What is "high speed"? (e.g. When do I need a ground plane) - Page 1
Forum section: electronics / projects

author: staze
author_rank: Frequent Contributor
timestamp: November 17, 2014, 08:30:32 pm
content: All,After trying, and largely failing at home etching a PCB (my clearances were apparently too small), I've decided I'm just going to build the damn thing on perf board if I can. My question is, will I have any problem doing a perf board build involving a 10Mhz OCXO? I can't seem to find any good info on when you start wanting/needing a ground plane for high speed design, but I would have to think it would be at least 50Mhz if not 100Mhz. Any thoughts? Obviously I realize a ground plane is always a good thing if you can do it. But at this point, I just want to get the damn thing built. =PThat said, the OCXO does output a CMOS/Square wave, so obviously harmonics are going to put it above 50Mhz pretty easily.Thanks!

author: Yansi
author_rank: Super Contributor
timestamp: November 17, 2014, 08:59:43 pm
content: 10MHz is relatively slow. Correctly designed circuit (eg grounding, decoupling, signal paths routing...) surely will work fine on perfboard even breadboard. 10Mhz is not really a concern, unless very bad design practiques happen. You can also slow the signal edges by adding a small (20-100 ohm) resistor in the path. It filters the highest harmonics and decreases possible crosstalk between signal paths.//Good luck making diy boards! Don't give up.

author: nfmax
author_rank: Super Contributor
timestamp: November 17, 2014, 09:40:39 pm
content: There are two, completely different reasons to need a ground plane. The first is applicable at high frequencies. A ground plane allows the return current for a signal trace to take a path a close as possible to the trace itself. This minimises the area of the loop formed by the current path, and hence the inductance. The second reason is to minimise the capacitance between tracks, at the expense of increased capacitance to ground. This can be important at very low frequencies (e.g. 50Hz) if signal levels are low and impedances high.In your application, provided there is a ground track closely parallel to the signal output trace, you probably don't need a ground plane at 10MHz, but if you have a voltage controlled trimming circuit (i.e. using a varicap diode to adjust the frequency) you might need to use one to minimise phase noise caused by coupling to this node.

author: tszaboo
author_rank: Super Contributor
timestamp: November 17, 2014, 09:54:36 pm
content: You will not find any useful information to just speed alone, because ground plane isnt just the matter of speed. I can show you circuits which work on 1Khz and they need a ground plane, and others at 100Mzh are just fine without one.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 17, 2014, 11:25:55 pm
content: Quote from: NANDBlog on November 17, 2014, 09:54:36 pma) Tunnel diode pulser with 25ps rise timeb) FM radio tunerWhat do I win?

author: staze
author_rank: Frequent Contributor
timestamp: November 18, 2014, 12:38:46 am
content: All good points. There's also things like automotive grounding, where the ground plane is the car chassis... and as anyone that has done automotive work on older Mazda's (and others) can tell you, good, solid, large ground paths are important.Thanks! I'm just going to try building this thing and see if it works. It worked relatively well breadboarded, I just wanted a nice PCB for the final product, but it's been way too long, and I want it off my bench and onto the shelf. =)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 18, 2014, 10:18:10 am
content: Perfboard is pretty meager stuff, but you can still try to help the signals: if it's the pad-per-hole kind, you can solder bare wire along pads as traces, and put ground traces either side of the signal trace. This makes a crude substitute for a "coplanar waveguide", which is a transmission line structure with useful impedance (in this case, probably around 100 ohms). Make sure to ground the ground traces at both ends, of course.Just routing things with loose wires (the way perfboard is generally used), you get transmission lines through space with wild impedances (100-300 ohms?) and high coupling between them. The high impedance means these wires act like inductors (or even antennas -- if the switching edges are fast enough for the electrical length of the wires), which more readily puts bounce and crosstalk into your signals.If you're literally just plopping down the OCXO to connect it to a transmission line, you won't have any other signals, and you'll have just one transmission line on a connector leaving the board. Keep that distance short and it won't matter. You may still want some resistance between squarewave output and connector, to help source terminate it (otherwise you have the hazard of reflections in the attached cable).Tim

author: tszaboo
author_rank: Super Contributor
timestamp: November 18, 2014, 11:21:55 am
content: Quote from: Alex Eisenhut on November 17, 2014, 11:25:55 pmWell, I was going for 6.5 digit DMM and single chip RF garage door opener.

author: DJohn
author_rank: Regular Contributor
timestamp: November 18, 2014, 12:37:50 pm
content: Quote from: Alex Eisenhut on November 17, 2014, 11:25:55 pma) has significant components in the tens of GHz. b) does not.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 18, 2014, 04:20:14 pm
content: Quote from: DJohn on November 18, 2014, 12:37:50 pmPrecisely the point.