Need to increase my 4 channel scope to 5 somehow - Page 1
Forum section: electronics / testgear

author: Gernby
author_rank: Newbie
timestamp: February 19, 2016, 05:15:15 pm
content: Hi, I just joined the forum, so please be gentle.I am working on a project that would greatly benefit from having a 5th channel in my oscilloscope (Rigol MSO1074S). Some things that seem like possible solutions would be:* connect a 2nd scope, and logically link them somehow* connect an external ADC to the scope via I2C* move the pulse signal to a digital channel, then connect an external ADC using the 7 other digital channels* leverage some other existing feature to free up the analog channel I'm currently using as a pulse triggerTo complicate things, I need to analyze the sample data in a post-process, so it would be difficult to merge data samples from 2 scopes. Also, the pulse input is not just a trigger for the scope as much as a position indicator for the other waveforms, so I'm not concerned about the resolution of that input as much as the location and timing of the pulses.Here is a screenshot of the data I'm currently capturing. I really need to have 4 analog waveforms plus the pulses. Thoughts or suggestions?

author: Cerebus
author_rank: Super Contributor
timestamp: February 19, 2016, 05:28:48 pm
content: How about the old fashioned chopper? At one time it was quite common to use an external chopper that switched two external channels, one with a DC offset, into one channel of the scope, alternating quickly between them.. There are obvious limitations related to bandwidth, triggering etc.

author: tggzzz
author_rank: Super Contributor
timestamp: February 19, 2016, 05:40:14 pm
content: Maximum and minimum voltages, and maximum and minimum frequencies, please. N.B. fundamental frequency != highest frequency, except in the special case of sinewaves.Are the waveforms repetitive or is single-shot operation required.

author: Kilrah
author_rank: Unknown
timestamp: February 19, 2016, 06:22:42 pm
content: Quote from: Gernby on February 19, 2016, 05:15:15 pmUh... I must be missing something, but is there anything preventing you from using one of your MSO's 16 logic channels, like... for that digital signal?

author: Gernby
author_rank: Newbie
timestamp: February 19, 2016, 07:08:07 pm
content: Quote from: Cerebus on February 19, 2016, 05:28:48 pmI haven't heard of anything like that, so I don't think I really understand how it would work. Is that a device that is readily available?

author: Gernby
author_rank: Newbie
timestamp: February 19, 2016, 07:12:53 pm
content: Quote from: tggzzz on February 19, 2016, 05:40:14 pmMost of the voltages have a ~4V DC offset, and have a ~50 mV AC component (the important part). The pulses are 5V, and 2 of the voltages could potentially be 90V AC. The pulses are coming in as fast as 300 HZ, so I would guess that anything under 1K Hz is all I'm really interested in.

author: Gernby
author_rank: Newbie
timestamp: February 19, 2016, 07:15:22 pm
content: Quote from: Kilrah on February 19, 2016, 06:22:42 pmNot sure if all MSO's do this, but mine has shared channels for Analog and Digital inputs. It can have 4 analog inputs, 3 analog inputs + 8 digital inputs, or 2 analog inputs + 16 digital inputs.

author: nctnico
author_rank: Super Contributor
timestamp: February 19, 2016, 07:17:17 pm
content: Quote from: Gernby on February 19, 2016, 07:15:22 pmThis is the first time I ever heard about an MSO having such a ridiculous restriction! I guess you are out of luck with the Rigol scope you have.

author: Cerebus
author_rank: Super Contributor
timestamp: February 19, 2016, 07:25:25 pm
content: Quote from: Gernby on February 19, 2016, 07:08:07 pmThey just multiplex two channels into one, alternating very quickly, adding a DC offset to one so that it appears offset on the screen. It works very well on an analogue scope but thinking about it might look 'orrible on a digital scope unless you've got intensity grading on.They used to be readily available and were a common staple of the projects in electronics hobby magazines. I've not seen a NEW one in years. You might find a second hand one in the usual places or if you're working at moderate frequencies cobble together one with an HC4053 and a handful of parts on a breadboard.

