How to Achieve Optimal Switching Speed with the NI PXIe-5654 and NI PXIe-5696

Publish Date: Jan 28, 2016 | 0 Ratings | 0.00 out of 5 | Print

Overview

An introduction to amplitude and frequency switching and how to optimize switching using the NI PXIe-5654 and NI PXIe-5696.

Introduction to Switching Speed and Why it’s Important

Synthesizer switching time is defined as the period of time it takes a synthesizer to change to the next frequency and power from the current frequency and power. Minimizing switching speed can reduce test times which can result in more DUTs tested, therefore reducing the cost of testing. To optimize switching speed, we must consider two main parameters: (1) amplitude settling time and (2) frequency settling time. This document will outline the tradeoffs that can be made to ensure the user gets the optimal performance from the NI PXIe-5654 and the NI PXIe-5696.

The figure below portrays the difference between frequency switching time and amplitude switching time:


Amplitude Settling Time Optimization

The table below displays the amplitude settling time differences for the standalone NI 5654, the NI 5654 in open-loop mode, and the NI 5654 in closed-loop mode.


There are five parameters that affect amplitude settling time:

  1. Crossing a Center Frequency of 250 MHz and Jumping into the 250M-2.4GHz Band

    There are two frequency bands, 250 kHz to 250 MHz (DDS band) and 250 MHz to 20 GHz (Non-DDS band). The amplitude settling time is determined by the final frequency:
    1. Add 1 ms to settling time values when entering the 250 MHz to 2.4 GHz frequency range. For frequency changes within the 250 MHz to 2.4 GHz range, no additional settling time applies.
    2. Add 2.5 ms to the settling time values when crossing 250 MHz.
  2. Automatic Leveling Control (ALC)

    Automatic leveling control can either be enabled or disabled. Enabling the ALC means that the ALC is closed, also known as closed-loop mode. Closed-loop mode provides greater amplitude accuracy across the entire frequency range. However, this improved accuracy might lead to a slower switching speed. Therefore, for faster switching speed, the ALC can be disabled, or the NI PXIe-5654 can be operated in open-loop mode. For more information on ALC closed and open loops, reference the following link in the RF Signal Generators Help file: RF Signal Generators Help: ALC Closed Loop Versus Open Loop.
  3. Transition Between the Low Harmonic Path and the High Power Path

    Add 2.5 ms to the settling time values when switching between the low harmonic and high power path. Reference the figure below for the power ranges for the Low Harmonic and High Power paths.

  1. Mechanical Attenuator State Change

    When you pair the NI 5654 with an NI 5696 AE, a 110 dB mechanical step attenuator extends the power adjustment range, and the power accuracy improves by engaging the ALC loop. Add 2.5 ms to settling time values when the mechanical attenuator value changes. Details regarding changes to the mechanical attenuator setting can be found in the PXIe-5654 Device Specifications as well as the NI RF Signal Generators Help.
  2. Automatic Power Search (applicable only with the PXIe-5696)

    Auto power search closes the ALC loop (closed-loop mode) to achieve an accurate power output. Once the specified accuracy is achieved, auto power search opens the loop (open-loop mode) between the PXIe-5654 and PXIe-5696; accuracy may drift over time and temperature. When this attribute is enabled, a power search is performed after the device is initiated, after output power is enabled, or when the frequency or power level changes while the device is generating. When this attribute is disabled, NI-RFSG does not perform a power search unless programmatically calling the function.

Another consideration for improving the switching speed is to turn off auto power search. Power searching does deliver a more accurate result; however, there is a tradeoff between accuracy and speed.

Frequency Settling Time Optimization

There are two device options for frequency settling time, Standard and Fast Tuning. The table below displays the settling times for each device option. These settling time values represent the settling time within 0.01 ppm of the target frequency, and these settling time values do not include amplitude settling times.

 
4Add 1 ms to the frequency settling time when transitioning from >250 MHz to <250 MHz..
Details regarding this can be found in the PXIe-5654 Specifications.

There are two parameters that affect frequency settling time:

  1. Clock Source
    For the best determinism and accuracy for frequency switching speed, use an external clock source as a trigger.
  2. Crossing a Center Frequency of 250 MHz
    There are two frequency bands, 250 kHz to 250 MHz (DDS band) and 250 MHz to 20 GHz (Non-DDS band). The frequency settling time is determined by the final frequency:
    1. For the fast tuning device option, the frequency settling time is 150 µs within the DDS band; the frequency settling time is 100 µs within the Non-DDS band.
    2. For either device option, add 1 ms to the frequency settling time when changing from the Non-DDS to the DDS band.

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