Instrumentation & Measurement Magazine 25-6 - 40

(U3020B) as shown in Fig. 1b. The high-power test-set consists
of four low-loss 20 dB directional couplers. To keep the
measurement signal levels within the linear region of the VNA
receivers, additional 10 dB attenuators on the port 1 receivers
and 20 dB attenuators on the port 2 receivers were used.
The connections between the VNA and the high-power test set
were made using semi-rigid coaxial cables. Port 1 of the test
set was connected to the DUT with a fixed 3.5 mm ruggedized
RF adapter, and Port 2 of the test set was connected to the DUT
with a flexible test port extension cable. For small-signal characterization
of the amplifier, S-parameters with uncertainty
were measured using the VNA with the following settings:
◗ Frequency: 100 MHz to 8 GHz
◗ Frequency step: 50 MHz
◗ Power: -5 dBm
◗ IF Bandwidth: 10 Hz
The first step was to characterize the sources of uncertainty
in the measurement as described in the VNA-DUO Uncertainty
Evaluation Tool section. The VNA was calibrated using
a mechanical calibration kit with a firmware short-open-loadreciprocal
(SOLR) calibration. A mechanical calibration kit
(Keysight 85052D) which had previously been characterized
for uncertainty using NPL's primary impedance microwave
measurement system (PIMMS) was used for calibration.
In our experiment, the four sources of uncertainty were
characterized as follows:
◗ The uncertainty due to the calibration standards was
based on the characterization of the calibration standards
at NPL using PIMMS.
◗ Uncertainty due to electrical noise was accounted for
by 20 repeat measurements of a match and a high reflect
standard on both ports (ports 1 and 2).
◗ Uncertainty due to cable flexing and connector repeatability
was characterized by 20 repeat measurements of
both a well-matched standard and a high reflect standard
on both ports. The flexible RF cable was moved,
and the standards were disconnected from the VNA
and reconnected with a different orientation between
measurements.
◗ The power meter uncertainty was accounted for by
uncertainty data provided by the manufacturer of the
power meter.
To characterize large-signal amplifier parameters, the
VNA was calibrated with an SOLR vector calibration and also
with source and receiver power calibrations using a Keysight
N1813A power meter and an N8485A power sensor. The uncertainties
in the vector calibration and in the power meter
were propagated by VNA-DUO to the power measured by the
receivers at ports 1 and 2 of the VNA.
The source and receiver power calibrations were performed
at a fixed source power of -5 dBm. It is only necessary
to connect the power sensor to port 1 of the VNA during calibration,
since the power calibration is then transferred to the
receivers on port 2 using the vector calibration. To capture nonlinear
parameters of amplifiers, the measurements were made
by sweeping the source power level between -15 dBm and
40
+5 dBm. Measurement results for a ZX60-83LN-S+ amplifier
from Mini-Circuits Corp. are presented in this paper. Measurements
were made over the frequency range between 0.5 GHz
and 8 GHz.
A Keysight N705B power supply was used to apply dc
bias to the amplifier and the voltage and current reading
were taken directly from the power supply unit. The receiver
powers together with the corresponding upper and lower uncertainty
limits were displayed as traces on the VNA screen
and all traces were saved as MDIF files.
The uncertainties in the measured amplifier parameters
, (u(PIN
such as PIN, GP and P1dB
), u(GP
) and u(P1dB
lated using (3), (4), and (6), respectively.
Measured Results
Small-signal Parameter Results
The uncertainty enabled SOLR calibration of the PNA-X was
carried out between 0.5 GHz and 8 GHz. The S-parameters of
the ZX60-83LN-S+ amplifier were measured and the measurement
uncertainties were evaluated. The amplifier was biased
with a dc voltage of +5V and a dc current of 60 mA. The measured
small-signal gain (S21
) and reflection coefficients (S11
S22
and
) and their corresponding measurement uncertainties are
plotted in Fig. 2. The results show that the uncertainty in S21
less than 0.045 dB. The uncertainty in S11 and S22
is
depends on
the value of the reflection coefficient. For measured reflection
coefficients up to -30 dB, the uncertainty is up to 1 dB.
Large-signal Parameter Results
Large-signal parameters of the amplifier were measured with
the excitation applied to port 1 of the DUT and with the excitation
source power set at levels from -15 dBm to +5 dBm.
The incident and scattered wave amplitudes (A1
, A2
B2
, B1,
and
) were measured together with their respective maximum
and minimum uncertainty limits. From the measured wave results
and associated uncertainties, the uncertainties in various
amplifier parameters were determined using methods previously
discussed in the Evaluation of Uncertainties section. The
amplifier's measured output power (POUT
) vs input power delivered
(PIN) at 2, 5, and 8 GHz are plotted in Fig. 3. The results
show that the amplifier gain starts to drop at an input power
between -5 dBm and 0 dBm and that it is well compressed at an
input power of +5 dBm for all frequencies. The power gain (GP
)
of the amplifier measured between 0.5 and 8 GHz at different
input powers is plotted in Fig. 4a. The results show that the amplifier
gain is compressed by more than 3 dB at PIN
of +5 dBm
confirming that the amplifier is operating in its non-linear
mode of operation for this value of input power. The measured
P1dB
, u(PIN
the uncertainty in POUT =|B2|2
), was calculated using (3) and
, u(POUT
), depends only on the
uncertainty in B2 because the amplifier was measured into a 50
Ω matched load. The uncertainty in GP, u(GP
), was calculated
IEEE Instrumentation & Measurement Magazine
September 2022
of the amplifier, along with the associated uncertainty, is
plotted in Fig. 4b. The upper and lower uncertainty bounds are
for a confidence level of 95%.
The uncertainty in PIN
)) were calcu
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