Probes plus software enhance impedance measurements

November 10, 2014 // By Martin Rowe
To minimise circuit loading, Picotest has developed two transmission-line probes that the company calls PDN probes. The company also introduced software for the Keysight E5061B VNA designed specifically for impedance measurements.

Characterising PCBs and components for signal integrity and power integrity often requires high-speed measurements in both the time and frequency domains. Because of high-frequencies, test equipment can load circuits and influence measurement results. The 1-port and 2-port PDN passive probes have, as you'd expect, 50 Ω impedance. According the Picotest, measurement of power distribution network (PDN) impedance for FPGAs generally requires the measurement of impedance levels in the milliohm range at frequencies over 1 GHz. The probes add less than 1 pF of capacitance to measurements and have a rated bandwidth of 1.3 GHz.

The unity-gain 1-port probe lets you measure ripple and noise produced by switching power supplies in power-distribution connectors on your board while maximising SNR (signal to noise ratio). You can use either the 1-port or 2-port probes to inject noise into your power supply, which lets you test sensitive circuits such as clocks and LNAs for how they perform under controlled noise conditions. The bidirectional 2-port probe can transmit a load-current step through one port while measuring the response from the other port.

The VNA software automates the process for making NISM (non-invasive stability measurements) on components such as amplifiers, filters, voltage references, voltage regulators, LDOs (Low Drop Out regulators), switching amplifiers, POL (point-of-load) power supplies, input filters, and all types of power converters through group-delay measurements even when the filter control-loop isn't accessible. Picotest describes the measurements as an alternative to using Bode plots. The software can calculate phase margin from an output-impedance measurement by using group delay.

The 1-port PDN probe costs $1495; 2-port PDN probe $2495, both for $3495. The NISM software for the Keysight E5061B VNA is priced at $795. When purchased with the 1-port PDN probe, the price is $1595 and with both the 1-port and 2-port probe is $3495.