Multipactor in Low Pressure Gas and in ... - of Richard Udiljak

In a typical test setup there can be many sources of noise, which could
result in ambiguous test results. On the one hand, the multipactor
threshold could be established at a too low value if non-multipactor
generated noise is misinterpreted as the result of a discharge. On the
other hand, a short-lived multipactor event could be disregarded and
lead to determination of a too high threshold based on a more distinct
indication. The AM detection method resolves this concern by only
signalling for true multipactor events.
Another advantage of the AM detection method is the fact that it
is particularly sensitive close to the multipactor threshold, since it only
responds to the signal difference between the input signal and the threshold,
as can be seen from relation 6.2. A weak amplitude modulation,
as shown in Fig. 6.10 where the single carrier signal has just passed the
multipactor threshold, will produce a very distinct modulated output
signal as indicated in Fig. 6.11. Even though the signal is very noisy,
the periodicity is very distinct. Not always will the periodicity be as
noticeable as in Fig. 6.11, but a FFT will reveal any periodicity in the
measured signal.
Amplitude [A.U.]
120
100
80
60
40
20
Input signal envelope, single carrier, 1%depth AM
Input signal
Multipactor threshold
0
0 0.5 1 1.5 2 2.5
Time
3 3.5 4 4.5 5
Figure 6.10: Example of a single carrier input signal envelope with a 1%
depth AM. The signal has barely exceeded the multipactor
threshold.
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One of the shortcomings of the AM detection method is that it re-