Good Scope for Using with ethernet? - Page 1
Forum section: electronics / testgear

author: mrpackethead
author_rank: Super Contributor
timestamp: September 14, 2015, 10:06:36 am
content: Hi, Any suggestions for a good scope that i could use on ethernet Differential pairs? I'm typically only working with 10/100 Mb.

author: nctnico
author_rank: Super Contributor
timestamp: September 14, 2015, 10:13:43 am
content: That kind of ethernet has a maximum bandwidth of about 120MHz so a 200MHz scope is more than enough. Don't know what you are after because there is not much too see without an ethernet bitstream / protocol decoder.

author: Fungus
author_rank: Super Contributor
timestamp: September 14, 2015, 10:15:15 am
content: Quote from: mrpackethead on September 14, 2015, 10:06:36 amA dedicated Ethernet analyser will be much better.

author: mrpackethead
author_rank: Super Contributor
timestamp: September 14, 2015, 10:21:52 am
content: Not so worried about the protocol level stuff, just wanting to ensure that i've got circuits and setups that are operating within spec.

author: nctnico
author_rank: Super Contributor
timestamp: September 14, 2015, 10:32:25 am
content: I use a spectrum analyser for that.

author: Fungus
author_rank: Super Contributor
timestamp: September 14, 2015, 10:46:20 am
content: Quote from: mrpackethead on September 14, 2015, 10:21:52 amThat's exactly what the protocol level stuff tells you. If data integrity is high, number of retries is low ... you're within spec!If it has a cable tester then it will tell you the weak points, too.

author: R_G_B_
author_rank: Frequent Contributor
timestamp: September 15, 2015, 02:36:07 pm
content: you can use one of these:Fluke 225C:• AS-i (EN50295, 166 kb/s);• CAN-bus (ISO-11898, up to 1 Mb/s);• Modbus 232 (EIA-232 up to 115 kb/s);• Modbus 485 (EIA-485 up to 10 Mb/s);• Foundation Fieldbus H1 (61158 type 1, 31.25 kb/s);• Profibus DP (EIA-485 up to 12 Mb/s);• Profibus PA (61158 type 1, 31.25 kb/s);• Ethernet 10Base2 (coaxial, 10 Mb/s);• Ethernet 10BaseT (UTP, 10 Mb/s);• Ethernet 100BaseT (100 Mb/s);• RS-232 (EIA-232, up to 115 kb/s);• RS-485 (EIA-485, up to 10 Mb/s).http://www.fluke.com/fluke/uken/portable-oscilloscopes/fluke-225c.htm?pid=56053R_G_B

author: helius
author_rank: Super Contributor
timestamp: September 15, 2015, 03:27:25 pm
content: Or a Fluke 683 or 686.

author: mrpackethead
author_rank: Super Contributor
timestamp: October 12, 2015, 04:08:56 am
content: Quote from: nctnico on September 14, 2015, 10:13:43 amWhat I was hoping to do was create a eye diagram and check to see if things look reletively healthy..It seems this is becoming a more expenive and complex problem than i had anticipated.A differnetial probe is goign to be expensive!

author: nctnico
author_rank: Super Contributor
timestamp: October 12, 2015, 06:31:56 am
content: You can use a balun to convert the ethernet pair into a 50 Ohm single ended output (assuming you can force your hardware to output a bit stream without a device connected).

author: mrpackethead
author_rank: Super Contributor
timestamp: October 23, 2015, 09:32:57 am
content: Quote from: nctnico on October 12, 2015, 06:31:56 amThat is a potentially very good idea. What sort of balun would I be looking for?

author: macboy
author_rank: Super Contributor
timestamp: October 23, 2015, 03:39:28 pm
content: Quote from: mrpackethead on October 23, 2015, 09:32:57 amHow about an Ethernet transformer :-)On the secondary side, connect to the center tap and one end (not end to end). That should get you a 2:1 ratio from your 110 ohm primary, so close enough to 50 ohm for a short run to the scope. Does your scope have a 50 ohm terminator built in?

