Hi speed connections between boards using coax? - Page 1
Forum section: electronics / beginners

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 30, 2015, 01:32:16 am
content: Im curious when sending things like clock signals from one module to another I would usually use a piece of coax and solder the braid to ground along with the signal to whatever component it needs to be soldered too. Here is an example of my style of coax interconnections that are permanenthttp://www.housedillon.com/?page_id=843After seeing this guy doing such an amazing project and terminating coax directly with soldier im a little less worried there is anything wrong with this method. But whenever I watch w2aew's videos or any home brew ham type stuff they always show that not terminating the coax with a proper connector or at least a 50 ohm resistor will cause standing waves and all kinds of issues. One of the reasons I bring this up is becuase I have a fewSI5351boards. I was listening to the soldier smoke podcast and they were talking about how you need to add the SMA connectors to the board and not just solder coax to the breakout. And on another blog a HAM developed a breakout board for these and terminated unused outputs with a 50 ohm resistor on top of using SMA's.Im thoroughly confused, If a TCXO in a shielded box a few inches from the clock distribution board and I am running 50 ohm coax between the two boards can I just directly soldier board to board if everything is permanent, or do I need expensive connectors terminating every end of any piece of coax? And if that is the case will a 50 ohm resistor at least work, this is much cheaper and more compact! I see no reason why connecting coax to a TCXO would need any kind of termination, I would think you would just be attenuating the oscillators signal whether it be a TCXO or something like an Si chip. I started running coax between modules for shielding not to make balanced transmission lines.Im intrested to get an answer to this or an argument on either side since I see people doing it both ways.

author: Vgkid
author_rank: Super Contributor
timestamp: December 30, 2015, 03:48:56 am
content: Very nice. What cable are you using?

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 30, 2015, 04:33:19 am
content: To clarify that wasnt my acuall project, just an example picture i found. I personally use 58 or the small semi rigid stuff shown in that spectrum analyzer. Sometimes rg59/75ohm depending on application.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 30, 2015, 07:18:18 am
content: Quote from: rwgast_lowlevellogicdesin on December 30, 2015, 01:32:16 amIf you are connecting a source & a load/input both of which are 50 Ohm impedance,there is nothing wrong with direct connection of the coax.Manufacturers do this all the time!Even when you don't have matched impedances,if the coax is very short compared to a wavelength at the highest frequency component of your signal,you can get away with it with no problems.Connectors make testing easier,but they don't automatically create a correct impedance match,the termination issue is just as valid in both cases.

author: LukeW
author_rank: Frequent Contributor
timestamp: December 30, 2015, 08:56:22 am
content: There is going to be a little bit of an impedance mismatch at the point where the braid separates from the coax and the centre conductor is connected to the board.How critical is it? It depends a bit on your application, what frequency, what impedance mismatch/VSWR you can tolerate etc.When you said coax soldered to a board, I was initially expecting to see something like this:http://homepage.ntlworld.com/rg4wpw/filter.jpgBut what you've got, with hand-formable semirigid coax actually through the board and soldered all the way around, is much nicer than that, it's neat and will behave well at higher frequencies. It kind of depends where the centre conductor is connected, though - it's not shown connected to anything in your photo.

author: German_EE
author_rank: Super Contributor
timestamp: December 30, 2015, 09:26:17 am
content: There is nothing wrong with connecting boards together directly, it just reduces serviceability. The secret is to use good PTFE insulated coax that doesn't melt when soldered.

author: tggzzz
author_rank: Super Contributor
timestamp: December 30, 2015, 09:33:06 am
content: Quote from: rwgast_lowlevellogicdesin on December 30, 2015, 01:32:16 amAre those illustrations your work, because the person doing them clearly knows about this topic and takes great care to minimise problems - and is building a spectrum analyser!If you don't have impedance matched, cable and terminations then you will get reflections. Whether the reflections are important will depend on the transmitter, receiver and operating frequencies.All you need at the receiver is a 50ohm resistor to match the 50ohm cable. The only problem is you can't buy a 50ohm resistor: nobody manufactures them.Youcanbuyapproximationsto resistors, which include parasitic capacitance and inductance. The amount thereof will depend on the resistor construction; you have to look at the manufacturer's datasheet. In general, physically smaller resistors have smaller parasitics.So to answer your question... Calculate the impedance of the parasitics at your frequencies, and see if they change the overall impedance significantly. From the impedance mismatch, calculate the magnitude of the reflection.Have a look at any ham radio source, or, for digital circuits,http://www.edn.com/collections/4435129/Bogatin-s-Rules-of-Thumb

