Measuring DOCSIS and AC current - Page 1
Forum section: electronics / beginners

author: kasoah
author_rank: Newbie
timestamp: September 13, 2013, 12:38:34 pm
content: Hello everyoneI was wondering if it was possible to measure the signals for my cable modem with an oscilloscopeThis was the oscilloscope I was planning on tryinghttp://www.amazon.com/Seeedstudio-DSO-Nano-Oscilloscope-v3/dp/B00BB4ETJW/ref=sr_1_3?ie=UTF8&qid=1379075637&sr=8-3&keywords=oscilloscopeNow, it says the analogue bandwidth is from 0-200khzModem signals for downstream are around 700mhz, and upstream at 200-400mhzI have a feeling that might be a problem, haha...When I looked for cable tv analyzers they are in the thousands of dollars.For the AC mains, could it get any sort of reading? I was interested if the powercleaner from monster actually did anythinghttp://www.amazon.com/Monster-HTS-800-Theater-PowerCenter/dp/B00003CWDQ/ref=sr_1_5?ie=UTF8&qid=1379075888&sr=8-5&keywords=monster+power+cleanThanks

author: Rerouter
author_rank: Super Contributor
timestamp: September 13, 2013, 12:52:20 pm
content: there is no way full stop for that thing to measure cable signalling,as far as mains, i myself would be cautious if it supplied the x10 probe if it even had a cat rating, and 9/10 times you will be more interested in the voltage waveform than the current, as switchmodes, pfc's and ccfl lights all colour the voltage through there odd current waveforms, where the end device usually just rectifying it to DC wont show it in its current waveform.equally i would say the monster board may have a dead simple lc filter and maybe a common mode choke if they where feeling generous which does not greatly help but may cut into some of the higher frequency switcher waveforms

author: ConKbot
author_rank: Super Contributor
timestamp: September 13, 2013, 01:19:58 pm
content: Check your cable signal levels by opening the webpage for the modem and see what it has to say. Free and though the accuracy may not be the best, its what matters in the endI'd also suggest keeping the DSO nano away from mains, and if youre checking the functionality of the filter, it might not even have enough bandwidth to accurately see the noise that it would be filtering.

author: kasoah
author_rank: Newbie
timestamp: September 13, 2013, 01:20:57 pm
content: Quote from: ConKbot on September 13, 2013, 01:19:58 pmI know I can visit the gui but I wanted to see the ripple.Is the general consensus the only way to view the ripple with a thousand+ dollar oscilloscope? ;_;Is there a particular reason the oscilloscopes are so expensive? Is the equipment in them really worth that much or is it just because of the niche market?

author: ConKbot
author_rank: Super Contributor
timestamp: September 13, 2013, 01:37:29 pm
content: Quote from: kasoah on September 13, 2013, 01:20:57 pmYou'd need a scope with at least 1GHz in bandwith and a calibrated pre-amp and filter if you wanted to view the waveform, probably $10k for thatIf you can filter your bands of interest (50MHz to 800 or 900 MHz for downstream iirc) you could get an RF power meter, or a log amp/RF level detector circuit would let you look at the power vs time.Scopes are expensive because of both reasons you mentioned. They are generally built to a spec, and all the engineering work to design, test, validate, make tooling etc, has to be amortized among the units sold. Samsung can sell a million cell phones, so the cost of that gets split up. But Agilent has a custom designed ASIC (big $$$ for design and production) high frequency analog front ends etc... and may only be selling tens of thousands of a particular oscilloscope.In short, what they are worth is upto you, when the perceived value matches the cost get oneFor checking out the mains filter performance, I'd start with a 20MHz analog oscilloscope from fleabay. See Dave's videos on the topic, and get one for slightly more than the nano DSO and have 100x the bandwidth.

author: kasoah
author_rank: Newbie
timestamp: September 13, 2013, 09:58:54 pm
content: Thanks for the recommendations and explanations.

author: kasoah
author_rank: Newbie
timestamp: September 23, 2013, 01:52:42 am
content: Bump but, would this do the trick?http://www.amazon.com/USB-2-Channel-PC-Oscilloscope/dp/B000LR4ZJY/ref=sr_1_1?ie=UTF8&qid=1379901103&sr=8-1&keywords=ghz+oscilloscope

author: BravoV
author_rank: Super Contributor
timestamp: September 23, 2013, 02:06:22 am
content: Quote from: kasoah on September 13, 2013, 09:58:54 pmQuote from: kasoah on September 23, 2013, 01:52:42 amDon't think you even understand a bit on above nice explanations given, what a waste.If you desperately need a scope, then yes, buy it, that is the best scope money can buy, hurry.

