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

the FFT. It was concluded that there is a positive correlation between
the modulation depth and the strength of the detected signal at the
corresponding frequency. By taking the time average of one of the test
sequences like in Fig. 6.3 and plotting it using linear scales on both axes,
it was found that there was a more or less linear relationship between
the input power and the resulting multipactor noise power (cf. Fig. 6.7),
which can be described by the following function:
Pnoise = k · (Pinput − Pth) [W] Pinput ≥ Pth (6.2)
where k = 5.3 × 10 −11 and Pth = 25.2 W is the multipactor threshold.
Noise Power [nW]
1.4
1.2
1
0.8
0.6
0.4
0.2
Linear Plot of Noisepower vs Input Signal Power
Input Signal Power [W]
26 28 30 32 34 36 38 40 42 44
0
50 55 60 65 70 75
Time (s)
80 85 90 95 100
Figure 6.7: Time average of the test sequence shown in Fig. 6.3 with a linear
scale on both axes. The straight line has been added to show the
close to linear relationship between input power and noise power.
Note: The input power is increased every 4 seconds, which is the
reason for the step like behaviour.
The reason why the small modulation was so noticeable in the multipactor
noise (see Fig. 6.4) is that the noise signal is a function of the
difference between the input power and the multipactor threshold, i.e.
no discharge noise is generated until the multipactor threshold has been
reached. Furthermore, since the decibel scale is a relative scale, the small
increase in absolute numbers becomes very noticeable in relation to the
existing noise floor. As a comparison, the first small steps in Fig. 6.7,
103