Quadrupole mass analyzer - Page 1
Forum section: electronics / rf-microwave

author: LukeW
author_rank: Frequent Contributor
timestamp: March 12, 2019, 11:35:05 pm
content: Has anyone ever built a driver for a quadrupole mass filter?Putting this in the RF forum, since it's largely an RF problem.Ideally, this would be a kind of agile system for different research uses, potentially including trapping, ion guides, etc.So there are two outputs that are out of phase, and the three variables that we're really interested in are the frequency, the RF amplitude and the DC bias amplitude applied to the two outputs (one is negative in DC bias, one is positive.)Frequencies of ~1-2 MHz are typical, the RF amplitude is typically hundreds of volts, and the DC bias amplitudes are typically tens or hundreds of volts. Ideally, they should be able to go as high as reasonably achievable (up to the order of a kilovolt?) for a research system.(The frequency is typically fixed at some value, say 1.2 MHz, in commercial instruments, but the ability to tune it would be nice.)There's a nice example of a Class-E 160m amateur CW system here, and this seems like a nice approach with high efficiency, and simplicity, and it's right at about the right frequency ballpark.http://people.physics.anu.edu.au/~dxt103/160m/One disadvantage of the Class-E is that the component values are tuned for resonance at the particular frequency desired - so it might be inefficient to use it in a variable-frequency way. How practical would it be to sweep the frequency a little bit without component changes?I would consider changing the output transformer to a centre-tapped transformer, centre grounded, and have two pi filter networks that feed the two output connectors to the rods. Then there would be a DC blocking capacitor and a RF choke (i.e. a bias tee) where the two DC power supplies are used to add the two DC biases onto the RF outputs.I'm tempted to use DDS for the clock source... if issues like jitter are not excessive.One advantage of the Class-E is the RF output amplitude can be varied (up to the MOSFET limit, which I would try and choose as high as possible) by just changing the power supply voltage.What do you think?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 13, 2019, 01:12:03 am
content: First of all, huge respect, cool project!Since you need two phased RF voltages, I guess you can either split your initial oscillator, phase shift each signal, amplify and offset each signal, or you can split and phase the high power signal right? I'd probably go for the former approach, since it'll mean less high-power components.I'm unsure what class of amplifier would be optimal, but since you're probably not gonna make a handheld quadrupole, is effecientness really that important?`Wouldn't more frequency independence trump that by far?At this stage I would worry less about your frequency generation - if you make the system modular (and I can't see any reason why not), swapping out different oscillators might be very insightful. Is there any studies on the waveforms used for the quadrupole? I'd say go analog and get as pure sine as possible.--Chris

author: David Hess
author_rank: Super Contributor
timestamp: March 13, 2019, 08:17:39 am
content: What is the load impedance? If it is high, then those specifications are not far from the capabilities of an early electrostatic CRT deflection amplifier for an oscilloscope.

author: LukeW
author_rank: Frequent Contributor
timestamp: March 13, 2019, 09:28:47 am
content: The load is really just a couple of rods in a vacuum chamber, plus cable parasitics.Call it say 50pf, predominantly capacitive.

author: David Hess
author_rank: Super Contributor
timestamp: March 13, 2019, 06:05:57 pm
content: Well, there you go then. A high impedance capacitive load is exactly what early oscilloscopes dealt with and they required high voltages to get sufficient deflection so that is where I would start. The vertical amplifiers they used are much faster than you require and the horizontal amplifiers have much higher linearity than you require (they used shunt feedback) but there is plenty of ground between them to make a slower but higher voltage implementation.Probably even more relevant are the high voltage video amplifiers used for z-axis drive in oscilloscopes and intensity in raster scan CRTs because they operated at lower power levels. They amount to a push-pull cascode output stage in a transimpedance configuration (current to voltage) with shunt feedback for both high voltage and high frequency.The broadband linear amplifier configuration would also of course work with the transformer reversed from what is typically done to drive a low impedance transmission line. Or a broadband transformer could be added (in reverse again) to an existing broadband amplifier to boost the output voltage.

author: awallin
author_rank: Frequent Contributor
timestamp: March 13, 2019, 06:11:51 pm
content: Quote from: LukeW on March 13, 2019, 09:28:47 amif you don't need to tune, one approach is a 'helical resonator' i.e. coil-in-a-box which is put in series with the 50pF of the electrode to get a resonance circuit. with a bit of tuning you can match a smaller coil inside the resonator to look like a 50ohm load and drive it with a normal RF-amp.Q-value can be as high as 1000, so you don't need that much RF-power (1-5W or so) to get 1000Vpp on the electrodes.for example this paper has pics of one design:http://www.sussex.ac.uk/physics/iqt/resonatorapb.pdf