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 30, 2015, 12:36:43 pm
content: Quote from: vk6zgo on December 30, 2015, 07:18:18 amIf I may modify your mental note:The rule of thumb " < 1/20 wavelength", or somewhere around that, assumes everything is at system impedance.If the circuit impedance is different from the characteristic impedance of the cable, then the restriction is proportionally shorter! (To be precise, for a ratio Z/Zo = x, the critical length is (x + 1/x) / 2 times shorter.)Note also that the wavelength is not what you think it is, but what the circuit thinks it is -- this allows for RF transistors oscillating at interesting frequencies.And of course, for wideband signals, the signal bandwidth includes the risetime, not the fundamental!This is why, for example, switching supplies are so critical of layout, because only a few cm is necessary to upset the switching edges, even though 1cm is 1/300th of 100MHz (a typical bandwidth for switching circuits). The impedance is almost always very low, making equivalent inductance a big deal!I need to condense this into a more memorable phrase, so the crude rule of thumb can begin to be replaced by something more accurate.Tim

author: lincoln
author_rank: Regular Contributor
timestamp: December 30, 2015, 06:24:20 pm
content: As others have said, no problems soldering coax directly, until it come time to make a change. The one nugget I will add is.....single ended clocks suck.That right, I said it. I say it for a very narrow use case, which happens to be where I live, LVDS or any other kind of balanced signal is much preferable.1. clock has to be digital. If you have a sine wave frequency source sure, you could use baluns to balance the signal but really, in the parlance of today's youth "Ain't nobody got time for that".2. It has to be a clock. Or rather there is not really any other modulation, ie IF stage in a radio, or reasons shown above.3. I have to distribute the clock in the same board or chassy some distance or near some thing nasty. If the clock has to leave the chassy it is going to leave on a SMA, BNC or N largely because thats what every one else dose. Now if the two parts are eclectically and physically close to each other, sure run a single ended trace and terminate the line. But if I have to go for more than a few cm or jump form one module to the next i will do what ever I can to use LVDS,Now the Si chip supports it so you have a source, to convert it back I have used things like SN65LVDS2 to convert it back to single ended a few mm away form the driven element.Anyway, it may be overkill for your project.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 01, 2016, 01:23:29 am
content: Suck .... for what?Engineering is always about purpose and suitability. I shake my head to no end with all the unqualified rules of thumb that float around!You wouldn't go out of your way for anything touching a microcontroller, they have RC clocks inside them for pete's sake. Ditto most digital logic things; and communications are almost all self-clocking, or PLL'd, so any interference only affects your floor SNR/BER.Where you would do that is on critical channels (with hard SNR/BER requirements), sensitive circuitry (e.g., spectrum analyzer bits, where the noise floor affects sidebands and dynamic range), and other sorts of precision timing, mostly special purpose (since most general purpose ones are self-synchronizing, like I said).Also, there's absolutely nothing that differential gains you, that you don't already have with coax -- indeed it may be worse, because having a pair out in the open (ha, nice mental image) potentially allows in more unbalanced noise than fully shielded coax (though common UTP may well be better than leaky types of coax, even in short lengths where the twist doesn't average out so well). Voltage always is a difference, so it's silly to reiterate "differential": it's a design and construction methodology, not an additional concept! You can use coax in a differential manner just as well; indeed, failing to do so yields the poor results that cause engineers to scream "this needs to be a differential pair"!Tim

author: lincoln
author_rank: Regular Contributor
timestamp: January 03, 2016, 09:09:19 pm
content: Quote from: T3sl4co1l on January 01, 2016, 01:23:29 amQuoteTo provide frequency references, most common thing we touch would be LTE-TDD. A board with have several gig or higher Ethernet ports and related sync E clocks, its own local oscillator which will be trained to ether a syncE, PTP, BITS or other clock source. The local oscillator will be used to drive one or more output clocks to ether be multiplied up to drive radios or some other customer purpose. We tend to measure clock stability in ppb or ppt.Great thing about advice is it can also be ignored if not fit for purpose.

