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.