Agilent Measurement Journal - Issue 3 - 2007 - (Page 52)

Using a VNA with CW time sweep — the wideband method As shown in Figure 2, the second method uses a VNA and a pulse generator, with the VNA set to a continuous wave (CW) time sweep. With this conﬁguration, measurement resolution and accuracy are usually much greater than is possible with an oscilloscope, and the device is measured under RF stimulus. Calibrating the analyzer to the device ports removes the effects of loss and mismatch in the measurement system for a more accurate measurement. While the CW time-sweep approach can be very accurate, it has an important drawback: Minimum time resolution is limited by the maximum IF bandwidth. If either the sweep time or timeaxis resolution at the maximum IF bandwidth is not sufﬁcient to capture the settling time, then another approach must be used. Applying pulsed-proﬁle S-parameter measurements — the narrowband method The third approach is shown in Figure 3. The hardware setup is nearly the same as the CW time sweep; however, the measurement method is completely different, using an approach similar to the pulsed-proﬁle S-parameter measurements often applied when testing radar components. Pulse generator Pulse generator Trigger out In VNA Control pulse DUT Out Trigger in In VNA IF gate Figure 2. Example measurement setup for CW time-sweep (wideband) method Control pulse DUT Out Averaging can be used to reduce measurement noise, though this is limited by measurement drift of the VNA. If averaging is used, it is necessary to trigger the VNA sweep on the control pulse. This can be done using the hardware trigger input usually found on the back panel of advanced VNAs.1 If triggering is not available, the sweep time can be adjusted to double the period of the control pulse. This will assure that at least one full pulse is measured somewhere within the sweep. In this case, averaging will not work because multiple sweeps will not be aligned. This setup has three key steps: 1. Determine the necessary sweep time and time-axis resolution 2. Select IF bandwidth, number of points, pulse width and pulse repetition frequency settings that meet the requirements of Step 1 3. Average to achieve the desired amplitude resolution Figure 3. Example measurement setup for pulsed proﬁle S-parameter (narrowband) method As before, the input is a CW signal and the switching action generates a pulsed RF output. A pulsed-proﬁle measurement ﬁlters and gates the IF outputs of the VNA receivers such that only a portion of the pulsed signal is measured. The gating window is then shifted sequentially in time to generate a pulsed proﬁle. (Please see Reference 2 for a more detailed discussion of the theory behind the narrowband method.2) This approach has three essential steps: 1. Determine the necessary sweep time and time-axis resolution 2. Select pulse width, pulse repetition frequency, IF gate width and number of points to satisfy the requirements of Step 1 3. Set the IF bandwidth and averaging to achieve the desired amplitude resolution 52 Agilent Measurement Journal

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

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