EE 41458/61558 Microwave Circuit Design and Measurements Lab

EE 41458/61558 Microwave Circuit Design and Measurements Lab
EE 41458/61558
Microwave Circuit Design and Measurements Lab
AMPLIFIER NONLINEAR PERFORMANCE
Lab #10
In this laboratory session, the nonlinear characteristics of your amplifier design
will be measured and analyzed. The amplifier will be characterized for two figures of
merit: the 1 dB gain compression point and the third-order intermodulation intercept
point. Both of these figures of merit provide insight into the performance of the amplifier
under large-signal conditions, and can provide an indication of the ultimate system
performance achievable with the amplifier.
1 dB Compression Point Characterization
The 1 dB compression point of an amplifier is the input power required to reduce
the gain (transducer power gain, GT) to 1 dB below its small-signal value. This value can
be measured by supplying a constant-frequency, ramped-power input signal to the
amplifier and measuring the output power as a function of available input power.
1.) Set up the measurement apparatus as shown in the figure below. We will use
the spectrum analyzer as a calibrated, tuned power meter. Set the signal source to your
amplifier’s center frequency at a power of -20 dBm. The center frequency of the
spectrum analyzer should be set to this same frequency, with a span of 10 MHz. Set the
spectrum analyzer's reference level to 30 dB to protect the input from excessive power
(setting the reference level automatically engages attenuators internal to the spectrum
analyzer).
2.) To find the loss in the cables at the design frequency, first use a femalefemale adapter as the DUT. In 1 dB steps, increase the signal generator power, and
measure the output power as indicated on the spectrum analyzer. Record the input
power/output power data points in your lab notebook. You may assume that the loss in
the adapter is negligible compared to the loss in the cables, and that the loss in each cable
is the same. How much loss is incurred in the cables?
3.) Reset the input power to -20 dBm, and replace the adapter with your
amplifier. Record the raw amplifier output power as a function of input power in the
same way as for step 2 above. Correct the data for the cable loss, and include a table of
the corrected Pavs vs. PL measurement in your lab notebook. Note that Pavs differs from
the synthesizer power setting by the loss in one of the cables, and that PL differs from the
reading on the spectrum analyzer by the additional loss due to the second cable.
4.) Compute and tabulate the transducer gain (GT) as a function of input power
(taking into account the cable loss), and plot the results in your lab notebook. Using this
data, find the input power that produces a gain that is 1 dB below that found for very low
input powers. This is the 1 dB compression point, and is specified as P1dB. You may
have to interpolate to find the value accurately.
Revised 11/2011
P. Fay
EE 41458/61558
Microwave Circuit Design and Measurements Lab
E4421B signal
generator
DUT
ESA-L1500A
spectrum analyzer
Measurement setup for 1 dB compression point measurement.
Intermodulation Performance Characterization
In the preceding section, we investigated the single-frequency performance of the
amplifier as the input power is changed. This measurement gives some insight into
absolute power levels that can be produced and handled by the amplifier, but it tells very
little about how the amplifier will respond to modulated (i.e. information-carrying)
signals at high powers. One way of characterizing the response of the amplifier to
modulated signals is the two-tone third-order intermodulation intercept point.
1.) Connect the measurement setup as shown in the figure below, but with the
female-female adapter as the DUT. Set the frequency of source 1 to 1 MHz below your
design frequency, and that of source 2 to 1 MHz above the design frequency. The power
level of both sources should be set to -20 dBm. Set the center frequency of the spectrum
analyzer to be your design frequency, with a span of 10-15 MHz to include the signals
and the responses induced by circuit non-linearities. Measure the power in the signals,
noting that the power splitter introduces considerable additional loss (~6 dB) above the
cable losses. Compute the loss on the input and output side of the DUT so that you can
properly compensate your amplifier measurements. Replace the female-female adapter
with your amplifier.
2.) For source powers from -20 dBm to 10 dBm in 1 dBm steps, find the output
power in the fundamental, Pd (i.e. the desired frequency components), and in the
intermodulation products, PIM (i.e. the spurious responses near the fundamental due to the
third-order non-linearity; these will lie 1 MHz above and below the primary signals).
The two sources should be kept at the same power level, and the magnitude of either
fundamental output can be recorded as Pd. Provide a table of the data in your lab
notebook, and graph PIM and Pd vs. Pavs for your amplifier. Compute the transducer gain
from this data; how does it compare with the data taken in with the VNA in Lab #9 and
the data collected in the compression measurement portion of the lab above?
3.) For each power level, compute the intermodulation ratio: IMR =
P IM ! Pin $
=#
&
Pd
" PIP3 %
2
.
This expression can also be used to find the input third-order intermodulation intercept,
PIP3. Note that this expression is only valid for input powers for well below the gain
compression point. Using the above expression, compute PIP3 for your amplifier. Using
the graph plotted in #2 above, extrapolate the data to find PIP3 graphically. How does this
agree with the result from the expression above? Comment on the origins of any
observed discrepancies. What is the significance of PIP3, and how is it useful?
Revised 11/2011
P. Fay
EE 41458/61558
Microwave Circuit Design and Measurements Lab
source #1
E4421B signal
generator
3' RG-8A/U
HP 11667A
power splittercombiner
DUT
source #2
E4421B signal
generator
Measurement setup for intermodulation measurement.
Revised 11/2011
P. Fay
8562E
spectrum
analyzer
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