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: September 24, 2013, 12:54:52 pm
content: Quote from: kasoah on September 23, 2013, 01:52:42 amI'm sorry if what I'm going to write doesn't sound nice, but I don't see any other way to say it:Imagine someone coming to a surgical forum and saying something like this"I've got headache and I want to open my head to check if my brain is OK. I've seen a surgery show one on TV and I'm thinking of buying this plastic knife (link here). Would it be good enough to open my skull and fix my brain?"And then:"Bump, so I've found this metal knife, it's not the sharpest, but it's better than plastic. Would it be OK?"That's how your question looks like.I mean, do you even have an idea what you should be looking for if you somehow do manage to get appropriate scope? How would you judge the signal quality? Would you be able to determine what's ripple and what's correctly modulated signal?Your question, assuming it's serious, tells me that you're not qualified to look at the cable signal, so there is no point in looking at it. Yes, a real tool costs thousands of dollars, but it's nor really all that expensive. Resources needed to train someone to be able to properly use such complicated measurement device and be actually able to interpret results of measurement are much greater.

author: madsci
author_rank: Regular Contributor
timestamp: September 24, 2013, 10:12:49 pm
content: Sampling rate DOES NOT equal bandwidth. Note that while it claims to sample at 1GHz (or 1GSa/s), its probes are60MHz.I doubt the scope is any more, it makes little sense to ship a high bandwidth scope with low bandwidth probes.Sampling rate is cheap these days. Fast DSPs are everywhere. Front end bandwidth is limited by the analog input amp and that stuff isn't cheap to fabricate or design. Very high speed circuitry is quite possibly the most difficult discipline in EE. The engineers are paid accordingly and when you add to the fact that test gear is a low volume industry compared to mobile phones and PCs, you're looking at some extremely expensive gear.Even if the front end was of infinite bandwidth, a 1GSa/s rate limits your scope's ultimate bandwidth. 500MHz is as fast as you can go with 1GSa/s, period. This is the Nyquist limit.Even a 500MHz scope will cost you some serious cash. My old (circa 1997) Tektronix TDS744A was nearly $700....and that's considered a good deal for this ancient DSO. GHz scopes in good working order are often considerably dearer. New ones are well over $10000 US.The Anritsu analyzer my cable company brought out to the house to test my lines costs around $15000. If they could get by with a sub $300 instrument, I know they would.There's a reason this scope is cheap. It's not a serious instrument....nothing from Velleman is.

author: johansen
author_rank: Super Contributor
timestamp: September 24, 2013, 10:46:30 pm
content: i have one of those scopes, if you want it you can have it for 100$http://www.amazon.com/USB-2-Channel-PC-Oscilloscope/dp/B000LR4ZJY/ref=sr_1_1?ie=UTF8&qid=1379901103&sr=8-1&keywords=ghz+oscilloscopei believe it is fixed at 50Mega samples per second regardless if both channels are on or not.sampling at 1GSPS is achieved by dithering the sampling buffers and assuming the signal is repetitive.anyhow, there's lots of things you can do with it, upload data to the computer continuously, it can also be used as a data recorder down to 2000 seconds per division, or use the dll to upload the data to a spreadsheet.anyhow, that's how much its worth to me but i don't really need it.you can measure up to 500MHZ with a Tek 2465B, which is a 400Mhz scope. my 2465 can register 500MHz but only with a very strong signal and its probably -6Db at that point or more. not to mention the probes i'm using are >200Mhz probeslike someone upline said: you need a pre amplifier to do signal level work with an oscope.

author: mmagin
author_rank: Frequent Contributor
timestamp: September 24, 2013, 11:31:01 pm
content: For cable modems, it's going to be a lot easier and cheaper to look at its own diagnostics, and if you suspect modem problems, swap one modem with another. If you really had a large budget and wanted to look at what's the signal levels are in operation, you'd probably want a good spectrum analyzer and a directional coupler (to extract some of the signal power in each direction while keeping most of the signal power and not producing an impedance mismatch).It doesn't seem like even many high-end oscilloscopes would be good for looking at what power level you're seeing on the relevant channels, because if the modem is attached to a normal cable TV system, there are many other signals on other frequencies. Short of an expensive spectrum analyser that can tell you "How much power is there between these two points in the spectrum?", I suspect some kind of calibrated receiver (akin to the old selective level meters which looked at audio channels in the 0 to 10 Mhz range typically) would be an ideal instrument if you were designing something from scratchNow, if you wanted something crude and cheap, I bet you could get a rough signal level by monitoring the AGC level in an ordinary TV tuner module (those ones found in any analog tv of the past 30 years). Good luck reverse engineering a random surplus one though. I expect that it is approximately what the cable modem does to tell you the signal level it's seeing on the downstream however.(Edit: I suppose the interesting and maybe overwhelming thing as a beginner is that it's remarkable how many inexpensive pieces of consumer electronics include some element which is difficult to really measure/test well without very expensive equipment, or at least improvising something that requires a lot of expertise. On the other hand, there's many things you'll find out there that are comparatively trivial.)

