Connectorized feed-though filters? - Page 1
Forum section: electronics / rf-microwave

author: Odysseus
author_rank: Regular Contributor
timestamp: January 19, 2019, 06:12:03 am
content: I'm familiar with the use of feed-through caps/filters on RF equipment to eliminate leakage over the incoming DC power, but everything I've seen uses soldered wire terminations. What options are there for feed-though filters which are connectorized? The best I've found is really intended for mains filtering and quite bulky.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: January 19, 2019, 06:18:59 am
content: How about a filtered D-sub? There are several kinds available. The simplest just use a puck of ferrite with holes drilled for each pin. More sophisticated types implement a C or pi filtering network on each pin. The pi network style tends to be very pricey.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2019, 11:02:30 am
content: Hmm, what kind of connectors were you hoping for?Form factor? Attenuation? Impedance, frequency range? Price?The filtered D-sub for example, it's filtered as such, yes -- but the roll-off is very gentle, a few dB to low 10s dB depending on circuit impedance. Maybe that's enough, maybe not.Tim

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: January 19, 2019, 07:20:11 pm
content: Quote from: T3sl4co1l on January 19, 2019, 11:02:30 amSimilar to what you'd expect from one of these, no? (I was assuming these were what the OP meant)The feedthroughs I've used range from 100pF to 10nF, I've seen similar values available for filtered D-subs which I've also used for one project.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2019, 07:35:26 pm
content: No, worse than that, assuming typical conditions (50 ohm source and load). Those ferrite plates likely offer ~100 ohms at 100MHz, or about 6dB, and not much more (mostly less) at any other frequencies. A 1nF cap rolls off at 6.3MHz (-3dB) and a feedthru worth its trouble should perform asymptotically above there (-20dB/dec).But it depends. If there are caps either side of the ferrite bead, the local impedance can be quite low, giving acceptable filtering.Tim

author: cdev
author_rank: Super Contributor
timestamp: January 19, 2019, 10:01:11 pm
content: Fair-rite makes cores that have six small holes through them.Quote from: radar_macgyver on January 19, 2019, 06:18:59 am

author: coppercone2
author_rank: Super Contributor
timestamp: January 19, 2019, 10:08:28 pm
content: well you can make an adapter that goes over it.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: January 19, 2019, 10:12:14 pm
content: Quote from: T3sl4co1l on January 19, 2019, 07:35:26 pmA little confused.This connectorfor example has a 1nF cap, so how would it perform worse than an equivalent feedthrough? Unless you were referring toone of thesewhich has just the ferrite puck.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2019, 11:04:10 pm
content: Yes, the ferrite plate type. The D-sub with caps wouldn't be much worse than a proper (coaxial?) feedthrough (or so I would hope).Sadly, neither one gives an impedance or s12 plot.Tim

author: Odysseus
author_rank: Regular Contributor
timestamp: January 20, 2019, 08:36:57 am
content: Thanks all! Filtered D-subs actually sound like a very good solution. I will have to keep this in mind on my next project. Yes, I'd consider ferrite only filters to be insufficient.I realized that youcoulduse just about any coaxial filter, but that seems like a painful mix-up waiting to happen.If there are any other filter/cap form factors you are aware of, I'd be grateful to know about them.

author: German_EE
author_rank: Super Contributor
timestamp: January 20, 2019, 06:15:30 pm
content: Why not use the wire-ended feedthroughs and add sockets on the wires, something like the female connectors on an RS-232 socket?

author: coppercone2
author_rank: Super Contributor
timestamp: January 20, 2019, 06:40:48 pm
content: for dsub there is a x2y capacitor bank you can use IIRC.