Real world purposes of 4Ch and high sensitivity scopes - Page 1
Forum section: electronics / testgear

author: pfm
author_rank: Regular Contributor
timestamp: August 05, 2014, 04:52:43 pm
content: I am curious for what real world purposes would it be useful to have 4 channels in a scope. Same question for high sensitivity (<1mv/div). For what actual purposes would anyone need to measure under 1mv ? Can anyone give some examples ?

author: Precipice
author_rank: Frequent Contributor
timestamp: August 05, 2014, 05:04:06 pm
content: Today, I was chasing problems with Wifi module on an MMC bus, which were suspected to be analogue woes, so no place for a logic analyser. 4 channels got me CMD, CLK and 2 data lines. 2 more channels got me the rest of the data bus. 3 more lines got me 1.8V and 3.3V to see if they drooped when sending data. one more channel got me IRQ, to see if the module was playing.(I have a 16 channel 'scope, which is usually enough. 4 channels is definitely limiting - you can get stuff done, but it#'s harder and slower than it could be, and debugging's hard enough without swapping probes around or hoping that runs are the same from go to go. TLS216 - Tektronix no longer make them, and they're insanely useful (sometimes), so I've hoarded a couple, and probes ))High sensitivity - some jobs require it, sensor stuff or power supply stuff for me. I just fire up a 7A22 in an analogue scope, which gets me down to 10uV/div and some high & low pass filtering that's often useful.

author: ovnr
author_rank: Frequent Contributor
timestamp: August 05, 2014, 05:27:36 pm
content: I'd never buy a 2-channel scope unless I had a need for somewhat ridiculously high bandwidth (think GHz and more). It's super convenient to have four channels (no need to always move your probes, can just clip on more), and it's absolutely essential if you're doing digital buses.As for high sensitivity, sensors is the big one, but also ripple measurement in low-noise circuits, etc. Some things get uppity if you have even a few mVpp noise, and your usual scope is not much use down in those ranges.

author: nctnico
author_rank: Super Contributor
timestamp: August 05, 2014, 06:31:51 pm
content: Once you have used a 4 channel scope you never want to use a 2 channel scope again. Life is so much easier if you can look at 3 or 4 signals at once.

author: pfm
author_rank: Regular Contributor
timestamp: August 05, 2014, 06:40:26 pm
content: as I suggested in my initial post - give me real examples like a couple given above. I have seen tons of opinions all over the world wide web like "you never look back" or "life is easier" with a 4 channel scope but the difference that is going to be about this thread is that it will contain only real actual field examples and no subjective ideas. Thank you.What were the last couple of things you used 4 channels for ?

author: farrell
author_rank: Contributor
timestamp: August 05, 2014, 06:45:49 pm
content: An example? How about power supply design. You might want to look at voltage in, voltage out, current in, current out, some waveforms in the feedback loop, etc. When reverse engineering it's also vital to be able to look at multiple analog waveforms when trying to understand how a circuit is working.Edit: to be even more specific, here's a boost converter circuit before I fixed ground bounce problems:

author: Precipice
author_rank: Frequent Contributor
timestamp: August 05, 2014, 06:48:47 pm
content: Also - power sequencing. When you've got a chip that requires the power supplies, oscillators and resets to come up in the right order, a multi channel scope is handy.DDR memory - clock+, clock- and something (anything) else... Then you often need a few extra channels to set a useful trigger.Come to think of it, extra channels are often useful for triggering - even if you have a scope with 2 full channels and 2 lesser channels (less capable attenuators, as some Tek scopes do), you're still onto a winner.Downsides: more channels = more probes, and probes can be horribly expensive.

