Attenuation

Dynamic Range Optimization—The RTG maximizes analog attenuation while minimizing digital attenuations to achieve the best dynamic range and signal-to-noise ratio (SNR).

Attenuation Sources—Attenuation sources include user-controlled, RTG-controlled, and fixed factors. Sources of Attenuation summarizes these sources.

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 optimizes dynamic range for accurate signal capture and regeneration. Optimal performance requires proper configuration of both external and common attenuation. Integrating the optional PXIe-5699 module further enhances performance by enabling dynamic common attenuation, which maximizes the use of analog stages and minimizes digital signal degradation.

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 these measurements in the External Attenuation parameter. Then the RTG can 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 these measurements in the External Attenuation parameter. Then the RTG can 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 parameter and the Common Attenuation parameter.

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, measure these external attenuations and enter those measurements in the External Attenuation parameter.

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 the following factors:

• Required absolute power levels expected at the DUT input interface.
• 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. Users cannot dynamically control this attenuation when the RTG is active. Therefore, this attenuation 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 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.

Note When using the PXIe-5699, the RTG leverages the internal loopback path of the device during the calibration routine. The RTG calibrates cables connecting the VST RF IN/OUT ports and PXIe-5699 ATTEN IN/THRU OUT ports. The THRU IN and ATTEN OUT ports are not used during the RTG calibration procedure. Therefore, cables connected to the THRU IN and ATTEN OUT ports are not included in the calibration routine.

Handling is automatically optimized by the RTG software to minimize digital attenuation.

Frequency Correction

Frequency correction addresses non-uniform amplitude flatness across a wide instantaneous bandwidth. Because this equalizes the system's response across the entire band, it inherently enables support for frequency-hopping scenarios.

The PXIe-5699 Agile Attenuator provides analog attenuation between 0 dB to 90 dB in 2 dB increments. When generating a target, the RTG software applies attenuation using the following steps:

1. Calculates the required variable attenuation by subtracting the fixed system attenuation from the total requested attenuation.
2. Maximizes the analog contribution by setting the Agile Attenuator to provide as much of the required variable attenuation as possible without exceeding the target value.
3. Applies the remaining required attenuation digitally.

   The final total attenuation delivered is the sum of the fixed analog, variable analog, and variable digital components.

Depending on the active personality, the RTG handles up to four simultaneous targets. When overlapping targets have different attenuation requirements, the software dynamically manages the shared hardware in the following ways:

• The system always scales the shared analog attenuation to match the target requiring the least attenuation (the highest power target).
• Targets requiring more attenuation make up the difference digitally.
• If a higher-power target begins while a lower-power target is already generating, the system dynamically drops the analog attenuation to accommodate the new target. Consequently, an analog attenuation state change may occur mid-generation.