author: tggzzz
author_rank: Super Contributor
timestamp: February 19, 2016, 07:34:23 pm
content: Quote from: Gernby on February 19, 2016, 07:12:53 pmThose are not major problems; it would be relatively easy to build something yourself. Alternatively you may be able to find a ready-made unit - typically called an analogue multiplexer, but there may be other names.The principle is to have a switch which connects one of the inputs to the output, and to rapidly switch between inputs; i.e. the inputs are multiplexed onto the output in succession. "Rapidly" means much faster than can be seen on whatever timebase you are using, so 10kHz or 20kHz is probably suitable. For each input you will also want to add a different DC voltage, so that the different signals are vertically eparated from each other on the display.The switch will be electronic rather than mechanical, and you may have to AC couple or divide down the input signal so that its voltage is doesn't harm the multiplexer inputs. Multiplexers will introduce noise, which will limit the minumum voltage you can observe.

author: Muxr
author_rank: Super Contributor
timestamp: February 19, 2016, 08:41:09 pm
content: Quote from: Gernby on February 19, 2016, 05:15:15 pmYou can save traces. If you can repeat the measurement multiple times then you could easily just save a trace and then move the probe to another signal you want to capture. And repeat the measurement trigger from the same event.

author: Kilrah
author_rank: Unknown
timestamp: February 19, 2016, 08:46:48 pm
content: Quote from: nctnico on February 19, 2016, 07:17:17 pmMe too! But thanks for that, being on the hunt for an MSO right now I'll be sure to check for such idiocies! Wouldn't ever have thought of that being a possibility and could very likely have been caught...EDIT: And it's even worse than this, it is absolutely NOT stated anywhere in the description, nor in the 12-page specs! Gotta read the full user manual to find a relatively obscure "note" about it, WTF!!Quote- 10 points in the balance for Rigol

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 19, 2016, 09:08:30 pm
content: Quote from: Gernby on February 19, 2016, 05:15:15 pmyou want to post-process and you cant merge them? what post-processing software you are using?Quote from: nctnico on February 19, 2016, 07:17:17 pmso whats the better bang per buck MSO that doesnt share A/D input?

author: Lightages
author_rank: Unknown
timestamp: February 19, 2016, 09:30:47 pm
content: This is one of the reasons I held on to my DS1052E after buying a DS1054Z. I can connect the trigger in on the DS1052E to the trigger out of the DS1054Z and have a "six channel" scope. It works quite well except for the pain of keeping things synced.

author: zapta
author_rank: Super Contributor
timestamp: February 19, 2016, 10:19:42 pm
content: If you are looking for post processing, you can feed one common pseudo random or time based signal to any number of scopes and then in post processing use that signal to do time matching between them.

author: Howardlong
author_rank: Super Contributor
timestamp: February 19, 2016, 10:25:08 pm
content: Quote from: Kilrah on February 19, 2016, 08:46:48 pmIn practical terms realistically it's very rarely a restriction... except in the OP's case! You can only put so much on that little screen as it is.For the OP, how about another scope? I used to frequently use two 2+16 MSOs with the triggers daisychained before I got a (proper!) 4+16.

author: Fungus
author_rank: Super Contributor
timestamp: February 19, 2016, 10:27:15 pm
content: Quote from: Gernby on February 19, 2016, 05:15:15 pmThat's what the 'trigger out' on your oscilloscope is for - you can connect it to an input channel on another oscilloscope and trigger off that. The two displays are then linked.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 20, 2016, 12:06:18 am
content: The downside of using a second scope is that you can't just look at the screen & see the time relationships between all the waveforms.You could display one waveform as a reference onboth'scopes & infer the time relationships between the others.This means you need a second two channel 'scope.

