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PicoVNA® 100 Series vector network analyzers

Quad RX four-receiver, single-sweep, architecture

The PicoVNAs integrate a fast-stepping sine-wave signal source

with a very fast-settling port transfer switch. Faster than dual-sweep

competitor VNAs, within a single frequency sweep, at each frequency

point, the PicoVNAs stimulate both ports in turn and twice measure

phase and amplitude of incident, reflected and transmitted waves at the

four receivers. This could be achieved with a degree of accuracy with

a single source, a transfer switch and two receivers; the latter inputs

being switched through a further pair of transfer switches. Alternatively,

three receivers can be used with an additional input transfer switch.

The PicoVNA, however, uses four receivers. This eliminates the receiver

input transfer switch errors (chiefly leakage and crosstalk) that cannot

otherwise be corrected. These residual errors are always present in two-

and three-receiver architectures and lead to lower accuracy than that of

the Quad RX design.

Receivers

Transfer switch

Source

Port 1

Port 2

DUT

Port 2

Port 1

Support for 8 and 12-term calibration and the unknown through

Almost all vector network analyzers are calibrated for twelve error sources (six for each signal

direction). This is the so-called 12-term calibration, which experienced VNA users are used to

performing fairly regularly. In a four-receiver design some error sources are so reduced that 8-term

calibration becomes possible, along with an important and efficient calibration technique known as

the unknown through. This gives the ability to use any through interconnect (including the DUT) during

the calibration process, vastly simplifying the procedure and reducing the number of calibration

standards that need to be maintained. Advanced vector network analyzer users will be pleased to

know that internal a-wave and b-wave data can be exported for diagnostic use.

SOLT (short, open, load and through) calibration

All vector network analyzers need to reference their measurements to well-known standard networks.

These need to provide a wide dynamic range of amplitude and phase (or delay time) so that

measurements between the given extremes become calibrated. PicoVNAs support SOLT calibration

of transmission and reflection, whereby the short, open and through provide known and opposing

extremes of phase, high-scale amplitude and transmit isolation. The load provides known low-scale

reflect amplitude and transmit isolation. For Pico calibration standards these are all fully and traceably

S-parameter characterized.

TRL and TRM (through, reflect, line and match) calibration

This is theoretically accurate because a machined air transmission

line can be fabricated more precisely than a good match can be

measured; certainly at higher frequencies. As this line standard can

also carry the burden of time (phase) calibration, the additionally

needed high-reflection standards, the shorts or opens, can also be

less well known.

TRL technique requires a line length of significantly more than 0º

phase delay and significantly less than 180°. Thus, a single TRL

line can only address a limited frequency band. The PicoVNA 108

supports one or two TRL bands and can account for line impedance

offset if required. A low-frequency TRM band can reference a readily

fabricated resistive match.

TRL and TRM calibration are popular choices when measuring

substrate-mounted DUTs, for example surface-mounted networks or

components. The line, match and reflections (shorts and opens in

the PicoVNA case) can all be readily fabricated on substrate and at

precise on-substrate measurement reference planes.

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