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Products => Test Equipment => Topic started by: Old Printer on March 29, 2019, 07:54:14 am

Title: Buying a 2CH vs 4CH Scope
Post by: Old Printer on March 29, 2019, 07:54:14 am
For the past couple years I have been following many of the "What scope should I buy" threads as I am looking for my first digital bench scope. I have a couple old analog TEK's and a couple Analog Discovery's to hold me until I finally pull the trigger. Most of the threads are consumed with the "Rigol vs Siglent vs ???" subject, but the "how many channels do I need" question shows up a lot, but dealt with only briefly. The general consensus leans towards the more the better, or are you going to look at digital signals?  Many beginners on a budget opt for 2 in the end because they can't see a reason for 4. I always cringe a bit, but keep my mouth shut because it's usually a done deal by then and I have been broke too. Anyway, over in the Repair forum member Sparky is trouble shooting an old HP 6227B PS (2 CH Tracking) link below. He is tracking down an oscillation and has the "need" to look at the signal at three different test points, and points out how nice it is to be able to look at them simultaneously on one screen. I am not saying this was a make or break for the repair, but it was an analog use for more than 2 channels. I am sure there are many more, if you know of one please add it here to help future scope buyers decide.

Maybe this thread will be a place to discuss and illustrate this topic, and keep it from being buried in the war of Chinese scope maker threads :) 

https://www.eevblog.com/forum/repair/hp-6227b-dual-dc-power-supply-ripple-in-master-channel/msg1658462/#msg1658462 (https://www.eevblog.com/forum/repair/hp-6227b-dual-dc-power-supply-ripple-in-master-channel/msg1658462/#msg1658462)
Title: Re: Buying a 2CH vs 4CH Scope
Post by: 0culus on March 29, 2019, 08:45:29 am
Having 4 channels available is almost a must IMO. I used all 4 on my 2465B when I was troubleshooting a problem in a 3325B function generator a while back. Ch3 and Ch4 are limited to 0.5 and 1 volt/div settings on this scope, but they are still handy to have. I'd rather have it and seldom need it than end up in a situation where I need it and don't have it. For multitudes of digital signals, I have a Tek TLA 715 logic analyzer.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: nctnico on March 29, 2019, 08:52:17 am
IMHO it depends on what you are working on. For most repair work 2 channels should do the job. When designing electronic circuits 4 channels (or even more) is very convenient. I can probably put an 8 channel scope to good use...
Title: Re: Buying a 2CH vs 4CH Scope
Post by: GregDunn on March 29, 2019, 08:52:39 am
Generally if I need more than one input, 4 is barely enough.  I kept my old Tek 922 just so I'd have spare inputs available for those eventualities.

Any time I'm troubleshooting an analog amp or a piece of test equipment, I need to look at the input and output for the section I'm working on, plus at least one more channel to follow the signal in between, and possibly control or feedback signals.  At work, doing computer stuff, we frequently ran out of channels looking at control signals and I/O; you can't use a logic analyzer to tell you if a signal has ringing or slow rise time.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: TK on March 29, 2019, 09:20:57 am
This is my experience... my projects are 99% digital (MCU):

1 Channel: 60%
2 Channels: 35%
3-4 Channels: 5%

It all depends how important your 5% is.  I used it mostly to do timing analysis between sensors and the output from the MCU based on some computation (performance tuning, minimum timing constraints, interrupt analysis, etc).

I recommend buying the 4-channel scope, because you will have those 5% moments when you will regret not having the extra channels.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: AG6QR on March 29, 2019, 09:22:05 am
I have used 2-channel and 4-channel scopes, and I can't recall a time when the extra channels made the difference between possible and impossible.  But I can recall many times they made the difference between easy and hard.

One example:  I was using a ham radio transceiver with an external amplifier, transmitting Morse code.  The output was changing in amplitude suddenly. (to reduce bandwidth, good Morse code transmitters avoid instantaneous on/off transitions, taking a few milliseconds to ramp the amplitude up or down.)

To diagnose the problem, I had to look at the RF input to the amplifier, the RF output, and the line between the transceiver and amp which tells the amp when to switch between transmit and receive mode.

At the time, I only had a 2-channel scope.  A 4-channel unit would have cut the diagnosis time to a fraction of what I needed with that 2-channel, because 2 channels forced me to go back and forth several times between the various signals to see the relationships.