author: rodpp
author_rank: Frequent Contributor
timestamp: August 05, 2014, 06:52:21 pm
content: Debugging a control system that uses a DAQ card in a computer. I measure the sensor signal before and after the low pass anti aliasing filter and measure the control signal on the DAQ output and after the power amplifier. It is in a very simple SISO system!Enviado de meu XT1058 usando Tapatalk

author: AdShea
author_rank: Contributor
timestamp: August 05, 2014, 07:26:48 pm
content: I work on power electronics and drives. Usually I end up with 2 or more 4-channel scopes as a 3-phase or more drive needs at least 3 channels.Now if only someone would make a nice 12 channel scope.

author: German_EE
author_rank: Super Contributor
timestamp: August 05, 2014, 07:30:13 pm
content: I used all four channels this evening during a repair:Channel 1 I2C Data to DACChannel 2 I2C Clock to DACChannel 3 DAC OutputChannel 4 DAC Reset lineWithout this ability (and the ability to decode the I2C signals on the scope) I would not have found the fault.

author: hans
author_rank: Super Contributor
timestamp: August 05, 2014, 09:18:25 pm
content: Today I was debugging an auto-ranging feature for a current meter. I was getting way too high A/D readings a few dozen samples after ranges were switched, so I probed the circuit to see how fast the front-end switches ranges (that was fast and clean enough) and what output code the A/D converter gave me at that time.With 4 channels I could trigger of the front-end range pulse, view the front-end analog output voltage and probe the SPI bus + decode the data to view the actual analog samples that were taken. With that I was able to deduct whether the A/D was at fault (which is one of those delta-sigma ones that can spit out kSPS at low OSR ratio's), or my software confusing ranges at the wrong timestamps.Just a while ago I was working on a DAC multiplexer circuit. I wanted the ripple to be as small as possible. A 12-bit DAC already has <1mV/LSB steps.. 14-bit is 0.25mV, etc. With a 1mV/div setting it's very hard to characterize the multiplexer circuit/filters at the desired frequency, because you're very quickly measuring noise. In that case I had to extrapolate measurements and assume the filter keeps on working in the microVolts range.

author: Smokey
author_rank: Super Contributor
timestamp: August 05, 2014, 09:25:41 pm
content: I know this isn't the concrete example you are looking for, but the fact is there is not some small set of problems where multiple channels on a scope are useful and they can all be listed here. This is the reasoning behind needing multiple analog channels, and you can in turn apply it to whatever application you might have. You have to make the decision yourself if 4 channels is worth the extra money for the things you are working on.One thing about completely "digital" signals is that... they don't exist (quantum effects aside). There is no such thing as an electrical signal that has an absolutely instantaneous rise/fall time and an absolutely DC minimum and maximum values.Every signal that you might consider "digital" has a TON of analog aspects to it which you can't monitor with a logic analyzer or multimeter. Some of these basic aspects are, rise time, fall time, over shoot, undershoot, ringing, ripple, runt pluses, low voltage levels, high voltage levels etc. And that's just general digital stuff. If you want to start talking about power electronics then you have issues like miller capacitance, and gate driver slew rate. Any time you need to see how one signal interacts with another you need at least two channels. It is very often the case that some signal will interact with MANY other signals and components and the only way to see that effect is to have many channels of monitoring capability.The one concrete example I will give is a PWM motor driver. Both transistors of one phase can't be on at the same time or you get "shoot through" where the current just goes from B+ to B- without going through the motor. Without the motor inductance/resistance to limit the current your drive will probably blow up. The time between the transition from high side to low side transistors being on is referred to as the "deadband". As an engineer you need to evaluate if your deadband time is sufficient to ensure both transistors are NEVER on at the same time. There are many physics issues that effect the time it takes a transistor to turn on and off and you really need a scope with many channels to monitor these signals at the same time. When evaluating a three-phase pwm motor amplifier 4 channels isn't even close to enough to monitor all the things at once. You need to evaluate it in pieces.Some of the things you would want to scope. This is usually switching at around 25khz:1) Low voltage signal into the gate driver2) Actual gate voltage of the transistor looking at miller effect on rise time (this is a big one)3) Opto prop delay and rise/fall time4) Current through the transistor5) Voltage across the transistorThat's just some of the stuff, and there are those sets of measurements for each of the 6 transistors. All that needs to be time correlated to see how the various components effect each other and the system as a total. If you screw up some aspect of the timing here things blow up, sometimes quite spectacularly. You could certainly work on a system like that with two channels of scope, but it will take longer to infer and composite together the same picture as a 4 channel scope would show on the screen.This is the beast we should all be lusting over!http://tmi.yokogawa.com/us/products/oscilloscopes/digital-and-mixed-signal-oscilloscopes/dlm4000-mso-series/

