ADL5961ACCZ-R2 datasheet(23/34 Pages) AD

23 / 34 page

Data Sheet

ADL5961

THEORY OF OPERATION

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Rev. 0 | 23 of 34

different calibration strategies are reported in literature to determine

either a subset of or all of the error coefficients (matrix elements).

One strategy to simplify the error model assumes that the crosstalk

between the VNA channels is negligible, that is, that contributions

of Port 1 to measurement errors in Port 2 and contributions of Port

3 to measurement errors in Port 0 are small. This assumption is

plausible in a VNA based on the ADL5961 because each VNA

port is realized by a separate device. Interaction between the VNA

channels can be minimized through careful PCB layout. For this

situation, only the block matrices on the diagonal in Equation 11

have nonzero elements, resulting in the following:

b2a2 =T23T31T01−1b0a0 =TMb0a0

(12)

where:

T31 represents the transmission matrix of the DUT itself (the quanti-

ty to be measured).

TM the transmission matrix measured by the VNA.

The error corrected transmission matrix can then be expressed as

follows:

TCORRECTED=T23−1TMT01

(13)

Many different calibration techniques are available to determine

the transmission matrices, T01 and T23. One of the simplest yet

effective methods is the SOLT calibration by which a one-port

calibration is applied to each port, followed by the measurement of

a thru connection (short between Port 1 and Port 3).

Multiport Calibration

Calibration of VNAs consisting of more than two ports can be

performed along similar procedures as the two-port calibration

discussed in the Two-Port Calibration section. The number of error

coefficients to be determined grows quadratically as 4n2, where n is

the number of ports. However, when interactions between the VNA

ports is neglected only the coefficients on the block diagonal of the

error model must be taken into account, resulting in 4n remaining

coefficients.

A practical problem arising with multiport calibration is that cali-

bration standards are usually either one-port (loads) or two-port,

whereas the calibration procedure, in principle, requires the meas-

ured n × n S-matrix and an n × n S-matrix for the actual standard

S-parameters. This problem can be addressed by constructing the

n-port S-matrix from a series of two-port measurements. Equation

14 illustrates the concept for a four-port system.

S= m

m12m13m14

m12m12m23m24

m13m23m13m34

m14m24m34m14

(14)

where mxy indicates from which two-port measurement the corre-

sponding S-parameter is determined.

For example, a two-port measurement using Port 1 and Port 2,

indicated by m12, can be used to determine s11, s12, s21, and s22.

Measurements on different combinations of two ports are needed

to fill the entire S-matrix. Some parameters are determined multiple

times (like s11) and can be disregarded in all but one measurement.

In general, the full set of n2 S-parameters can be determined with

n(n − 1)/2 two-port measurement sessions.

When the SOLT calibration method is applied to an n-port VNA,

for example, it requires measurement of three loads on each port,

followed by a thru standard measurement between all combinations

of two ports. The total number of measurement runs required for

this is

3n + n(n − 1)/2 = n(n + 5)/2

(15)

Rejection of IF Spurious Tones

Besides the desired output signal, a variety of other spurious

tones and mixing products are generally present in the IF output

signal spectrum. Some of these undesired tones appear at the

same frequency as the desired IF signal, and therewith reduce the

measurement accuracy. The techniques described in this section

can be used to reduce the impact of such undesired tones and

enhance the measurement accuracy.

The LO interface configurations that use the OF interface are most

vulnerable to IF spurious tones. Harmonics, subharmonics, mixing

products between the LO and OF, and partially suppressed image

frequencies contribute to spurious tones in the IF output spectrum.

The impact of spurious tones is most pronounced when the DUT

at the RFOUT port is well matched, such that the desired signal

in the reverse IF output channel is small. Figure 58 illustrates

the impact of spurious tones at the IF frequency, comparing a

return loss measurement result corrected for IF spurious tones and

a raw, uncorrected result. As apparent from Figure 58, spurious

tones introduce ripple vs. frequency in the measurement result

and reduce the measurement sensitivity, particularly at frequencies

below 5 GHz.

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