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

cally occurring discharges will be difficult to identify in a FFT plot. By
using a source of free electrons in the test setup, e.g. a hot filament
or a UV light [14], seed electrons will be abundant and a multipactor
event is likely to occur each time the envelope exceeds the threshold for
a long enough time. If the envelope is amplitude modulated, the discharge
events will vary in strength with the AM frequency. This periodic
variation will appear as a peak in the FFT plot of the detected signal at
the same frequency. By applying a weak, 1-5% depth, synchronised AM
to the input signals, the multipactor threshold can be determined with
high accuracy and without risking ambiguous test results. A 1% AM
corresponds to less than ±0.1 dB variation in the input signal and will
have no significant effect on the measured threshold.
The detected signal must then be processed by a computer or a similar
tool in order to reveal the periodicity, as in the single carrier case.
In Fig. 6.9 an example of a possible test setup is given. In order to
achieve a good AM in the multicarrier case, all the signals should be
modulated using the same reference signal of modulation. Many signal
generators have a signal reference input, thus allowing the user to synchronise
several signal generators. To perform a successful multicarrier
experiment, the phases have to be stable in relation to each other and
thus a common reference signal will have to be used in any case. Another
way of achieving synchronised modulation could be to modulate
the gain adjustment of the high power amplifier.
In Fig. 6.9 it is suggested that the third harmonic should be monitored
and that is probably the best choice when studying multicarrier
multipactor, since third harmonic detection is fast and sensitive. Closeto-carrier
noise detection is a possible alternative, but it may not be as
sensitive and thus weak multipactor events may be overlooked.
6.2.3 Main achievements
A method of multipactor detection has been devised, which can be used
to obtain accurate and unambiguous measurement results for both single
and multicarrier multipactor. The method does not aim to replace any of
the existing methods of detection, rather it can serve as a complement
to the other methods to improve accuracy and confidence in the test
results.
Close-to-carrier noise and third harmonic detection are two fast and
sensitive methods of multipactor detection. Both methods rely on noise
generation, which makes them prone to non-multipactor generated noise.
106