WO2010136828A2

WO2010136828A2 - A method for the control of phase locked rf generators in the operation of mass spectrometers

Info

Publication number: WO2010136828A2
Application number: PCT/GB2010/050928
Authority: WO
Inventors: Kenneth Worthington
Original assignee: Micromass Uk Limited
Priority date: 2009-05-29
Filing date: 2010-06-01
Publication date: 2010-12-02
Also published as: WO2010136828A3; WO2010136825A1; WO2010136827A1; WO2010136826A1; WO2010136824A1

Abstract

A method of producing high frequency high amplitude AC voltages with control of the phase angle between outputs from a plurality of generators having individual DC current controlled variable inductors which method comprises controlling the timing of the circuit to allow inputs at the same frequency to have their phase angle preserved. All the outputs are phase locked and synchronised to a common external timed signal and programmed to the same output frequency.

Description

A METHOD FOR THE CONTROL OF PHASE LOCKED RF GENERATORS IN THE OPERATION OF MASS SPECTROMETERS

This invention relates to a method of producing high frequency high amplitude AC voltages from a plurality of generators having individual DC current controlled variable inductors.

It is known that resonant circuits allow for high frequency, high amplitude AC voltages, or RF voltages to be generated without excessive power consumption.

Variable inductors using magnetic saturation are used to achieve high voltage resonance at a particular frequency as disclosed in US 5 684 678 and 5 737 203 to Barrett. Electronics control of frequency and phase between a low voltage input signal and a related output signal is known from US 3 046 490 to Marshall et al . Variable inductors, where a saturation field is applied via a single DC control winding, have been applied to power supply circuits as also is known from the Barrett disclosures.

As a general proposition an untuned load causes excessive power dissipation. Manually tuned circuitry may drift, requiring either excessive power or manual intervention. Changing the phase or amplitude on physically adjacent load elements results in the effective load capacitance changing. This in turn causes the resonant frequency to change .

Oscillators which use the resonance between a fixed inductance and the load to determine their frequency will not have controllable phase. However, multiple high voltage AC, or RF generators with individual DC current controlled variable inductors allow the resonance of those circuits to be automatically tuned to the same frequency. By feeding more than one of these circuits with the same frequency but at different phases, will cause their outputs to have a controllable phase shift between them.

In mass spectrometry recently developed methods of the analysis of ions requires three or more RF generators that are phase locked in all modes of operation of the mass spectrometer. In particular, for one method of separation provided by a mass spectrometer called ScanWave, there is a requirement for variable phase and amplitude but the phases are still locked. It is therefore desirable to provide a method for producing high frequency, high amplitude AC voltages from a plurality of generators having individual DC current controlled variable inductors whilst the phase of the RF generated is accurately locked.

One aspect of the present invention provides a method of producing high frequency, high amplitude AC voltages with control of the phase angle between outputs from a plurality of generators having individual DC current controlled variable inductors which method comprises controlling the timing of the circuit to allow inputs at the same frequency to have their phase angle preserved

According to a feature of this aspect of the invention, all the outputs from the plurality of generators may be phase locked and synchronised to a common external timed signal and programmed to the same output frequency. Preferably, the phase of each individual generator is programmed independently.

According to another feature of this aspect of the invention a direct digital synthesiser (DDS) may provide an AC reference signal to each generator which reference signal is programmable in both frequency and phase and locked to said common external timed signal and synchronisation signals shared by the other generators. Preferably, the DDS output is amplified and applied to the output transformer primary.

According to a further feature of this aspect of the invention, the output circuit may be matched to the capacitive load of an ion guide and its inductance may be controlled by applying a DC saturating field to the timing inductor in order to achieve resonance. Preferably, the phase difference between the reference signal and output may be compared and a timing error signal may be produced, amplified and applied to the DC control winding thereby forming a closed loop automated phase control system.

Another aspect of the invention provides a mass spectrometer incorporating ion guiding means controlled by a method according to any one or more of the four immediately preceding paragraphs.

Yet another aspect of the present invention provides a multiplicity of high voltage AC or RF generators with individual DC current controlled variable inductors to allow the resonance of those circuits to be automatically tuned to the same frequency by feeding more than one of the circuits with the same frequency but at different phases so that their outputs will have a controllable phase shift between them.

A still further aspect of the present invention provides a mass spectrometer incorporating ion guiding means powered by a multiplicity of generators according to the immediately preceding paragraph.

Additional and variable phase delays, due to the power amplifier are compensated for by including the amplifier within the control loop.

If two adjacent cells are set at widely different amplitudes, coupling between the adjacent end plates may cause the lower of the two to be higher than the requested value. This is because, even though the amplifier output is zero, coupling alone from the higher amplitude output is sufficient to exceed the set value. It is possible to cancel some of this coupling by adding a compensating capacitor between the plate and AC phase opposite to that of the adjacent plate.

The variable inductor DC control winding is shielded against residual flux, that has not been removed by magnetic cancellation. The shield can be a conductive metal foil, close to, or surrounding the control winding.

An embodiment of the present invention will now be described with referecnce to the accompanying drawings, in which

Figure 1 is a circuit diagram of a frequency control system according to the invention; Figure Ia is a RF generator block diagram;

Figure 2 is a diagram of a tuning inductor winding according to the invention;

Figure 3 is a diagram illustrating the positioning of a compensating capacitor according to the invention,

Figure 4 is an example of the potential use for the invention to power an ion guiding means of a mass spectrometer, and

Figure 4a is a block diagram showing three generators with individual DC current controlled variable inductors powering an ion guide of a mass spectrometer.

