Some of these sources are controllable and others are not, as shown in the following figures. The goal is to apply the required attenuation for a target accounting for all losses between the radar generation and acquisition ports. The RTG accounts for all sources of attenuation—fixed and variable—and applies settings to the variable attenuation components to achieve the desired attenuation with high accuracy and maximized dynamic range.

An optimized dynamic range keeps the signals of interest away from the unwanted system noise.

• Analog attenuation preserves the dynamic range, but it comes at a cost of resolution and synchronization.
• Digital attenuation provides fine resolution, but it affects dynamic range.

The RTG optimizes dynamic range by attempting to maximize analog attenuation and only uses digital attenuation as needed.

Sources of attenuation are as follows:

External Attenuation—The RTG system includes attenuation from the DUT to the RTG and from the RTG to the DUT. Typically there is attenuation in the path from the DUT to the RTG that, among other things, provides power protection to the RTG’s input. Ideally the input power to the RTG should be in the range of ±10 dBm. The RTG has no method to measure external attenuation. In order for this to be accounted for, the user must determine the total external attenuation and provide this value to the RTG.

RTG Fixed Analog Attenuation—The RTG uses a VST for RF acquisition and generation. The VST’s input reference level should be set to optimize dynamic range based on the supplied power from the DUT (accounting for external attenuation). With the reference level fixed, adjusting the VST’s output power level provides a set amount of analog attenuation. This attenuation cannot be changed while actively processing targets because it is unsynchronized. This setting can be used to account for minimum scenario attenuation, allowing the dynamic attenuation components control within scenario changes.

Digital Attenuation—Regardless of the presence of analog attenuation, digital attenuation is needed to provide the additional loss required to match what is specified in each target’s configuration. Large amounts of digital attenuation result in lower signal-to-noise ratio (SNR) values. Digital attenuation provides any fine adjustments to attenuation needed to reach specific values. It also provides any attenuation differences between each target and the target with the least amount of attenuation.

Common Attenuation—The sum of all analog attenuation; applied to all targets equally.

Analog Attenuation (PXIe-5699) (Optional)—The RTG software adds an optional analog attenuation solution through the PXIe-5699 module. This module provides agile analog attenuation in the RTG. Based on the current target configurations, the RTG software chooses an optimized analog attenuation. After the optimal analog attenuation is applied, the digital gain is compensated to provide the correct per -target attenuation.

On-the-Fly (OTF) Correction

Use the on-the-fly (OTF) gain correction for frequency-hopping scenarios. Enable OTF to measure the input radar pulse frequency and calculate the fixed analog attenuation for the measured frequency. This provides improved accuracy on target power level.

However, this capability comes with a limitation—the time it takes to measure the frequency and determine the power level correction prevents it from working with low-delay targets. Only targets with a total delay greater than or equal to the minimum delay can use OTF correction; refer to the following table. In cases where OTF correction is enabled and the target delay is too small, a correction is made based on a single, default user-supplied frequency.

Table 3. Minimum Delay for OTF Correction

VST	Minimum Delay
PXIe-5830/5831/5832	System Minimum Delay + 250 ns
PXIe-5841	System Minimum Delay + 250 ns
PXIe-5842	System Minimum Delay + 250 ns

Optimizing Analog Gain

Integrate the PXIe-5699 Agile Attenuator module into the RTG system to provide 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 going over the desired attenuation amount.
4. Digitally applies the remaining attenuation.

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

The RTG supports up to three or four targets (depending on 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.