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: August 05, 2014, 11:21:41 pm
content: I have never found anything that couldn't be diagnosed properly with 2 probes. That said, sometimes I´ve used the 2 probes plus the trigger input, and sometimes if I had had 4 or six or 8 channels surely I would have used them and it would have made my life slightly easier, for example instead of floating my scope´s ground, each couple of single ended channels make a nice differential one. In my opinion more than 2 channels is always a nicety but very rarely a requisite.

author: nctnico
author_rank: Super Contributor
timestamp: August 06, 2014, 12:17:54 am
content: I've just been busy debugging an RS485 interface and it is very nice to be able to show the transmit enable signal besides RX and TX on a 4 channel scope.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 06, 2014, 12:51:22 am
content: the logic is: if i already have 4 channel with my money, why would i get another 2 channel scope? but if i cant afford 4 channels, whats wrong with 2 channel?about the sensitivity lowish 1mv/div. if you are analog guru or audiophools you'll crave for it, otherwise doing wallwart smps just to make something alive there is no reason to get one unless well, if you have too much to waste or think for the future.

author: Electro Fan
author_rank: Super Contributor
timestamp: August 06, 2014, 02:26:44 am
content: Silly question, but could some of the examples of signals needing more than 2 channels described in this thread be accomodated on a scope with 2 analog channels + a 16 channel analyzer (like the Rigol MSO2000)?

author: tautech
author_rank: Super Contributor
timestamp: August 06, 2014, 02:30:50 am
content: Quote from: Electro Fan on August 06, 2014, 02:26:44 amOnly some. Most MSO have a low max input voltage/ch.

author: EEVblog
author_rank: Unknown
timestamp: August 06, 2014, 03:08:54 am
content: Quote from: Precipice on August 05, 2014, 06:48:47 pmAny complex digital product these days (e.g. with an FPGA) might have 4-5 different power rails.You'll always have a need to use one probe free to probe your circuit, so with a two channel scope you only have one other channel to probe your power rails, another signal, etc. But you can only pick one. 4 channel of course means you can pick 3 other signals.I had to use two 4 channels scopes at once to measure many different power rails plus various digital signal to see is there was any correlation during a failure event. Even with 8 channels I still didn't have enough channels and had to pick which ones were likely more important.

author: VK5RC
author_rank: Unknown
timestamp: August 06, 2014, 03:49:25 am
content: Used 4 channels fault tracing a hp6632b power supply, as crowbar circuit would kick in, so needed to watch input voltage (into control section), crowbar circuit, also PS unit has a separate pull down circuit (not the correct term I am sure), 4 channels were helpful getting the sequence of events correct (or incorrect as it was!)

author: David Hess
author_rank: Super Contributor
timestamp: August 06, 2014, 04:30:55 am
content: Some oscilloscopes allow 4 channels to be used to make 2 differential measurements simultaneously without the expense of dedicated differential probes.Higher sensitivity can allow better settling time measurements assuming the oscilloscope is not overloaded. Most oscilloscopes fail miserably as far as overload recovery though.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: August 06, 2014, 05:05:30 am
content: Quote from: ovnr on August 05, 2014, 05:27:36 pmAlmost everything high bandwidth has four channels unless you're looking for something antique (CRO) or one of the slow PC sampling 'scopes'.With scopes, 2 channel is predominantly found in the low end and the portable market only.