The MSA for
RLC Analysis

Updated Dec 16, 2009

Many circuits and components can be modeled as a series or parallel combination of a resistor, capacitor and inductor. This applies to resonant LC circuits with parasitic R, as well as individual components. A capacitor has parasitic series resistance and inductance. An inductor has parasitic parallel capacitance, and parasitic resistance that at least in some cases can be viewed as parallel. If a DUT with a resonant frequency is scanned in Transmission mode so as to show the resonant frequency, and at least one of the -3 dB points.

 The following image shows such a scan for a 2 nF capacitor with long leads.




The markers in the above graph were actually placed in the next step, by the RLC Analysis module. The resonant dip is at 20.1 MHz; the lower -3 dB point is at 3 MHz. The upper -3 dB point is off the right end of the graph. Note that for a resonant dip, the -3 dB point is exactly at -3 dB, not 3 dB from the bottom of the dip. For a resonant peak, it would be 3 dB below the peak of the dip.


With this scan in place, we invoke the Analysis/RLC menu item, which opens the following dialog:




The dialog allows us to specify whether the RLC components are to be considered in series or in parallel, and in either case whether the DUT is in a series or shunt fixture. We can also specify R0 of the fixture, and for a shunt fixture we can specify the one-way delay of the connection between the fixture and the DUT.


With the options selected, we click Analyze, and the values at the bottom of the dialog are displayed. They include Fs (the resonant frequency), the R, L and C values, and the Q values at resonance—unloaded (QU) and loaded (QL).


To summarize the steps in using RLC Analysis:


1. Set the video filter to Narrow (large capacitor) and the wait time to at least 50 ms. (Both the hardware video filter switch and the software Video Filter combobox must be set.)


2. After any required calibration, perform a scan of the DUT that shows the resonant peak or dip, and at least one of the -3 dB points. In a Series Fixture, a parallel resonance will produce a dip, and a series resonance will produce a peak; in a Shunt Fixture they are reversed. For a peak, the -3 dB points are 3 dB below the peak. For a dip they are at a fixed value of -3 dB, independent of the depth of the dip.


3. Select the AnalysisàRLC menu item.


4. Select whether the RLC components are in series or in parallel. For an LC tank, this depends on their actual connection. When one or more of the components are parasitics, it depends on the appropriate model for the device. A capacitor is a series RLC device. An inductor is a parallel RLC device.


5. Select whether the DUT is mounted in a Series or Shunt Fixture, and specify the fixture R0, which is the impedance seen by the device looking toward the TG output or the MSA input.


6. For the Shunt Fixture you may specify the connector delay, in ns. This is the delay between the main signal line and the actual DUT. It is intended to compensate for very short distances, such as the length of an SMA connector. A value of 0.125 ns per inch is typical. 0.115 ns is good if the fixture has an SMA connector and the DUT will be attached to the back side of a mating SMA connector.


7. Click Analyze. The component values will be measured and displayed in the box at the bottom of the dialog. The resonant frequency (Fs or Fp) and the R, L and C values will be displayed, along with unloaded and loaded Q (QU and QL). These Q values are the Q of the resonant circuit formed by L and C.


8. You may attach another DUT and click Analyze again. When done, click the Close Box in the upper right corner of the window.


Inductors with large losses, or any components whose self-resonant frequency interferes with measurement, are better handled by the RLC analysis available in Reflection mode.