ADL5961ACCZ-R2 datasheet(21/34 Pages) AD

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Data Sheet

ADL5961

THEORY OF OPERATION

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CALIBRATION AND ERROR CORRECTION

The accuracy of S-parameter measurements is prone to a variety

of errors introduced by the hardware in VNAs. Systematic errors,

those that are repeatable and predictable, can be eliminated from

the measurement results using calibration and error correction

techniques. The types of systematic errors that can be eliminated

(to a certain extent) include the following:

►

Impedance mismatches

►

Gain and insertion loss differences between channels

►

Crosstalk between channels

Various error correction and calibration techniques have been de-

veloped over the years (see, for example, D. K. Rytting, "Network

Analyzer Accuracy Overview," 58th ARFTG Conference Digest,

2001, pp. 1-13, doi: 10.1109/ARFTG.2001.327486) that differ in

their effectiveness to eliminate certain errors, complexity, and cali-

bration standards used. The general principals that are applied,

however, are the same for all. The systematic errors are determined

by measuring the VNA response for a set of DUTs with accurately

known S-parameters, the calibration standards. Comparing the

measured S-parameters with the known S-parameters allows calcu-

lation of the error coefficients. These error coefficients, combined

into what is often referred to as an error adapter, can then be

used in a postprocessing step to calculate the error corrected

S-parameters from the measured S-parameters of an unknown

DUT. Figure 55 illustrates the concept.

Figure 55. Concept of Error Correction in VNAs

Different error correction techniques require the use of different

sets of calibration standards. Some of the most well known include

short, open, load, thru (SOLT), thru, reflect, line (TRL), and thru, re-

flect, match (TRM). The standards within a set are chosen to have

significantly different S-parameters, such that they span a large

area on the Smith chart. The calibration standards themselves, par-

ticularly the ones with extreme S-parameter values such as short

and open standards, are typically not completely ideal either and

usually include a manufacturer supplied model for the S-parameters

vs. frequency. In general, calibration standards can be subdivided

into the two following categories:

►

One-port calibration standards, measured at each VNA port

►

Two-port calibration standards, measured for each combination

of VNA ports

The calibration procedure for a VNA built from ADL5961 devices

is similar to that for any other VNA system. For maximum accura-

cy, it is important to note the frequencies used. Additionally, the

ADL5961 SPI gain, bandwidth, and frequency multiplier and/or

divider settings used during calibration must exactly match the

settings used during an actual DUT measurement. Calibrate the

VNA for multiple different configurations if the settings are expected

to be different during an actual measurement because gain and oth-

er settings are subject to device-to-device and channel-to-channel

spread (mismatch).

Although S-parameters are the result of a power ratio calculation

and, in principle, independent of absolute power levels, it is often

important to accurately control the RF source power vs. frequency

during a measurement. When the DUT exhibits nonlinearity across

the applied RF power levels, such as semiconductor devices, a

change in power level can result in a change of the DUT behavior

that affects the measurement accuracy. The ADL5961 forward IF

channel can be used to monitor the power levels in the RF signal

path, particularly when the port is terminated with 50 Ω, such as

during calibration of the system with a load standard. The conver-

sion gain of the ADL5961 itself does exhibit roll-off vs. frequency

as well, which must be taken into account to achieve an accurate

power measurement.

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