Referring to the drawings, the frequency control system of the present invention comprises two or more high voltage AC (or RF) generators 10, 12 and 14 respectively, each of which resonates with its output load. All the outputs are phase locked and synchonised to a common external clock signal from a quartz clock 16, and programmed to the same output output frequency. Howevever, the phase of each individual generator output may be programmed independently. The operation of each of the generators is identical, as described below.

Referring more specifically to Figure 1 and Ia, a DDS 18 (Direct Digital Synthesiser) , is used to provide an AC reference signal. This is programmable in both frequency and phase, and locked to the common clock and synchronsisation signals shared by the other generators.

The DDS output is amplified by RF amplifier 20 and applied to the output transformer primary 22. Since the output circuit must be matched to the capacitive load of the ion guide cell 24, its inductance must be controlled by applying a DC saturating field from a DC amplifier 26 to the tuning inductor 28, in order to achieve resonance.

At resonance, the frequency reference and output voltage are in phase and by comparing the phase difference by a phase detector 30 between the reference signal and output, a tuning error signal is produced. This is amplified by DC amplifier 26 and applied to the DC control winding, forming a closed loop automated phase control system. Since the power amplifier 20 is included within this loop, any phase delays due to this are effectively removed.

Resonant operation is most important at high amplitudes. When this is the case the signals available at the phase detector input are relatively high, and correct tuning is easier to achieve. At low amplitudes, the loop is interrupted and the control current held at the last value. Assuming that the load capacitance does not change, this level should be correct when the output returns again to high amplitude.

Since the DDS frequency reference is independent of the output amplitude, the frequency will remain stable at low amplitudes. However, if two adjacent cells are set at widely different amplitudes, coupling between the adjacent end plates may cause the lower of the two to be higher than the requested value. This is because, even though the amplifier output is zero, coupling alone from the higher amplitude output is sufficient to exceed the set value. It is possible to cancel some of this coupling by adding a compensating capacitor to the ion guide (see below) .

By repeating this circuitry and having a common frequency source for each DDS, the frequency and/or phase difference between the output windings can be controlled.

Referring to Figure 2, the tuning inductor 28 is arranged with the DC control winding 32 around the centre limb 34 of the core 36, and the two AC windings 38, 40 around the outer limbs 42 and 44 respectively. Both the AC windings are wound in opposite directions, so that the field from these does not pass through the centre limb 34 and does not induce an AC potential in the DC control winding 32. This is important, as otherwise due to the high turns- ratio, a high voltage would be induced.

The AC windings can be connected in either series or parallel, provided that their magnetic fields are opposite with respect to the DC control winding.

Further protection is obtained by shielding the DC control winding against any residual magnetic flux because shielding the DC control winding prevents residual flux inducing RF in the DC circuit.

Referring now to Figure 3, the Ion Guide 24 is divided into sections; section 1 and section 2. There is a stray capacitance between electrode plates at the boundary between sections. A change in amplitude or phase in one section, causes a disturbance to the adjacent sections, due to this capacitance. Where both sections are fed from balanced AC outputs, this disturbance effect can be minimised by including a compensating capacitor 46, equal in value to the stray capacitance. The compensating capacitor 46 is connected between the end plate of one section and the opposite AC phase is connected to the adjacent section end plate.

In the particular example shown in Figure 3, in each section, adjacent plates are fed from opposing AC phases. Therefore, the compensating capacitor is connected between the plate 50 adjacent to the end of section 1 and the end plate 48 of section 2.

Figures 4 and 4a give an example of the potential use for the generator circuits and method according to the present invention. In this example, the ion guide cell 24 may be powered by three generators connected in circuitly as described above.

Thus, the Direct Digial Synthesiser (DDS) 18 provides a variable phase reference signal. The non-linear characteristic of the ferrite material core 36 of the tuning inductor 28 creates a current controlled variable inductor. This resides in the phase control loop which compares the output with the reference signal phase and tunes the RF power circuit. Such closed loop phase control ensures repeatable phase at the Ion Guide 24 because the tuning (variable) inductor 28 automatically tunes to the Ion guide load capacitance.

Moreover, the inclusion of the RF amplifier 20 within the control loop improves accuracy and the compensating capacitor 46 between ion guide sections, described with reference to Figure 3, improves dynamic performance.

Claims

1 A method of producing high frequency high amplitude AC voltages with control of the phase angle between outputs from a plurality of generators having individual DC current controlled variable inductors which method comprises controlling the timing of the circuit to allow inputs at the same frequency to have their phase angle preserved.

2 A method according to claim 1 wherein all the outputs are phase locked and synchronised to a common external timed signal and programmed to the same output frequency.

3 A method according to claim 2 wherein the phase of each individual generator is programmed independently.

4 A method according to claim 2 wherein a direct digital synthesiser (DDS) provides an AC reference signal to each generator which reference signal is programmable in both frequency and phase and locked to said common clock and synchronisation signals shared by the other generators .

5 A method according to claim 4 wherein the DDS output is amplified and applied to the output transformer primary.

6 A method according to claim 5 wherein the output circuit is matched to the capacitive load of an ion guide and its inductance is controlled by applying a DC saturating field to the timing inductor in order to achieve resonance.

7 A method according to claim 6 wherein the phase difference between the reference signal and output is compared and a timing error signal is produced, amplified and applied to the DC control winding thereby forming a closed loop automated phase control system.

8 A mass spectrometer incorporating ion guiding means controlled by a method according to claim 1.

9 A multiplicity of high voltage AC or RF generators with individual DC current controlled variable inductors to allow the resonance of those circuits to be automatically tuned to the same frequency by feeding more than one of these circuits with the same frequency but at different phases so that their outputs will have a controllable phase shift between them.

10 A mass spectrometer incorporating ion guiding means powered by a multiplicity of generators according to claim 9.