Agilent Technologies AN 1287-6 Application Note page 8

The uncertainty associated with the measurement
will depend upon the quality of the match between
the booster amplifier and the DUT. In addition, any
drift or power fluctuations associated with the
booster amplifier will appear to be from the DUT
since the reference signal is not measured after the
booster amplifier. This configuration is best for
analyzers that do not allow direct access to the R
(reference) channel and when the accuracy of the
measurements is not important. If the analyzer
allows access to the R channel and the reference
signal is coupled after the amplifier, a configura-
tion that allows more accuracy is possible (see
Configuration 3).
CH1 S 21 &M log MAG 2 dB/ REF 11 dB
PRm
Cor
LINE TYPE
1
1
True response
CENTER 2.500 000 000 GHz
Figure 1. Ripple is caused by a mismatch between the
DUT and the booster amplifier.
8
18 Dec 1997 16:16:33
1_: 13.704 dB
2.512 960 000 GHz
True response
with
mismatch ripple
SPAN 4.000 000 000 GHz
Calibration
Only transmission response calibration is possible
with this setup. The location of the booster
amplifier does not allow the analyzer to make
transmission measurements in the reverse direction,
making a full two-port calibration impossible.
Perform a response calibration by connecting the
output of the booster amplifier to the attenuator
or coupler on port 2. A basic response calibration
does not remove mismatch errors due to the DUT
— only the frequency response errors and any
mismatch associated with the booster amplifier,
the attenuator, and the analyzer. Since mismatch
is not corrected for during calibration, even
measurements made with a response calibration
are limited in their accuracy.
In this setup you can improve the source match by
adding an isolator between the booster amplifier
and the DUT, or by adding attenuation. Adding an
isolator between the output of the booster amplifier
and the input of the DUT will remove the effects of
any mismatch. If the booster amplifier has enough
gain, a 3-dB or 6-dB attenuator at its output can
be added to improve its output match. Be sure
to include the isolator or any attenuation in the
calibration if you include it in the measurement
setup. Also make sure that the isolator or attenuator
can handle the power level being tested. Instead of
using a high-power isolator, a circulator with a
high-power load on its third port can be used.