Random phase set 2
3rd order IMD
-60.22 dB
6
Generating this signal implies that each discrete tone in the composite test signal is
phase aligned so that the individual tone powers add 100% constructively. Consequently,
the DUT is subjected to the maximum peak-to-average ratio it will experience when
presented with similar loading conditions. Although this test does result in a robust
device, it is not representative of the most common operating conditions. In practice,
the spectral components in the passband of the DUT are continuously changing over
time in a random fashion, particularly when you consider phase-modulated signals.
As more tones are added to the test signal, the probability of all the tones aligning in
phase is significantly reduced. As the probability of the peak power occurrence decreases,
it becomes increasingly unreasonable to make design tradeoffs based on this event. As
a result, the design effort required to accommodate the maximum peak-to-average ratio
condition may not be warranted. Even so, if you would like the security of knowing your
device can handle the worst-case peak-to-average ratio, the vector stimulus technique
discussed later facilitates aligning phase to simulate this event.
More importantly, a high peak-to-average ratio does not imply high levels of IMD at all
frequencies. As shown in Figure 4, the IMD levels generated by a device under test vary
depending on the phase relationships of the multitone test stimulus. Notice that when
the initial phase of the tones are aligned (i.e. worst case peak-to-average ratio) the
3rd order IMD product is –52.89 dBc. However, when the tones have a random initial
phase setting, the 3rd order IMD product is –48.65 dBc with random phase set #1 and
–60.22 dBc for random phase set #2. This clearly shows that the worst-case peak-to-
average ratio (i.e. equal phase set) does not result in the highest 3rd order IMD level.
If tests were only performed using phase-aligned tones, the IMD performance issues
associated with random phase set #1 would not be revealed.
Figure 4. Three different IMD measurement results using test signals with different phase sets.
Equal phase set
3rd order IMD
-52.89 dB
Random phase set 1
3rd order IMD
-48.65 dB
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