The Non-Invasive Stability Measurement (‘NISM’) software brings the unique stability measurement capability to the Keysight, Rohde & Schwarz, and Copper Mountain vector network analyzers and to various simulation tools (Keysight ADS and Cadence PSpice). The software add-on allows the instrument to perform stability margin measurement of voltage references, voltage regulators, LDOs, switching amplifiers, POLs, op-amps, input filters and all types of power converters through their output impedance.

NISM is an alternative to a Bode plot measurement that allows the user to assess circuit or filter stability even when the control loop is not accessible. It determines phase margin from an output impedance measurement. The software guides the user through a few simple steps to obtain the stability margin from a circuit’s output impedance. The measurement results can be saved or printed as with other E5061B measurements.

VNA users can use one, two, and three port impedance measurements (1-port reflection, 2-port shunt thru, 2-port series thru,3-port voltage/current, etc.). The software supports the gain-phase frequency and high frequency S parameter ports to assess the stability of all types of the aforementioned circuits ranging from mohms to kohms and up to the full frequency range of the VNA.

NISM computes stability from output impedance. Click here for a list of test methods and equipment that you can use to measure impedance.

Testing Stability Where No One Has Gone Before
Very often you cannot make a Bode plot for a number of reasons . For example, the stability of linear or switching regulators, whose control loops are not exposed externally from the IC, cannot be assessed using a traditional Bode plot test. In those cases, the non-invasive stability margin measurement is the only way to verify stability for these types of circuits. Whether traditional loop breakage is not possible or not, NISM can perform the stability assessment and now Keysight, Rohde & Schwarz, and Copper Mountain VNAs have the capability.

Picotest PDN Probes Eases Output Impedance Testing
In many instances, the power converter’s output capacitor is very small and the stress of soldering a cable might lift the capacitor from the PCB, assuming there is even sufficient physical access for the cable. In the case of an opamp, test connections can alter the measurement. Fortunately, now there is a better way to gather impedance data. Picotest’s 1-port and 2-port PDN handheld probes allow easy access to the output test point and maintain a true 50O transmission line characteristic from the VNA to the part being measured (i.e. no loading of the measurement). The 1-port probe can be used to measure the impedance using the 1-port reflection method and the 2-port probe can be used to measure the impedance using the 2-port shunt or series thru method. Once the impedance is measured, the NISM software provides the associated phase margin.

Performing a 1-port impedance measurement on the CMT Compact VNA

Performing a 2-port shunt-through impedance measurement on the CMT Compact VNA