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Another important parameter we can measure when phase informa
-
tion is available is group delay. In linear devices, the phase change
through the DUT is linear-with-frequency. Thus, doubling the fre
-
quency also doubles the phase change. An important measurement, es
-
pecially for communications system users, is the rate of
change-of-phase-vs.-frequency (group delay). If the rate of
phase-change-vs.-frequency is not constant, the DUT is nonlinear. This
nonlinearity can create distortion in communications systems.
Measurement Error Correction
Since we can measure microwave signals in both magnitude and
phase, it is possible to correct for six major error terms:
q
Source Test Port Match
q
Load Test Port Match
q
Directivity
q Isolation
q Transmission Frequency Response
q Reflection Frequency Response
We can correct for each of these six error terms in both the forward
and reverse directions, hence the name 12-term error correction. Since
12-term error correction requires both forward and reverse measure-
ment information, the test set must be reversing. “Reversing” means
that it must be able to apply the measurement signal in either the for-
ward or reverse direction.
To accomplish this error correction, we measure the magnitude and
phase of each error signal (Figure 3-19). Magnitude and phase infor
-
mation appear as a vector that is mathematically applied to the mea
-
surement signal. This process is termed vector error correction.
Summary
A vector network analyzer is similar to a scalar network analyzer. The
major difference is that it adds the capability for measuring phase as
well as amplitude. With phase measurements comes scattering, or
S-parameters, which are a shorthand method for identifying forward
and reverse transmission and reflection characteristics. The ability to
measure phase introduces two new displays, polar and Smith Chart. It
also adds vector error correction to the measurement trace. With vec
-
tor error correction, errors introduced by the measurement system are
compensated for and measurement uncertainty is minimized. Phase
measurements also add the capability for measuring group delay,
which is the rate of change-of-phase vs. frequency (group delay). All in
all, using a network analyzer provides for making a more complete
analysis of your test device.
37xxxE OM 3-11/3-12
NETWORK ANALYZERS, A PRIMER NETWORK ANALYZERS
VECTOR ERROR CORRECTION
THEN THE RESULTANT VECTOR IS
APPLIED MATHEMATICALLY, HENCE
MAGNITUDE AND PHASE OF
EACH ERROR SIGNAL IS MEASURED
MAG
PHASE
Figure 3-19. Magnitude and
Phase