Functional Description
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___________________________________________________________________ MS Detector
_______________________ Finnigan LTQ Hardware Manual _____________________
2-37
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The main RF voltage generation involves the following components:
• RF oscillator
• RF Voltage Amplifier PCB
• Low Pass Filter PCB
• RF voltage coil
• RF voltage detector
• Mass DAC
• Integrating amplifier
The RF oscillator on the Digital PCB provides a 1.2 MHz sine wave
reference signal that it uses to produce the RF voltage.
The RF Voltage Amplifier PCB produces the RF primary voltage for the RF
voltage coil. To produce the RF primary voltage, the RF Voltage Amplifier
PCB takes the sine wave reference signal and amplifies it by an amount based
on a 0 to 10 V dc RF modulation signal from the integrating amplifier.
The Low Pass Filter PCB removes second and third harmonics from the RF
primary voltage.
The RF voltage coil amplifies the RF primary voltage to produce a secondary
voltage of 0 to 10,000 V ac (peak to peak) that is supplied to the rods of the
mass analyzer.
The RF voltage detector senses the 0 to 10,000 V RF voltage signal applied to
the rods of the mass analyzer and converts this sensed signal into a 0 to
-10 V dc output signal.
The integrating amplifier (also called an error amplifier) produces the 0 to
10 V dc RF modulation signal that is used by the RF Voltage Amplifier PCB.
The magnitude of the RF modulation signal is proportional to the difference
between the detected RF signal and the mass set signal requested by the Mass
DAC (digital-to-analog converter). The integrating amplifier adjusts the RF
modulation signal until the detected RF signal equals the requested RF signal.
The Waveform DDS (direct digital synthesizer), which is part of the Digital
PCB, provides the reference waveforms that are used to create the ion
isolation waveform voltage, resonance excitation RF voltage, and resonance
ejection RF voltage. The reference waveforms are amplified by a waveform
amplifier on the Analog PCB. The amplified waveforms are then passed
through the low pass filter, and they are finally combined at the RF voltage
coil with the main RF for the exit rods.
The embedded computer, which is also part of the Digital PCB, is where the
computations that are required to produce the waveforms take place.
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