System detuning after strong RF pulse. Component hysteresis? - Page 1
Forum section: electronics / rf-microwave

author: Andrew_K
author_rank: Contributor
timestamp: October 03, 2024, 01:59:56 am
content: Hi,I'm working with some high-power RF in a scientific instrument. In this system there are kW of RF energy.I'm seeing some odd intermittent behavior where tuning changes slightly, immediately following an RF pulse. It stays in this slightly different state until the next high-power RF pulse. The issue I'm trying to root-cause is completely intermittent, not able to reproduce reliablyI've tried many different methods of instrumentation, many different combinations of disabling/bypassing subsystems. The effect almost resembles something mechanical, in that, the system stays in this perturbed state with no visible time-constant or decay (nothing observed over minutes), until the next RF pulse. I would be open to the idea of something moving, but I seriously doubt that 100's of MHz could induce physical movement.I'm wondering if there are some "lesser known" non-ideal behaviors, like hysteresis, that could be possible with these types of components. The system uses pin diodes, air-core inductors, ceramic HV capacitors, and other passives. I wouldn't expect hysteresis in any of these components, but odd things happen at these high power levels.Would really appreciate if anyone has ideas, or even just general advice on troubleshooting such intermittent issues.

author: ftg
author_rank: Frequent Contributor
timestamp: October 03, 2024, 08:05:34 am
content: How are the mechanical connections from the air-core inductors, ceramic HV capacitors, and other passives in the system made?Having a kilowatt of RF energy means that there can be quite a bit of heating in spots that have a bad connection and thus losses.And the parts might extend and contract a bit mechanically from the heating.And that then moves them and their connections a little bit.If you can measure the system in a safe, unpowered state, then I'd connect it up to the test equipment used to determine the change of tune and go around gently tapping on things with the plastic handle of a screwdriver and see if there are any particularly problematic places.If the system did not do this previously, it might be fixable just by tightening some screws.

author: Andrew_K
author_rank: Contributor
timestamp: October 03, 2024, 01:48:57 pm
content: Connections between the high-power RF subsystems are made using properly rated cables. Within each subsystem, components are connected on circuit boards. Other subsystems have protection circuits, and are likewise connected by circuit boards and properly rated cables.I like the idea of tapping on things mechanically and is something I will try.Still, it seems odd that an RF pulse could heat things enough when pulses are short (<1mSec). And there's no apparent thermal time constant / decay that is visible.This is my list of potential root-causes...1) Mechanical motion caused by thermal effects - Possible, but hard to prove.2) Mechanical motion caused by high currents/fields - no motion has been observed yet.3) Hysteresis in some component (Caps, Pin Diodes) due to high currents/voltages - Haven't been able to find much.As far as I've been able to find, all diodes/pin diodes have a path to discharge. Still, I'm wondering if a pin-diode isn't exhibiting some odd behavior. Or maybe a capacitor. I know that at extremely high powers/voltages, components stop acting ideal. But maybe I'm focusing too much on that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2024, 11:40:31 pm
content: Hmm, interesting. What components are detuning? In what way, by how much? Perhaps you can run a swept or broadband signal through the system, or between tapping points within the system, to observe the tuning effect more precisely?What is this, plasma system perhaps? Ionization should be long gone after some ms, but perhaps there's a... rectification effect, or bias from somewhere else, in any case spraying charge onto who knows what, and that's detuning something?Presumably you have limiters and switches to the receiver, so you can rule out detuning there. Doesn't hurt to test and confirm this.Tapping on connectors, apparatus, tuning components, etc. isn't a bad idea, just to help rule out mechanical things. Surfaces can get oxidized, especially nickel and silver plated connectors are notorious for generating harmonics (well, Ni moreso than Ag, being that Ag2S is somewhat conductive, but a tarnished, poorly-wiped contact might have CM emissions and other extenuating effects, rectified or not). A loose sparking contact might have spooky intermittent behavior at RF, who knows; should just be a glow discharge or arc under load, but the resulting debris can pack the joint, making it dirty, intermittent, and perhaps rectifying, or other nonlinearity.Anywhere you have enough bandwidth to tap off the waveform plus harmonics, also something to see, as far as detecting distortion (or if this is a multi-tone or modulated pulse, then IMD of that).I suppose you'll need a capable instrument to measure harmonics; scope might not be sensitive enough (low THD almost invisible), but a DSA style (ADC + FFT) spec. an. might be able to acquire a brief enough window, on trigger, to see it.Tim