It turns out there was a firmware issue with the amplifier, where it would switch on amplification late, but only if it had recently been switched off.  A string of rapid-fire Morse dits would attempt to toggle the amp on and off too fast.

I emailed the amp designer a description of the problem, with screen captures from the scope, and less than 24 hours later, he sent me updated firmware that fixed the problem!

Details here:
https://ag6qr.net/Radio/HR50Clicks/
Title: Re: Buying a 2CH vs 4CH Scope
Post by: james_s on March 29, 2019, 11:43:31 am
If you have the budget, 4 channels can be nice. I have a 4 channel scope but I very rarely use more than 2 channels. Better to have channels you don't need than need channels you don't have. Depending on your use case though 2 channels may be enough but I would only go that route if budget mandates other priorities.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: Fungus on March 29, 2019, 12:00:27 pm
Three channels is probably the optimum but they don't make many of those.  :popcorn:

Better to get four, just in case.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: tautech on March 29, 2019, 12:21:50 pm
IMO it's not just 2 vs 4 channels but what you get in a DSO's total package as to if the 2ch and 4ch models offer the same features.
If you already own a 2 ch scope then there are ways to use another 2 ch unit to get the info you need and still get close timing relationships with Trig In/Out etc.

I can count on one hand the times I've needed 4 channels and I wouldn't have needed 4 if I used 2 scopes.
For example, a digital control circuit triggers some current draw on an attached part of a design and you need to monitor PSU behavior when it's at both levels of current draw; idle vs a digital event.

For full duplex decoding, 4 channels is a must while just 1 bus decoded with a 2 ch model still can be useful to see if you've got the code right.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: SilverSolder on March 29, 2019, 12:34:21 pm
An alternative to a 4 channel scope is a 2 channel + digital MSO.   

The mix of analog and digital on the same display is an awesome capability in its own right, but here's the thing:   The digital channels have adjustable trigger levels - so they can be used to keep track of analog signals too, in many cases.

All of a sudden, you have a LOT of channels to play with!

A MSO is a perfect tool for playing with Arduinos etc. with ADC, DAC, PWM, etc. peripherals.  For example, you can dedicate one of the digital channels to read a pin on the micro that you flip from software, so you can sync what you see on the scope with what your code is doing.  Did that analog voltage stabilize completely before you read it?  -  is it marginal?  - the MSO will tell you.  (So would a 4 channel scope in this example.  But let's say we have 10 PWM channels.  The MSO can show when all 10 pass a certain voltage, for example - in parallel!)
Title: Re: Buying a 2CH vs 4CH Scope
Post by: Muttley Snickers on March 29, 2019, 12:41:54 pm
Only on one occasion have I needed more than two channels, I was working on a timer/ counter project and ended up saving a reference waveform in memory and then used the live signals to compare against. I would expect that most current scopes have this feature and ability to recall and overlay different signals, my old Tektronix TDS-1002 has this feature along with a real time clock which some today don't even have. 
Title: Re: Buying a 2CH vs 4CH Scope
Post by: slodat on March 29, 2019, 12:51:47 pm
I’m tuning servo drives on a cnc machine. Monitoring an analog output on four motor at once, looking for the following error on each, at the same time. Completely impossible on a 2 channel scope. I use oscilloscopes for this sort of field work the most. I vote for the four channels with logic inputs.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: tautech on March 29, 2019, 12:55:06 pm
An alternative to a 4 channel scope is a 2 channel + digital MSO.   

The mix of analog and digital on the same display is an awesome capability in its own right, but here's the thing:   The digital channels have adjustable trigger levels - so they can be used to keep track of analog signals too, in many cases.

All of a sudden, you have a LOT of channels to play with!

