High speed signal transmission - Page 1
Forum section: electronics / projects

author: Smilinvamp
author_rank: Newbie
timestamp: July 13, 2012, 04:58:48 pm
content: Hi there,Question for Dave: Do you have a video topic about building signal transmission lines?I have many times worked worked with short run signal lines. Like say 3-10 meters of coax. Most of the time my signal gets all messed up, etc. I do understand why... impedance miss match, capacitive loading, reflection, etc....Although I have dealt with this issue many time, I still don't fully understand what I can do to minimise the problem. Seems that all my designs are build it now and fix it later by trial and error.Are there any tips and tricks to help my square waves stay square?Thanks!-Richard(hope I don't get trolled)

author: jahonen
author_rank: Super Contributor
timestamp: July 13, 2012, 06:44:52 pm
content: Well, there are basically four "big" parameters which determine the signal transmission properties: 1) Impedance 2) Loss 3) Crosstalk 4) Delay.As you are dealing with a coax, crosstalk is not probably a problem. Impedance means that you'll need termination for your transmission line. Best termination is "both ends"-termination, it tolerates imperfections favorably but disadvantage is that signal voltage gets divided by two, which may complicate receiver design. Loss is that you'll loose signal more and more when frequency and line length increases (via skin effect and dielectric loss). It makes your otherwise sharp-edged signals somewhat rounded/tilted. Delay is just that signal takes time to reach the other end. Not a problem in a serial link otherwise than via latency.Regards,Janne

author: Smilinvamp
author_rank: Newbie
timestamp: July 13, 2012, 07:35:47 pm
content: Right, thanks... I think were I get stuck is Capacitive Loading Effect.I am transmitting a 10MHz reference square wave through a coax 10 meters. It appears as a near triangle at the Rx end. I loose all my rise time.

author: jahonen
author_rank: Super Contributor
timestamp: July 13, 2012, 08:06:38 pm
content: That shouldn't be too difficult at all. Have you terminated your coax correctly? What is the load at the other end? Lumped capacitance of the line is not significant parameter by itself, as signal does not see it (transition of your signal does not physically occupy whole line at once).Simplest termination for point-to-point loads is to add a series resistor at the driving end so that combined impedance of the driver and resistor adds up to 50 ohms. Of course, this does not help if you use a line driver which can't drive 50 ohm line at first place. Your observation that you have triangle wave suggests this. If you would have overshoots, then the driver impedance is smaller than line impedance.There are dedicated line driver ICs but even something like 74VHC1G125 should be ok for that purpose. Logic gates sometimes have irritating property having different output impedances in '0' and '1' states so single series termination resistor can't terminate the line perfectly, but one can usually get acceptable result with some tweaking (some reflections on both transitions but within acceptable limits).

author: w2aew
author_rank: Super Contributor
timestamp: July 13, 2012, 08:35:03 pm
content: Quote from: jahonen on July 13, 2012, 08:06:38 pmI would agree with this. What device, IC, circuit, etc. are you connecting to the coaxial line?A properly terminated coaxial cable appears resistive. An improperly terminated coaxial cable will have an input impedance that will depend on many factors (frequency of operation, line length, complex termination impedance, line loss, etc.) - a lot more complicated than "capacitive loading effect".

author: Smilinvamp
author_rank: Newbie
timestamp: July 13, 2012, 08:46:05 pm
content: My transmitter is an Agilent 33521A - function generator, sync output.My transmission medium is 15 Meters (50 Feet) Belden 9239 Coax, connected on both ends via BNCMy Rx side is a 50 ohm resistor to ground (basically) - then goes to a Analog Devices EVAL-ADF4350EB1Z Synthesizer Board and a custom modulator circuit that is deigned by a colleague.Not much there that I have designed .... <shrug> ...I figure this should be relatively simple. But I am getting really odd issues with it. Once I figure it out I think I shall slap my forehead repeatedly...

author: w2aew
author_rank: Super Contributor
timestamp: July 13, 2012, 09:16:49 pm
content: Quote from: Smilinvamp on July 13, 2012, 08:46:05 pmStrange - I just checked the specifications for the Sync output of that generator, and it's spec'd to have a 50ohm output impedance, and should be able to deliver 1.5Vpp into a 50 ohm load. You're going to get a few dB of attenuation at 10MHz over the 50ft for that thin coax. However, in the Belden catalog, they specifically note that this coax is not recommended for RF use... ...it is designed for low noise applications, not RF. It uses a conductive PVC insulator layer around the center insulation. There are other RG-174/U type coax's that are better suited for RF applications, such as Belden 7805. My guess is that's where your trouble lies...

author: krenzo
author_rank: Regular Contributor
timestamp: July 13, 2012, 11:42:58 pm
content: Wikipedia has a good article on different kinds of coaxial cable.Make sure your cable is suitable for your frequency and matches your impedance. You'll also want to switch to SMA or N connectors when you work with higher frequencies. BNC is only rated up to 3 GHz.

author: Smilinvamp
author_rank: Newbie
timestamp: July 16, 2012, 02:25:58 pm
content: Right, thanks for your replies, I appreciate it.My signal is a standard 0-5volt square wave... only 10MHz...

author: hlavac
author_rank: Frequent Contributor
timestamp: July 16, 2012, 07:39:57 pm
content: Are you sure you don't have double termination on the end, one external and one internal?Also if your signal gen can only do 1.5V p-p into 50 ohm, that is too weak for the 5V you expect... you may have to adapt the input of the device to restore the signal level...