, because it reduces the SNR too much to accurately measure system performance.

Handling: Not included in RTG System Self-Calibration. Users must measure attenuation outside the RTG loopback calibration and include them in the External Attenuation parameter for the RTG to consider and compensate for these attenuations.

Cable Losses—Signal losses in cables between the DUT and RTG (input and output).

Handling: Not included in RTG System Self-Calibration. Users must measure losses outside the RTG loopback calibration and include them in the External Attenuation parameter in the RTG software for the RTG to consider and compensate for these losses.

Small Power Reduction—An input signal conditioned with a small power reduction attenuator, in addition to the Large Power Reduction. This reduction helps with placing the input signal within the RTG’s ideal input range of ±10 dBm.

Handling: Users can include Small Power Reduction in the RTG System Self-Calibration if it is less than 10 dB.

Note Attenuation greater than 10 dB in this path can result in a failed RTG System Self-Calibration.

Acquisition—Incoming signals digitized by the acquisition engine of a VST through the RTG software. Set the reference level close to the maximum input power of the incoming signal—ideally between ±10 dBm—to achieve the best dynamic range.

Handling: The RTG software configures the VST to the specified reference level.

Digital Attenuation—RTG data path attenuation. During operation, Digital Attenuation is used to achieve the specified attenuation per target. Excessive use impacts SNR; typically, Digital Attenuation exceeding 60 dB results in a target below 0 SNR.

Handling: Used minimally to complement analog attenuation.

RTG Fixed Analog Attenuation—Analog attenuation applied based on the VST and the RTG’s calibration status to achieve a 0 dB Loop Gain by default.

Handling: The output power of the VST is adjusted according to the External Attenuation and Common Attenuation parameters.

External Attenuation—The total attenuation between the RTG and the DUT beyond the signal plane accounted for by system self-calibration.

Handling: To maintain a 0 dB loop gain including External Attenuation, users must measure these external attenuations and enter them in the External Attenuation parameter in the RTG Software.

Note Specifying External Attenuation causes the RTG software to add gain to the system inside the VST to compensate. This behavior may or may not be desirable depending on required absolute power levels expected at the DUT input interface and the operating range of the VST.

Common Attenuation—Analog attenuation applied to all targets to reduce the loop gain and limit the need for excessive digital attenuation per target. By default, a calibrated RTG maintains a 0 dB loop gain. In some situations when the loop gain must be less than zero, users can set the analog Common Attenuation to apply to all targets.

Handling: Users must set the Common Attenuation parameters before the RTG data path is active. This attenuation cannot be dynamically controlled when the RTG is active, thus it is common to all targets in List Mode or during a Static Target Mode session. To change the loop gain, the VST decreases the output power commensurate with increasing the Common Attenuation.

Agile Analog Attenuation (Optional PXIe-5699)—The PXIe-5699 Agile Attenuator module provides agile analog attenuation (0 dB to 90 dB in 2 dB steps). The RTG intelligently selects the optimal value for analog attenuation on the PXIe-5699 on a per-target configuration basis. Digital attenuation is added to each target specified in the configuration as needed.

Handling: Automatically optimized by the RTG software to minimize digital attenuation.

On-the-Fly (OTF) Gain Correction

On-the-fly (OTF) gain correction enables frequency-hopping scenarios by dynamically adjusting the variable attenuation to compensate for the variations in the fixed analog attenuation based on the radar pulse frequency. The fixed analog attenuation is measured and stored when RTG is calibrated. Without RTG calibration data, OTF gain correction will not work. Refer to the Attenuation Sources figure.

Integrating the PXIe-5699 Agile Attenuator module into the RTG system provides analog attenuation between 0 dB and 90 dB (nominally) in 2 dB steps. When the RTG software generates a target, the following general steps occur:

1. Calculates the total amount of variable attenuation that is needed by subtracting the fixed attenuation from the total attenuation.
2. Applies the variable attenuation through a combination of analog and digital settings.
3. Based on the needed attenuation, chooses an analog attenuation setting that, through the Agile Attenuator, applies as much attenuation as possible without exceeding the desired attenuation amount.
4. Digitally applies the remaining attenuation.

   The final total attenuation is then the sum of fixed (analog) attenuation and variable (analog and digital) attenuation.

The RTG supports up to three or four targets (depending on the selected personality). The RTG software supports overlapping targets with different attenuation configurations in the following ways:

• Chooses the analog attenuation corresponding to the lowest target attenuation, requiring the targets that need more attenuation to apply the remaining attenuation digitally. This can also cause an analog attenuation change while a target is actively being generated.
• If a low power (high attenuation) target is generating when a higher power (lower attenuation) target begins—causing an overlap—the RTG software changes the analog attenuation to support the lowest attenuation needs.