High power dc pulsed coaxial cable split - Page 1
Forum section: electronics / rf-microwave

author: LoneWolf6912
author_rank: Contributor
timestamp: July 09, 2025, 09:05:47 am
content: Hi everyone,I’m working on a project where I need to feed four plasma cathodes from a single source using a Type-N coaxial cable. My initial approach was to simply solder four coaxial cables directly onto one, creating a kind of passive 1-to-4 split. However, during testing I observed some unwanted resonances and coupling effects between the outputs.I'm now looking for more robust and electrically sound solutions to split the RF signal cleanly into four outputs and assuring good isolation between them. Ideally, I'd like something that minimizes cross-talk or reflection issues.Has anyone dealt with a similar situation or can recommend a suitable splitter design or method? Commercial 4-way power dividers? Custom RF circuitry? Impedance matching tips?In my application (sputtering deposition) each plasma are about 300 -350 W at 200kHz.I seen most commercial device are more about 4 input and one sum port. But i need exactly the inverse.At time, i'm a little bit lost as the application is kind of... special...

author: Marco
author_rank: Super Contributor
timestamp: July 09, 2025, 12:38:47 pm
content: That's not RF unless you are using km of cable or km wide chambers.The cross talk is because the impedance of the source is high relative to the impedance of the loads, so either poor high impedance source or poor low impedance loads (arcing).

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 10, 2025, 08:31:53 pm
content: Quote from: LoneWolf6912 on July 09, 2025, 09:05:47 amYou need to describe more clearly what you mean by "resonance" and "coupling" effects.What are the impedances of the source and load(s)?QuoteThe carrier frequency is 200kHz, or the pulse rate is 200kHz?If the carrier frequency was just 200kHz, then I don't think the cables are the reason for the "resonance" and "coupling" you observed...QuotePassive combiners/splitters are generally reciprocal devices, i.e. any combiner will also work as a splitter, and vice versa.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: July 10, 2025, 08:50:16 pm
content: How long are the cables. What is the risetime on the pulse. There will be reflections from impedance mismatch which could cause the resonance. The one cable to 4 is going from 50 ohms to 12.5 ohms.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: July 10, 2025, 08:51:37 pm
content: Are the four cables the same length? Are the four plasma loads the same. Plasma probably is non linear.

author: TimFox
author_rank: Super Contributor
timestamp: July 10, 2025, 10:37:45 pm
content: Guessing about what you are doing, a plasma load can exhibit electronic negative resistance, so it could form an oscillator with a resonant coaxial cable.There is literature about the "Poulsen arc", improved into the "Federal Arc" used as a CW transmitter before the invention of the vacuum tube.An arc discharge is inside a strong magnetic field to give a stable negative resistance in its I-V curve.To use as a Morse transmitter, the arc cannot be turned on and off quickly enough, so frequency-shift keying was used by switching a capacitor into the resonant tank circuit with a keying relay.(Lawrence's cyclotron used an armature from a Federal Arc, found in a scrap yard, to apply the magnetic field for his accelerator.)

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2025, 04:00:15 pm
content: The usual solution that I see is to use separate output amplifiers.I think there is a hybrid transformer configuration which could be used to direct reflections from the load into a termination.What I might try first is to add LC filters immediately after the splitter to improve the impedance match and reflect harmonics back to the load.

author: LoneWolf6912
author_rank: Contributor
timestamp: July 14, 2025, 12:15:58 pm
content: Hello,Thx for all your answer. Attached i joined the current in each plasma cathode. As you can see we always have one cathode who is totally oscillating. Of course it greatly impact our deposition rate.For information the pluse rate is 200kHz. The power supply is a 10kW Pulsed DC Plasma Generator RPDG-100 form MKS.The cable is about 3 meter lenght. We split after this cable in four cable of 50 cm each.

author: David Hess
author_rank: Super Contributor
timestamp: July 14, 2025, 01:39:06 pm
content: Does each plasma cathode have a ballast? If they are all connected in parallel, then without a ballast I would expect small differences in strike and operating voltages to cause unequal current distribution.Adding a resistive ballast to each one would also lower the Q of the transmission line and lessen or remove the effects of negative resistance.

author: LoneWolf6912
author_rank: Contributor
timestamp: July 14, 2025, 02:38:37 pm
content: It's not really a normal plasma. It's sputtering cathode. Currently a Kurt J lesker Torus magnetron plasma cathode.https://www.lesker.com/newweb/deposition_sources/torus_sputtercathodes_circularmagkeeper.cfm

author: coppercone2
author_rank: Super Contributor
timestamp: July 14, 2025, 07:24:38 pm
content: I always suspected that getting these things to run right would be more challenging then it looks. It's nice to know what problems you might run into if you try to implement one of these processes.

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 15, 2025, 12:16:38 pm
content: Odd that the oscillations is mostly on one output, as opposed to being distributed between multiple outputs.Without some sort of electrical model, it's hard to give any specific advice...Having a proper splitter with good isolation between its ports would probably help. One can make low frequency broadband power splitters using simple transformers and resistors.See figures 3 and 4 in this app note.. Though for your working voltage/power levels, the design of such a splitter might be quite challenging.Another option would be to add some sort of snubber circuit to the load(s). Like a series R+C circuit in parallel with each load. So long as the oscillation frequency is far from the intended operating frequency, such a circuit shouldn't degrade operating efficiency by much. But would need some sort of model to suggest component values for the snubber...Does the driver have some sort of defined output impedance?

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2025, 03:54:08 pm
content: Quote from: LoneWolf6912 on July 14, 2025, 02:38:37 pmI would at least try adding resistive ballasting at the sputtering end of each transmission line. There could be a couple of things going on that this would help with.If the sputtering load has a negative resistance characteristic, the added resistance will reduce the negative resistance reducing the gain for purposes of oscillation. The series resistance will also disconnect the sputtering load from the charge line formed from the transmission line, and the resistance will increase isolation between the lines.The chosen resistance will depend on the voltages and currents involved, and what power dissipation in the resistance can be tolerated, so we are not talking about a lot of resistance or power dissipation, and the voltage drop will be inconsequential.Power inductors could also be used without any power loss. This is how the arc is stabilized in DC welding. But I would start with resistors.