Biomedical Engineering Reference
In-Depth Information
The RF-generator (designed by Krohne Messtechnik GmbH) delivers about 1 W
at a frequency of 2.45 GHz (IMS-band). Due to the very low impedance of the
plasma presumably most of this power is dissipated between oscillator an plasma
chamber. Once the chip will be packaged in a small housing the RF-power can be
significantly reduced to <100 mW and the generator be further integrated. Obviously,
the impedance of the plasma changes dramatically from ignition to stable operation,
which complicates impedance matching and presently also makes an oversized
RF-source necessary.
The DC-Bias-Voltage on the RF-electrode of the plasma chamber is used during
the plasma ignition procedure to monitor a successful ignition and stop the pulse
train by software. The DC-Bias-Voltage is also a measure for the intensity of the
plasma. As the plasma state changes abruptly during this process as shown in
Fig. 23b, c , the DC-Bias-Voltage drops abruptly.
6.6
Traveling Wave Generator
Complementary to the physical design of the chip the signal generator driving SIS-
separator governs the measurable mass range and resolution of the system. As
described in Sect. 3.5 , the traveling wave generator (TWG) generates rectangular
signals. These are applied to the finger electrodes to generate the local electric fields
in the separator which deflect all ions, which are not meant to be detected.
The TWG is one of the most challenging components of the electronics. In order
to analyze a wide spectrum of masses within a few seconds the time to change from
one selected mass to the next, i.e., from one stable TWG-frequency to the next is in
the order of a few milliseconds. The mass-to-charge ratio depends on frequency
according [ 33 ] :
1
mU
2
2
qU
=
mv
⇒=
,
2
q
2
()
df
U is the ion acceleration voltage and d the distance of the finger electrodes driven
by the same signal of the generator. For a linear sweep of the mass spectrum, the
frequency sweep has to be nonlinear in time, i.e., an independent controller for a fast
computation of the sweep is necessary.
The minimum requirements for the TWG are:
￿
Two 180 ° phase shifted rectangular signals with 50 % duty cycle.
￿
Programmable signal amplitude from 2 V
p-p to 5 V p-p , to adapt different electrode
configurations. (In future fixed amplitudes may be sufficient).
Rise and fall time of the signals have to be very short, since a fast switching of
￿
the electric field within the separator determines its mass resolution. The actual
generation of TWG achieves rise and fall times less than 0.5 ns at 5 V p-p
amplitude.
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