What does matter is to get the two closely spaced signals summed
together without the two sources talking to each other through the power
combiner with a possibly non-linear output impedance. Any slight output
stage nonlinearity can actually create IM2/IM3 terms in the combined
source signal. Some isolation can be achieved simply by padding down the
inputs and/or using a power combiner with high port to port isolation.
For the ultimate in source to source isolation, the test signal for
ISL55210 buffered each generator output with a very high intercept
(linear output impedance) RF amplifier (ref. 4) before the power
combiner as shown in fig. 4
Click on image to enlarge.
Figure 4. Source summing structure for <-120dBc IM3 test signals (power cables removed).
MiniCircuits Hela-10D amplifier used here are +12V supply requiring
525mA quiescent current (6.3W/each) but provide 11dB gain with an OIP3
at a matched 50Ω load >48dBm. The ZFSC-2-4-S+ power combiner in fig. 4
quotes approximately 30dB port 1 to port 2 isolation and about 3dB
insertion loss to the summed output pins. The Hela-10D amplifiers were
modified with an output resistive pi attenuator to generate a 0dB gain
stage from the inputs at the Hela-10D’s, to the combiner output pins.
This helps to keep both the loading on the Hela-10’s ok as cables were
connected/disconnected and allowed the source programmed power to be
placed as a test input power level to the DUT.
testing on the combined output 2-tone signal failed to expose a 3rd
order spurious (down to -120dBc) even to very high (4dBm) output test
signals. Recall this summed output is intended to drive an amplifier
stage, so these 4dBm single tones combine to a 2Vpp envelope and should
be adequate for most measurements. If used for ADC testing, this very
low IM3 source will need to be filtered by a narrow bandpass filter to
limit the out of band spectral (noise and distortion) artifacts as the
ADC will fold all of those into Nyquist. Normally, much lower test
source levels are required for amplifier testing. The intended
application span was approximately 10Mhz to 200Mhz with the measured
frequency response for each of the two port inputs of fig. 4
shown in fig. 5
. The unused input in each test needs a 50Ω termination for this measurement.
Click on image to enlarge.
Figure 5. Source summing frequency response.
Measuring the output SFDR for a 2 tone IM3 spurious
we can generate a very clean input signal to test a modern, low power,
FDA like the ISL55210, we are still left with the issue of resolving
what might be more than -110dBc separation from the test tones to the
spurious. Most spectrum analyzers are limited to about -85 to -90dBc
dynamic range. While there are a number of tricks that can be used to
extend this measurement range, the one selected here essentially creates
a super narrowband notch filter on one of the output test tones from
the DUT. This allows the spectrum analyzer attenuation to be greatly
reduced without creating internal IM3 terms that would mask the DUT’s
performance. Backing out the attenuation to the spectrum analyzer mixer
will of course create very poor single tone HD terms, but those are not
of interest in this test.
Prior to applying the DUT output
spectrum to a spectrum analyzer, the first step is to attenuate to where
the 2-tone power levels will not create meaningful IM3 if passed back
through another Hela-10D amplifier. With a 48dBm intercept in the
Hela-10D, and a -120dBc measurement aspiration, this implies taking the
DUT output powers down to approximately -12dBm on each tone at the
output of the Hela-10D. This implies approximately -23dBm for each tone
at the inputs using eq. 2 and solving for P0. This impedance isolated
DUT output spectrum (that is being passed through another HELA-10D),
is then applied to another power combiner structure identical to fig. 4
where the 2nd input is now a 3rd low phase noise lab signal generator.
This 3rd phase locked source can be combined with the DUT’s output spectrum as shown in the complete test structure of fig. 6
where here the fixed resistive pi attenuator is not used on the outputs
of the HELA-10D’s in this output side power combiner to keep the
spurious as high as possible. The output of the 2nd power combiner then
goes to a spectrum analyzer with minimal internal attenuation. The
remaining uncancelled test tone gives an indication of P0, while the
spurs at +/-3Δf can be measured by zooming in on a small span, with a
low RBW, and sweep averaging.
Click on image to enlarge.
Figure 6. Complete source and output tone nulling structure for <-110dBc IM3 measurements
the 3rd generator with phase tuning, it is then possible to set the
amplitude and phase of a cancelling signal in this 2nd power combiner to
null out one of DUT output tones. Actual results showed about a -50dBc
selective attenuation on one of the tones using the HP8644 source. This
proved adequate to extract the measured results down to the -120dBc
level of fig. 2. Since all of these sources and spectrum analyzers are
available under GPIB control, this test methodology could be expedited
by programming it under Labview control.
Summary and added projection techniques to estimate extremely low IM3 stages
measurements here have shown<-110dBc IM3 performance on one of the
emerging very high SFDR, low power, signal chain solutions. These
devices have been applied successfully to both ADC interface (ref. 5)
and IF applications saving significant power where suitable. While not
used here explicitly, there are a number of other techniques that allow
measurements in one range to be projected to others. For VFA based
amplifiers, reducing the loop gain with a resistor across the inputs can
be used to artificially increase the distortion terms during test, then
project them to what they would be with the resistor removed. In
theory, the SFDR should increase by the dB delta in noise gain. Also,
once a solid measurement is made, such as those above 150Mhz in fig. 2
if the amplifier has a 20dB/dec loop gain rolloff, the distortion terms
can be projected to run on that slope up and down in frequency.
finally, if the amplifier can be shown to have an intercept
characteristic, (i.e., step the test tone powers by 1dB and the 3rd
order intermodulation terms should change by 3dB, 2dB change in SFDR)
getting that intercept at relatively high output power levels will allow
the SFDR to be projected at lower, possibly un-measurable, levels.
- ISL55210 Wideband, Low Power, Ultra-High Dynamic Range Differential Amplifier
- ISL55210 data sheet
- Designers guide to the ISL55210/55211 eval board
- MiniCircuits HELA-10 amplifiers
- Ultra Low Power 8 to 14bit data acquisition platform