author: kwass
author_rank: Frequent Contributor
timestamp: February 20, 2016, 06:26:13 am
content: Quote from: Cerebus on February 19, 2016, 07:25:25 pmThe Thurlby OM358 is an 8-to-1 scope multiplexer. Heathkit sold the ID-101 Electronic Switch, a really ancient 2-to-1 multiplexer for recurrent sweep 'scopes.Tektronix used to make plugins for their scopes that were simple multiplexers, but they were expensive. In college I had a great job designing and building electronics for a neurology professor. One of my projects was to create a 4-to-1 multiplexer plugin for a tek scope using all CMOS with a 4052 at the core.

author: Andreas
author_rank: Super Contributor
timestamp: February 20, 2016, 07:29:10 am
content: Why bothering with 5 channels if you can have 8?https://www.picotech.com/oscilloscope/4824/8-channel-oscilloscopewith best regardsAndreas

author: Howardlong
author_rank: Super Contributor
timestamp: February 20, 2016, 11:57:14 pm
content: Quote from: vk6zgo on February 20, 2016, 12:06:18 amThat's what the trigger out's for, connected to the other scope's ext trigger.Just remember to set the timebases the same, and for anything faster than a few 100ns/sweep, take into account the skew.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 21, 2016, 01:01:06 am
content: Even if they are triggered together,theyareon different displays,so the ability to directly compare their timing is seriously impaired.Of course,if you don't actually look at the waveforms,& just look at the readouts,that may not be a problem.

author: artag
author_rank: Super Contributor
timestamp: February 21, 2016, 01:05:31 am
content: Quote from: Andreas on February 20, 2016, 07:29:10 amFor rather a lot less money (and a lot less quality and capability) you could get a Hantek 1008http://www.ebay.co.uk/itm/201241040655

author: tautech
author_rank: Super Contributor
timestamp: February 21, 2016, 01:14:28 am
content: Quote from: vk6zgo on February 21, 2016, 01:01:06 amThat's called skew, the amount can be identified and adjusted for.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 21, 2016, 02:01:56 am
content: Quote from: tautech on February 21, 2016, 01:14:28 amI'm not saying it can't be done,or that the "skew" can't be adjusted for.I'm only pointing out that using two displays has problems.If one channel is duplicated on both 'scopes it makes the timing relationships easier to determine.

author: tautech
author_rank: Super Contributor
timestamp: February 21, 2016, 02:15:33 am
content: Quote from: vk6zgo on February 21, 2016, 02:01:56 amExactly.

author: Fungus
author_rank: Super Contributor
timestamp: February 21, 2016, 11:09:29 am
content: Quote from: vk6zgo on February 21, 2016, 02:01:56 amYep. You can also send the triggering signal to both 'scopes and do it that way.To solve the skew problem you just stack the scopesverticallyso the traces line up.

author: Howardlong
author_rank: Super Contributor
timestamp: February 21, 2016, 01:49:33 pm
content: Quote from: Fungus on February 21, 2016, 11:09:29 amExactly!

author: Tom45
author_rank: Frequent Contributor
timestamp: February 21, 2016, 06:06:13 pm
content: Since you need to work with the signal values anyway, how about just using a data acquisition interface such as one of these:http://www.mccdaq.com/products/product-listing.aspx?oid=1596&r=12

author: Gernby
author_rank: Newbie
timestamp: March 02, 2016, 07:25:38 pm
content: Wow! Thanks for all the replies! I didn't even know all this discussion was going on, since I didn't receive any email notifications from the forum...There are a lot of great suggestions, and looking back, I see now that a USB DAQ might have been a much better fit for my needs, since I'm mostly using the scope as a datalogger, then analyzing and archiving the data in Excel.That said, I did come up with a decent solution to my particular issue. I decided to use the 4th analog channel in the 8 bit digital channel mode, and use 3 of those digital inputs to monitor 3 signals that I was using analog channels for. The pulse trigger input was simple, but the 2 others are actually computer generated waveforms from a microcontroller. What I decided to do was to duplicate the ouputs from the microcontroller as both true analog AND PWM, so that I could use the digital scope inputs to record the PWM version.