ADL5960ACCZ-R2 datasheet(21/32 Pages) AD

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Data Sheet

ADL5960

THEORY OF OPERATION

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Rev. A | 21 of 32

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, in which a one-port

calibration that is applied to each port, followed by 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 Two-Port Calibration. 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 can

be neglected only the coefficients on the block diagonal of the error

model need to 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 indicate 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, sub-harmonics, 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 very 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 this figure, spurious

tones introduce ripple vs. frequency in the measurement result,

and reduce the measurement sensitivity, particularly at frequencies

below 5GHz.

Figure 58. Return Loss Measurement of a 50 Ω Load, With and Without

Correction of IF Spurious Tones

The following simple procedure can significantly reduce the ripple

due to spurious tones:

1. Measure the IF output signal with RF present.

2. Calculate the complex fast Fourier transform (FFT) frequency

component at the IF output frequency.

3. Measure the IF output signal with the RF signal off, or set to a

very low level.

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