MM0474
(2) Mismatch Error. The following discussion uses the terms conjugate
power, Zo available power, conjugate match and mismatch, and Zo match and
mismatch. These basic terms are defined as follows:
Conjugate power is the maximum available power. It is dependent on
a conjugate match condition in which the impedance seen looking toward the
thermistor mount is the complex conjugate of the impedance seen looking toward
the RF source. A special case of this maximum power transfer is when both the
RE source and the thermistor mount have the same impedance as the transmission
line.
Zo available power is the power a source will deliver to a Zo load.
It is dependent on a Zo match condition in which the impedance seen looking
into a transmission line is equal to the characteristic impedance of the line.
(3) In a practical measurement situation, both the source and thermistor
mount have SWR, and the source is seldom matched to the thermistor mount
without the use of a tuner.
The amount of mismatch loss in any measurement depends on the total SWR
present.
The impedance that the source sees is determined by the actual
thermistor mount impedance, the electrical length of the line, and the
characteristics impedance of the line, Zo.
(4) In general, neither the source nor the thermistor mount has Zo
impedance, and the actual impedances are known only as reflection coefficients,
mismatch losses or SWR.
These forms of information lack phase information
data. As a result, the power delivered to the thermistor mount and hence the
mismatch loss can only be described as being somewhere between two limits. The
uncertainty of power measurement due to mismatch loss increases with SWR.
Limits of mismatch loss are generally determined by means of a chart such as
the Mismatch Loss Limits charts in Figures 4 and 5.
(5) An example may explain how imperfect match affects the uncertainty
of power measurement. A typical Zo available power measurement situation can
involve a source with an SWR of 1.7 (ps = 0.26) and a thermistor mount with an
SWR of 1.3 (pm = 0.13). Figure 6 shows a plot of power levels and mismatch
power uncertainties that result from source and thermistor mount mismatch. The
source Zo mismatch results in a power loss of -0.29 dB from the maximum power
that would be delivered by the source to a conjugate match. The power level
that results from this loss is the Zo available power. The thermistor mount Zo
mismatch causes an additional power loss of -0.07 dB.
However, on the
thermistor mount Zo mismatch loss is an uncertainty resulting from the unknown
phase
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