author: kasoah
author_rank: Newbie
timestamp: September 25, 2013, 03:22:26 am
content: Quote from: AndrejaKo on September 24, 2013, 12:54:52 pmWell in the Cisco pdfs they show what the upstream ranges and downstream curves should look like, was just curious is all. I mean I know it uses 64 qam for upstream and what frequencies I should look around, 253mhz, 306mhz, and 370mhz since there's 3 upstream channels in my area.Just wanted to see what the oscilloscope records when my netgraph ingame gets wonky.

author: BravoV
author_rank: Super Contributor
timestamp: September 25, 2013, 03:39:56 am
content: Quote from: kasoah on September 25, 2013, 03:22:26 amSo all of these trouble buying an oscilloscope capable of reading cable modem signal is just for looking for the culprit that caused your internet gaming problem ?

author: kasoah
author_rank: Newbie
timestamp: September 25, 2013, 03:41:37 am
content: Quote from: BravoV on September 25, 2013, 03:39:56 amyou bet =)

author: madsci
author_rank: Regular Contributor
timestamp: September 25, 2013, 07:12:36 am
content: My usual thing to do here would be to graph network latency and do a few trace routes to see where the latency is. It's very possible that it's not your modem or ISP at all.Broadband Reports can generate a line quality test....go check that out. It tests latency to various points on the Internet over a period of 20 minutes.http://www.broadbandreports.com/toolsIf your modem has management page you can access, check the signal as stated above. Your cable company will be able to tell you what the average is. Also, your cable company can do remote diagnostics as well. Their telemetry is pretty good.If all that fails, they'll be sending someone out with an analyzer that costs as much as a small car. If you ask nicely, they'll let you look at it.

author: kasoah
author_rank: Newbie
timestamp: September 25, 2013, 07:24:25 am
content: I've done a lot of them. Problem is intermittentFor the VOIP test all the jitter is on the upstreamhttp://myspeed.visualware.com/indexvoip.phpI was planning on asking for my uSNR, but I have a feeling the value will be quite good since the issue is intermittent.Not looking too forward for when a tech has to come though.Modem signal are perfect on my endtried a sb5101 as well, same thing but the jtiter pattern is different.

author: madsci
author_rank: Regular Contributor
timestamp: September 25, 2013, 07:50:41 am
content: Not sure what "good" values for errors are but two of those channels look odd in that regard.Check your splitters but usually jitter is a network problem. How high does it go?

author: kasoah
author_rank: Newbie
timestamp: September 25, 2013, 09:16:48 pm
content: 300ms jittermy netgraph goes from 0 jitter at 20 ping to 400 and then I keep freezing at 999then it goes away and comes backI checked all the splitters, they're too tight to be loosened by hand.Those two channels have errors, but that's most likely from the LTE bands riding on those two frequencies. I can put my phone on my 5341j and the errors skyrocket up on those two channels. My ingame netgraph starts dropping packets, but it does not cause any jitter, it just drops packets.

author: c4757p
author_rank: Super Contributor
timestamp: September 25, 2013, 09:44:19 pm
content: Quote from: kasoah on September 25, 2013, 09:16:48 pmThat only means the shield is securely connected. Unplug them and reconnect them.

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: September 26, 2013, 12:36:01 am
content: Quote from: c4757p on September 25, 2013, 09:44:19 pmDoes it? The shield will be properly connected due to tight connections only if the shield was properly connected to the connector in the first place and and has remained properly connected. We have no guarantee of that.

author: johansen
author_rank: Super Contributor
timestamp: September 26, 2013, 01:22:46 am
content: those cheap aluminum shielded and copper plated steel coax cables are subject to all manner of nonsense.when i lived in north carolina i had some issues with the internet dropping out, they send the guy out and he checked the signal levels, he ended up replacing all of the coax connectors all the way out to the street. (at least 6 of them) to correct the problem

author: madsci
author_rank: Regular Contributor
timestamp: September 26, 2013, 04:51:12 am
content: Quote from: kasoah on September 25, 2013, 09:16:48 pmThen what does that say about your shielding?