author: nctnico
author_rank: Super Contributor
timestamp: October 24, 2015, 12:37:29 am
content: Quote from: mrpackethead on October 23, 2015, 09:32:57 amSomething like this:http://uk.farnell.com/pro-signal/ab4-5m88/balun-bnc-rj45-pin-4-5/dp/1268361

author: mrpackethead
author_rank: Super Contributor
timestamp: October 24, 2015, 02:03:00 am
content: QuoteDo you think its got enough bandwidth to see a 125Mhz through it?

author: mrpackethead
author_rank: Super Contributor
timestamp: October 24, 2015, 10:38:53 pm
content: Quote from: macboy on October 23, 2015, 03:39:28 pmThis too is an interesting idea, and one that seems to ahve some merit. I have a Rigol 2072A-S, 300Mhz, and it has 50ohm inputs. It would be easy ( and cheap ) to build this all onto a small PCB, which would keep it all very tidy, and probably can make it so it can almost plug onto the scopes inputs direclty.ANyone else have any thoughts on this?

author: mrpackethead
author_rank: Super Contributor
timestamp: October 26, 2015, 03:04:14 am
content: Quote from: macboy on October 23, 2015, 03:39:28 pmDo you mean something like this.. i thought it should be terminated? I was thinking that it would ust make it very convienet and tidy to make this on small pcb, with connectors etc.Scope is 50ohm or 1Mohm inputs..Have done both sides, as sometimes things are MDIX-auto so no guarentte which pair the TX signal will be..

author: nctnico
author_rank: Super Contributor
timestamp: October 26, 2015, 09:06:54 am
content: You'd need to terminate the RD- and RX- with 50 ohms to ground. Now it is unbalanced.

author: mrpackethead
author_rank: Super Contributor
timestamp: October 26, 2015, 06:44:00 pm
content: Quote from: nctnico on October 26, 2015, 09:06:54 amI was thinking that the Scope provide the 50ohm termination?Are you suggesting somethign like this;

author: nctnico
author_rank: Super Contributor
timestamp: October 26, 2015, 07:13:02 pm
content: Quote from: mrpackethead on October 26, 2015, 06:44:00 pmAn ethernet transformer needs to see 50 Ohms on each leg because ethernet over twisted pair is 100 Ohm differential. So if you terminate one leg with 50 Ohms to ground and the other one at the oscilloscope the impedance is 100 Ohms differential.I meant RX and TX legs BTW.

author: mrpackethead
author_rank: Super Contributor
timestamp: October 26, 2015, 07:33:45 pm
content: Quote from: nctnico on October 26, 2015, 07:13:02 pmQuote from: nctnico on October 26, 2015, 07:13:02 pmSo like this?

author: nctnico
author_rank: Super Contributor
timestamp: October 26, 2015, 08:02:33 pm
content: Yes, exactly!

author: mrpackethead
author_rank: Super Contributor
timestamp: October 26, 2015, 09:08:43 pm
content: Thanks Nico.. that makes sense. I'll knock this up on a little PCB this week. PCBs are so cheap and quick now, that its just not worth the effort to "bodge" wire it up.Hoping that i'll be able to get some good eye diagrams.In an earlier reply you said you used a spectrum analysier.. What do you use that for and how?( there is so much to learn here, i'm greatful for the knowledge you guys share! )

author: nctnico
author_rank: Super Contributor
timestamp: October 26, 2015, 09:48:20 pm
content: I used a spectrum analyser because I had problems with passing emissions testing on an ethernet interface. I used the spectrum analyser to check the effectiveness of the additional filtering versus attenuation of the ethernet signal.

author: mrpackethead
author_rank: Super Contributor
timestamp: October 27, 2015, 04:16:49 am
content: Quote from: nctnico on October 26, 2015, 09:48:20 pmI guess you could use the FFT on a scope ,but mine is only 300Mhz, so its not going to see much at all, ( assuming that we'll probalby see odd order harmonics from the switching )....What other cheap hacks can be done...