Instrumentation & Measurement Magazine 25-6 - 38

saturated output power (Psat
), 1 dB compression point (P1dB
),
etc. depend on the power in the incident and scattered waves.
Measurement of the incident and scattered power requires an
absolute calibration of the VNA using a power meter in addition
to the usual S-parameter (vector) calibration of the VNA. VNADUO
evaluates the uncertainty in the VNA power measurement
from the uncertainty in the power meter reading as well as the
uncertainty due to systematic and random effects in the VNA.
Uncertainties in Amplifier Parameters
An active RF device such as an amplifier responds linearly to
sufficiently small input signals. In this case, the output signal
is proportional to the input signal and the output power
is much smaller than the saturated output power (Psat
) of
the amplifier. The small-signal performance of RF amplifiers
is usually characterized using a VNA which measures
the complex-valued S-parameters of the amplifier-i.e., the
magnitudes and phases of its reflection and transmission coefficients.
Fig. 1 shows a typical block diagram of a 2-port
VNA for measurement of a 2-port device under test (DUT). To
minimize systematic errors, the VNA is calibrated using an Sparameter
(vector) calibration such as Thru-reflect-line (TRL),
short-open-load-thru (SOLT), etc. For larger input signals, the
amplifier responds non-linearly i.e., the output power is not
proportional to the input power. In this case, S-parameters
alone do not fully describe the amplifier behavior. To measure
large-signal amplifier behavior, the VNA requires an additional
power calibration. The uncertainties in small-signal and
large-signal amplifier parameters are discussed below.
Uncertainties in Small-signal Amplifier
Parameters
The amplifier operating in linear mode can be evaluated with
S-parameter measurements. However, for evaluation of the
measurement uncertainty, the methods used in [10]-[13] can
fail when the parameters of the DUT can change due to dc supply,
thermal self-heating, changes in active junction regions,
electromagnetic influence due to proximity, environmental
changes, etc. Tools such as VNA-DUO which are pre-characterized
for uncertainty are more suitable for such measurements.
For S-parameter measurements, VNA-DUO considers three
sources of uncertainty [15]: noise in the VNA (noise floor and
trace noise); cable and connector repeatability; and the calibration
kit. These sources of uncertainty are characterized before
the measurement of the DUT, and then the corrected S-parameters
of the DUT together with their uncertainties are evaluated
and displayed on the VNA screen in real-time. A detailed study
of VNA-DUO has been reported in [15]. Small-signal amplifier
parameters, such as the input and output return losses,
small-signal gain, and reverse isolation, together with their uncertainties
can be measured using VNA-DUO.
Uncertainties in Large-signal Amplifier
Parameters
Some large-signal characteristics of an amplifier such as GP
P1dB, and gain compression can be measured using a VNA but
38
Fig. 1. (a) A 2-port VNA block diagram showing the four receivers: A1
. (b) The measurement setup.
, B1
and B2
this requires a power calibration of the VNA with a power
meter. To determine the uncertainties in the subsequent measurements,
the uncertainties associated with the power meter
must also be known. VNA-DUO enables the uncertainties in
the power meter and the uncertainties in the S-parameters to be
propagated to the VNA receiver measurements. From the measured
receiver powers and their uncertainties, the amplifier's
other parameters can be fully determined, and the associated
uncertainties can be evaluated. Methods to evaluate the uncertainties
in some large-signal parameters are explained in the
section entitled " the evaluation of uncertainty. "
,
VNA-DUO Uncertainty Evaluation Tool
VNA-DUO considers the following four sources of uncertainty
for S-parameters and power measured using a VNA:
◗ Uncertainty due to the definition of the calibration standards.
The uncertainty can be based on fully characterized
IEEE Instrumentation & Measurement Magazine
September 2022
, A2
,
Instrumentation & Measurement Magazine 25-6

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 25-6