author: langwadt
author_rank: Super Contributor
timestamp: January 03, 2016, 10:04:32 pm
content: Quote from: LukeW on December 30, 2015, 08:56:22 amI prefer it flat on the board similar to this:http://www.w6pql.com/hf/coax-done.jpgno pigtails, no vias and it is easy to go straight into a 50R strip line

author: tggzzz
author_rank: Super Contributor
timestamp: January 03, 2016, 10:15:40 pm
content: Quote from: T3sl4co1l on January 01, 2016, 01:23:29 amJust so.Mind you, overly dogmatic and general statements are a good early warning that the writer is juvenile, unaware of the rich range of possibilities, and is just repeating things without deep understanding.

author: natbuk
author_rank: Contributor
timestamp: January 03, 2016, 11:46:14 pm
content: Hmm.The trouble with these direct-soldered methods is that you may easily (inadvertently) be changing the impedance at the interface. Making PCBs constant impedance requires detailed knowledge of PCB dielectric and track spacing. This can all be worked out. but doing it in 3D is harder. The people that make the connectors have done this in 3D to make properly-matched impedances, taking into account the 3 dimensional aspects and the dielectrics, cable terminations etc. If you just come along and solder the cable down, you havn't really done the homework to know it's all right. In many applications, it probably doesn't matter, but taking care of all the details, at least to me, looks like much harder work than necessary, given the low cost of the connectors available, especially when bought from ebay and shipped from china,If you need really low size, I've used those tiny connectors used on 2.4/5GHz network cards - they'r pretty cheap - you have to buy the tails/leads already made up and the connectors are only specified to be good for a few dozen mating cycles, buy they are impedance (largely) and good to a few GHz, and tiny...Farnell (and others) can supply both the SMD connectors and the leads.I have often used them as 'rf test points' for things like clock jitter measurments, where they take up hardly any space, and can be left-off the production units...Best regards,Nat

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2016, 04:41:41 am
content: Quote from: natbuk on January 03, 2016, 11:46:14 pmNot necessarily.Notoriously, BNC is quite awful. Yeah, it can be used into the GHz, but not for clean pulses with << 1dB error. More like RF where a maximum SWR of 1.3 is acceptable.AFAIK, the SMx (x = A, B, C) connectors are pretty good into the 10GHz range, but that's partly due simply to being smaller. Which is a necessity anyway: the practical upper cutoff frequency of a coax structure is when 1/2 wave fits inside the shield, where TE/TM modes start popping up and you get different propagation modes, velocities, couplings and so on. It's also necessary to avoid TEM modes between discontinuities (e.g., step changes in diameter, even if the coax diameter *ratios*, and therefore average impedance, do remain constant). For which, short connector lengths are desirable as well.Ultimately, it's all about dimension. Smaller is faster. If you can poke a 50 ohm coax through a shield, and "instantly" transition it into a 50 ohm microstrip (i.e., within a mm or two -- basically, the width of the trace itself!), then youwill do better than most connectors, period, even if the approximate dimensions within the transition aren't very close to 50 ohms.PCB layout example: changing the angle of a microstrip trace. You avoid sharp (square) 90 degree corners, because most of the energy 'short-circuits' around the inside corner, making the outside corner looks like dead weight -- added capacitance, or more accurately, a lower impedance. The duration of that reduced impedance is less than a trace width's, though, so it has very little effect until frequencies comparable to the trace width itself (in which case, again, you're getting quite close to non-TEM modes of the transmission line, and quirky behavior!).So you can lop off most of the corner (there's actually an ideal chamfer for this situation), but it's only important to do so in critically high frequency applications, or more likely, high precision applications (where the impedance change is small, but that small change is very important).For everyone else, there are 45 degree bends, or arc segments.Tim