What's this symbol for? - Page 1
Forum section: electronics / projects

author: gmc
author_rank: Regular Contributor
timestamp: December 13, 2020, 09:16:04 pm
content: I'm busy trying to repair a old retro Macintosh computer from the 80'sOn the schematic there is a crystal oscillator (tin can) connected to the CPU which I've traced out and is all connected fine.However on the schematic, between the CPU and crystal (between pin 3 and 51) there is a symbol which I've never been before, and looks like a cap with a line through going to ground. However this component does not have a reference and is nowhere to be found on the board. There are 3 oscillators and they all have this symbol.Curious as to what this is for? If there is no component for this, is it something that was just left out of the final PCB design?

author: bob91343
author_rank: Super Contributor
timestamp: December 13, 2020, 09:19:22 pm
content: That is a symbol for shielded or coaxial wire.

author: TimFox
author_rank: Super Contributor
timestamp: December 13, 2020, 09:35:40 pm
content: Usually, a coaxial cable is shown as a circle or ellipse around the wire. This symbol is too close to that of a feed-through capacitor, and is confusing. There are SMT "feedthrough" capacitors that have the through-conductor from end to end along the long dimension, and ground pads on each of the long edges that have similar function to a conventional feed-through capacitor, where the ground outer thread or case is coaxial with the center through-conductor. However, it would be strange to put a capacitor across the output of the oscillator module.

author: Benta
author_rank: Super Contributor
timestamp: December 13, 2020, 09:45:49 pm
content: My guess is that it's a hint for the PCB layout artist to treat this trace as a stripline.

author: TimFox
author_rank: Super Contributor
timestamp: December 13, 2020, 10:48:08 pm
content: Or maybe to bury it between two ground layers.

author: Benta
author_rank: Super Contributor
timestamp: December 13, 2020, 10:52:36 pm
content: Quote from: TimFox on December 13, 2020, 10:48:08 pmThat's what a stripline is.https://en.wikipedia.org/wiki/Stripline

author: TimFox
author_rank: Super Contributor
timestamp: December 13, 2020, 10:56:33 pm
content: Yes, technically "microstrip" has ground on only one side.

author: gmc
author_rank: Regular Contributor
timestamp: December 14, 2020, 09:26:45 am
content: Quote from: TimFox on December 13, 2020, 10:48:08 pmI think this may be it. Its at least a 3 layer (or more) board and one of the visible layers is ground. Maybe one of the inner layers is also ground.

author: veryevil
author_rank: Unknown
timestamp: December 14, 2020, 10:48:41 pm
content: Could it be a 3-Terminal Feedthrough Capacitor?

author: phil from seattle
author_rank: Super Contributor
timestamp: December 15, 2020, 01:44:25 am
content: It does look like a grounded shield. But that doesn't exactly make sense because you want the crystal to be very close to the input pin. My other thought was a guard ring - common around xtals in some designs. Though, I would think that the engineer would just make a note to the PCB designer rather than a funky symbol.

author: mc172
author_rank: Frequent Contributor
timestamp: December 15, 2020, 02:07:41 am
content: I also think feedthrough capacitor.

author: tonyh88
author_rank: Regular Contributor
timestamp: December 15, 2020, 02:56:37 am
content: Ground guard ring around oscillator/crystal traces?

author: Bud
author_rank: Super Contributor
timestamp: December 15, 2020, 03:11:23 am
content: Feedthru capacitor.

author: james_s
author_rank: Super Contributor
timestamp: December 15, 2020, 03:31:13 am
content: This could easily be confirmed by a picture of the PCB. I suspect it's nothing more than a guard trace.

author: sam[PS]
author_rank: Contributor
timestamp: December 20, 2020, 04:51:39 pm
content: I doubt it's any sort of capacitor as it would have a value associated as well as a part number, there is none.I would also bet for some sort of ground shielding/guard. It state for 57,xxx Mhz so a real impedance guaranteed micro-strip should not be needed in that range but surely you want it somehow shielded if you have any analog part in your product.Also it looks like a scan of an old print so that schematics is probably old, you should not attach too much importance to the conformity to standards on this kind of document. Look at the quartz oscillator symbole, the frequency is stated in MCS (mega cycle per second) and not in MHz as it should if standard where of any concern.just my 2cts...

author: mikerj
author_rank: Super Contributor
timestamp: December 20, 2020, 11:12:37 pm
content: Quote from: sam on December 20, 2020, 04:51:39 pm100% this, it doesn't even have a reference designator so clearly isn't a component. I'd go with Benta/Tims idea that it's to tell the layout guys to keep this trace well screened by routing it under ground planes. Not necessarily "stripline" since that implies an impedance controlled interconnect.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 20, 2020, 11:20:20 pm
content: Feedthru capacitor.

author: mc172
author_rank: Frequent Contributor
timestamp: December 21, 2020, 02:45:41 am
content: Quote from: mikerj on December 20, 2020, 11:12:37 pmI've designed products before that used threaded feedthrough capacitors that ended up as part of the mechanical BOM, without a designator or value assigned on the schematic.

author: free_electron
author_rank: Super Contributor
timestamp: December 21, 2020, 03:42:36 am
content: guard ground on the board. it has no designator so it is not a component.

author: TimFox
author_rank: Super Contributor
timestamp: December 21, 2020, 04:01:01 am
content: I don't see any reason why a feedthrough capacitor would be connected at that node. Normally, you see them on power or DC bias.

author: m3vuv
author_rank: Super Contributor
timestamp: December 21, 2020, 09:05:31 am
content: isnt it a feedthru cap?

author: TimFox
author_rank: Super Contributor
timestamp: December 21, 2020, 02:39:27 pm
content: The symbol suggests a ft cap, but there is no capacitance value shown (1 nF is typical), and it would be a strange place for one, since it is a high frequency signal, not DC.

author: mikerj
author_rank: Super Contributor
timestamp: December 21, 2020, 11:28:43 pm
content: Quote from: mc172 on December 21, 2020, 02:45:41 amWhy would you hang a capacitor off a clock line?

author: cdev
author_rank: Super Contributor
timestamp: December 22, 2020, 06:55:37 pm
content: What model/kind of Mac is it, and could you include more of it? It says "G3" is it a G3 Power mac?If you could upload a larger shot that includes more area and also a little higher resolution that would be helpful.There seems to be no logic to why it would be placed like that if it was a feedthrough cap.So I think we need more info.