NISM software is now available. Bringing the unique non-invasive stability test to the Keysight E5061B vector network analyzer.

The software add-on, available now for new or existing customers, allows the instrument to perform non-invasive stability margin (“NISM”) measurement.

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 via group delay.

This capability, formally only available in one other VNA instrument, allows the user to measure the stability of op amps, voltage references, voltage regulators, LDOs, switching amplifiers, POLs, input filters and all types of power converters through their output impedance.

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.

E5061B users can use one, two, and three port impedance measurements which the software can convert to phase margin. The software supports the low frequency and high frequency ports to assess the stability of all types of the aforementioned circuits ranging from 1mohm to 100kohm up to the full frequency range of the VNA.

The software is available immediately and is priced at $795. Options add Picotest’s 1 and 2 port PDN transmission line probes bundled with the software which greatly ease the measurement of output impedance due to their small form factor and minimal circuit loading (< 0.2pf). The NISM software with the 1 Port PDN probe is $1,595. The NISM software with both the 1 Port and 2 Port PDN probe ‘bundle’ is $3,595.

Testing Stability Where No One Has Gone Before
The stability of linear or switching regulators, including regulators with no exposed control loops, cannot be assessed using a traditional Bode plot. “Very often you cannot make a Bode plot for a number of reasons. Either the loop is not exposed or frequently newer devices have multiple loops, which are not all accessible. The non-invasive stability margin measurement is the only way to verify stability for these types of circuits”, stated Steve Sandler, CEO of Picotest. “A huge benefit of the measurement is that because it is based on impedance, it can be measured ‘in-situ’ with a one probe measurement and non-invasively, that is without impacting the result.”

In fact, given opamp and regulator dependency on loading, and the shrinking size of ICs as depicted by the TI LDO image next to a kernel of corn, in-circuit is often the only access available. Traditional loop breakage is not possible.

What is NISM?
Picotest has published many articles and application notes related to the assessment of stability without access to the control loop, since Picotest introduced the method in 2011. The method has proven to be a popular and simple technique for determining the exact level of stability and phase margin.

The mathematics behind the non-invasive measurement was first incorporated into a VNA in late 2011. Bode plot measurements and non-invasive results are generally within 1 degree of each other up to and including a phase margin of 65 degrees.

Is the Non-Invasive Stability Measurement As Accurate as a Bode Plot?
Yes. In fact, it’s accurate in cases where Bode plots are invalid and is not subject to some of the issues that plague Bode plot’s accuracy. “In many cases, the NISM is a better assessment of stability than a Bode plot”, states Sandler.