Using De-embedding Techniques to Flatten Test System Frequency Response
When a signal generator outputs a modulated signal, the components inside the signal generator and an external test network comprised of mixers, filters, and cables, contribute frequency responses that degrade modulation quality. These responses occur at different frequencies and output levels and include both amplitude and phase response. You need to minimize the impacts of frequency responses in order to make accurate measurements.
Frequency Response Inside a Signal Generator
Vector signal generators (VSG) support an internal calibration routine. This routine collects correction data for both baseband and the RF magnitude and phase errors. The correction data covers entire RF frequency ranges and extends across all power level ranges. It also includes parameters of the correction filter that are applied to baseband waveforms in real-time. You can learn about the impacts of frequency responses and how to improve RF system flatness in my earlier post, “Confronting Measurement Uncertainty in Signal Generation - Part 6: Frequency Response.”
Frequency Response of External Networks
A wideband VSG provides accurate output power levels and flat frequency responses at the well-calibrated reference plane (the output port of the VSG). Often, there’s an external network between the reference plane and the measurement plane – the device under test’s (DUT) input port – as shown at the top of Figure 1. Use de-embedding techniques to isolate the DUT performance and external network (red box) impacts on the measurement results.
De-embedding Improves Measurement Accuracy
De-embedding is a mathematical process that removes the effects of the external network that are embedded in the measured data by subtracting the network characteristics between the reference plane and DUT. To characterize the network, make measurements at the measurement plane with a power meter and sensor, or a spectrum analyzer in order to obtain a corrected filter for the de-embedding as shown at the bottom of Figure 1.
Keysight VXG M9384B and M9383B provide a Measure Corrections Block Wizard as shown in Figure 2, which can guide you through the process of measuring and calculating corrections for an external network of cables, connectors, and other passive components between a signal generator and a DUT. Configure the power measurement instrument that you will use for this correction, such as a power meter and sensor or a spectrum analyzer, then measure the correction data.
Once you characterize the desired topology, you can remove the effects from the output signal by moving the effective reference plane to the point at which the power sensor or spectrum analyzer was connected. If you have multiple networks with different correction files, Keysight VXG also allows you to cascade the corrections (A and B blocks) as shown in Figure 3. The supported formats of the correction files include .s2p, .csv, and .uflat.
Modern signal generators provide not only wider modulation bandwidths, but also ample flatness performance across the entire signal bandwidth. When you extend the measurement plane for wideband signal generation, you must take external network’s frequency responses into consideration. The responses in the amplitude and phase of the modulated signal degrade modulation quality. The internal and external channel corrections remove the frequency responses inside the signal generator and the external network which improve measurement accuracy.
For the impacts of modulator imperfections on wideband signal generators, you can refer to my earlier post "Confronting Measurement Uncertainty in Signal Generation - Part 4: I/Q Impairments." You can adjust I/Q impairments and perform baseband I/Q calibration to minimize the impacts.
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