Agilent Measurement Journal - Issue 3 - 2007 - (Page 53)

It is important to note that a tradeoff exists between the time-axis resolution and the accuracy of the measurement. Better time-axis resolution can be achieved by narrowing the IF gates; however, a narrow gate admits less energy into the VNA IF data acquisition circuitry, resulting in more measurement noise. A typical compromise for the IF gate-width setting is a 5 to 15 percent duty cycle, though at this setting the time-axis resolution is usually too coarse for rise-time measurements. Averaging can be performed on measured datasets to reduce measurement noise. The minimum possible time-axis resolution is determined by the minimum width of the IF gate switching circuitry (typically tens of nanoseconds). A note on settling-time calculations Settling time can be determined from the measurement using manual and automated methods. The manual approach relies on a visual conﬁrmation that the signal has settled. This typically requires 10 to 15 measurement points to determine whether the device has settled. Next, a marker is placed in the settled area and a second marker is moved back in time until the value reaches the speciﬁed settling difference. The readout of the second marker provides the settling time of the switch. This technique can be automated by transferring the trace data to a program that performs the same procedure. However, determining whether or not the switch has settled is often easier to do visually than programmatically. One useful approach is a smoothed derivative (e.g., derivative of the moving average) of the settling measurement: If the derivative reaches zero, it usually means the switch has settled. Some devices, however, may have a settling plateau, giving a false indication that the switch has settled. In such cases, prior knowledge of this behavior must be taken into account when determining if the measurement has actually settled. In order to successfully determine the settling time, the peak-to-peak trace noise should be less than the settling time criteria. For example, if 0.01 dB settling time is to be measured, the peak-to-peak trace noise also should be kept below 0.01 dB — ideally two to 10 times less. Settling time of return-loss measurements can be different since very low signal levels are typically being measured. Therefore, rather than measuring a very small deviation from a very small ﬁnal value, it is usually more appropriate to measure the time it takes for the return loss to settle below a desired value. Determining measurement accuracy With so many variables in the measurement, it is often difﬁcult to calculate accuracy. Fortunately, when measuring settling time, only the relative amplitude accuracy between the ﬁnal value and a speciﬁed difference from the ﬁnal value is important. Errors in the time scale are typically very small and need not be considered. Because only the relative amplitude accuracy is important, the measurement is dominated by trace noise, which is simple to measure. In contrast, absolute measurement accuracy depends on calibration quality (not discussed here).3 A simple way to test the trace noise on, for example, an Agilent PNA Series microwave network analyzer is to set up the system for the measurement and close the switch (or turn on the device). Then, set up a CW time sweep for the desired parameter and turn on the trace statistics function. This capability will provide the peak noise and standard deviation, which can be used to determine if the measurement can achieve the desired relative accuracy. If the trace noise is not acceptable, parameters such as IF bandwidth, averaging or IF gate width can be adjusted until the trace noise is within acceptable limits for the measurement. Using this simple method, amplitude noise can be less than 0.001 dB, allowing settling time measurements at this resolution. Agilent Measurement Journal 53

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 3 - 2007

Applying Ingenuity to the Measurement of Emerging Technologies Contents Emerging Innovations Department Delivering Bigger Benefits by Optimizing Customer Workflows Measuring Material Properties at the Nanoscale Utilizing In Situ Atomic Force Microscopy in Life Science, Pharmaceutical and other Bio-Related Applications WiMAX™: Plotting a New Path to Global Mobility Addressing the Triple Complexity of Triple-Play Networks What Next for Mobile Telephony? Exploring the Inner Workings of Tire-Pressure Monitoring Systems Developing, Assessing and Applying a High-Resolution Thin-Film Magnetic Probe Making Accurate Settling-Time Measurements Using a Vector Network Analyzer Applying Metabolomics Methods to the Study of Bacterial Leaf Blight in Rice Plants Measuring Stream Dynamics with Fiber Optics survey

Agilent Measurement Journal - Issue 3 - 2007

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.