HPMX-2005 Typical Data
Measurement
Direct measurement of the ampli-
tude and phase error at the output
is the most accurate way to evalu-
ate modulator performance. By
measuring the error directly, all
the harmonics, LO leakage, etc.
that show up in the output signal
are accounted for. Figure 3 below
shows the test setup that was
used to create the amplitude and
phase error plots (figures 19 and
21).
Amplitude and phase error are
measured by using the four chan-
nel power supply to simulate I and
Q input signals. Real 1.5 V
p-p
I and
Q signals would swing 0.75 volts
above and below an average 2.5 V
level, therefore, a logic “high”
level input is simulated by apply-
ing 3.25 V, and a logic “low” level
by applying 1.75 V to the I and/or
Q inputs.
Amplitude and phase are meas-
ured by setting the network ana-
lyzer for an S
21
measurement at
the center frequency of choice.
Set the port 1 stimulus level to the
LO level you intend to use in your
circuit (-12 dBm for the data
sheet).
By adjusting the V
i
and V
q
settings
you can step around the I/Q vec-
tor circle, reading magnitude and
phase at each point. The relative
values of phase and gain (ampli-
tude) at the various points will
indicate the accuracy of the
modulator. Note: you must use
very low ripple power supplies for
the reference, V
Imod
, and V
Qmod
supplies. Ripple or noise of only a
few millivolts will appear as wob-
bling phase readings on the net-
work analyzer.
The same test setup shown below
is used to measure input and out-
put VSWR, reverse isolation, and
power vs. frequency. V
Imod
and
V
Qmod
are set to 3.25 V and the
appropriate frequency ranges are
swept. S
11
provides input VSWR
data, S
22
provides output VSWR
data and S
12
provides reverse
isolation data. S
21
provides power
output (add the source power to
the S
21
derived gain).
LO leakage data shown in figure
17 is generated by setting V
Imod
=
V
Qmod
= V
Iref
= V
Qref
= 2.5 volts then
performing an S
21
sweep. Since
phase is not important for these
measurements, a scalar network
analyzer or a signal generator and
spectrum analyzer could be used.
7-57
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
