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dc2496aaf
DEMO MANUAL
DC2496A-A/DC2496A-B
operation
Setup Signal Sources and Spectrum Analyzer
The LTC6432-15 is an amplifier with high linearity per-
formance. Therefore, the output intermodulation prod-
ucts are very low. Even using high dynamic range test
equipment, third-order intercept (IP3) measurements can
drive test setups to their limits. Consequently, accurate
measurement of IP3 for a low distortion IC such as the
LTC6432-15 requires certain precautions to be observed
in the test setup as well as the testing procedure.
Set Up Signal Sources
Figure 7 shows a proposed IP3 test setup. This setup has
low phase noise, good reverse isolation, high dynamic
range, sufficient harmonic filtering and wideband imped-
ance matching. The setup is outlined below:
a. High performance signal generators 1 and 2 (HP8644A)
are used. These suggested generators have low har-
monic distortion and very low phase noise.
b. High linearity amplifiers are used to improve the reverse
isolation. This prevents cross talk between the two
signal generators and provides higher output power.
c. A low pass filter is used to suppress the harmonic
content from interfering with the test signal. Note that
second order inputs can “mix” with the fundamental
frequency to form intermodulation (IM) products of their
own. We suggest filtering the harmonics to –50dBc or
better.
d. For improved VSWR, distortion and isolation, the
H-6287-10 signal combiner from Werlatone covers
the 100kHz to 50MHz band. It features >30dB isolation
over this band. From 20MHz to 1400MHz, we suggest
the H9 signal combiner from MA/COM which features
>40dB isolation. Passive devices (e.g. combiners) with
magnetic elements can contribute nonlinearity to the
signal chain and should be used cautiously.
e. The attenuator pads on all three ports of the signal
combiner will further support isolation of the two input
signal sources. They also reduce reflections and promote
maximum power transfer with wideband impedance
matching.
Set Up the Spectrum Analyzer
a. Adjust the spectrum analyzer for maximum possible
resolution of the intermodulation products’ amplitude
in dBc. A narrower resolution bandwidth will take a
longer time to sweep.
b. Optimize the dynamic range of the spectrum analyzer
by adjusting the input attenuation. First increase the
spectrum analyzer’s input attenuation (normally in
steps of 5 or 10dB). If the IM product levels decrease
when the input attenuation is increased, then the input
power level is too high for the spectrum analyzer to
make a valid measurement. Most likely, the spectrum
analyzer’s 1st mixer was overloaded and producing its
own IM products. If the IM reading holds constant with
increased input attenuation, then a sufficient amount
of attenuation was present. Adding too much attenua-
tion will bury the intended IM signal in the noise floor.
Therefore, select just enough attenuation to achieve a
stable and valid measurement.
c. In order to achieve this valid measurement result, the
test system must have lower total distortion than the
DUT’s intermodulation. For example, to measure a
53dBm OIP3, the measured intermodulation products
will be –102dBc below a –13dBm/tone input level and
the test system must have intermodulation products
approximately –108dBc or better. For best results, the
IM products and noise floor should measure at least
–112dBc before connecting the DUT.