A MSO is a perfect tool for playing with Arduinos etc. with ADC, DAC, PWM, etc. peripherals.  For example, you can dedicate one of the digital channels to read a pin on the micro that you flip from software, so you can sync what you see on the scope with what your code is doing.  Did that analog voltage stabilize completely before you read it?  -  is it marginal?  - the MSO will tell you.  (So would a 4 channel scope in this example.  But let's say we have 10 PWM channels.  The MSO can show when all 10 pass a certain voltage, for example - in parallel!)
Great points.  :-+

However when budget constrained to where a 2ch model is a so much cheaper option, don't forget to factor in the additional cost of;
1 Logic probe HW
2 MSO licensing
These additional costs will make the 4ch model the cheaper option especially in the entry level range of products.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: Fungus on March 29, 2019, 01:11:37 pm
A MSO is a perfect tool for playing with Arduinos etc. with ADC, DAC, PWM, etc. peripherals.  For example, you can dedicate one of the digital channels to read a pin on the micro that you flip from software, so you can sync what you see on the scope with what your code is doing.  Did that analog voltage stabilize completely before you read it?  -  is it marginal?  - the MSO will tell you.  (So would a 4 channel scope in this example.  But let's say we have 10 PWM channels.  The MSO can show when all 10 pass a certain voltage, for example - in parallel!)

Yep, I use my 4-channel 'scope as a debugger. eg. Flip an I/O pin in an interrupt handler when something occurs, watch it on the scope to make sure it's happening (and when).

It's not 100% about "necessity". When you have four channels you simply have more room to do stuff, it makes life easier.

Title: Re: Buying a 2CH vs 4CH Scope
Post by: Old Printer on March 29, 2019, 01:32:30 pm
Thanks for all the opinions and examples. Many first time scope buyers are not even sure why they need a scope or what features they need in it. Without a specific use for a scope, the 2ch - 4ch choice is even less clear. I was hoping to get actual examples of when the 2 extra channels came in handy or necessary, or didn't, so that beginner EE buffs could see what might be down the road for them. The responses have been great, please keep it up.  :-+
Title: Re: Buying a 2CH vs 4CH Scope
Post by: Brumby on March 29, 2019, 01:46:08 pm
This is what I ended up doing for more than 2 channels of scope:
(https://www.eevblog.com/forum/testgear/buying-a-2ch-vs-4ch-scope/?action=dlattach;attach=690303;image)

On the other hand, it's been quite some time since I've had any need to go that far.

However, if I had the funding to do so, I'd be looking at modern scopes with a definite bias towards 4 channel - even thought the vast majority of occasions, the need has only been for a single channel.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: james_s on March 29, 2019, 03:55:41 pm
The last time I used more than two channels I was looking at the clock, data and strobe lines out to some shift registers. Of course I could have used a logic analyzer, arguably a better tool for the job. At least 70% of the time I'm using only a single channel, most of the remaining is two channels.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: EEVblog on March 29, 2019, 04:03:31 pm
Go for 4CH because it's just so cheap these days.
Just being able to do basic stuff like monitor one or two power rails whilst checking some digital signals is super valuable.
Tons of stuff has more than two power rails these days, so right there a 2CH scope is limiting.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: Psi on March 29, 2019, 04:22:06 pm
Having a 2CH will annoy you to no end when you need to compare more than 2 signals.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: maginnovision on March 29, 2019, 04:44:36 pm
If you can afford it, go 4 channel. If you can't afford it, just get 2. Really recommend saving for 4 though. For the difference in price it's sort of a no brainer because just one time you need 3 or 4 channels and it's worth it.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: jackbob on March 29, 2019, 05:57:33 pm
One of the most compelling reasons to go for 4 channel (at least for me) is to get differential measurements on the cheap. Differential probes often cost more than scopes and surely more than any budget scope. With 4 channels you can get a "semi-differential" measurement using two channels and the math function to obtain the difference. Having 4 channels available means you can get two differential measurements simultaneously, with a two channel scope you can only have one. This is especially useful if you want to plot voltage and current on the same screen and don't have unlimited cash for a current probe. You can use one probe to measure voltage and two other channels to measure the voltage difference across a shunt resistor and obtain a current measurement. While you can achieve this with a two channel scopes in very specific circumstances, it is often useful to have the freedom to make differential measurements anywhere in your circuit . In any case, keep in mind all the ground clips are common.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: EEVblog on March 29, 2019, 06:24:40 pm
If you can afford it, go 4 channel. If you can't afford it, just get 2.

A 4CH scope is only US$375
Title: Re: Buying a 2CH vs 4CH Scope
Post by: jackbob on March 29, 2019, 06:48:13 pm

A 4CH scope is only US$375
I must agree with Dave here. You can pick up a "classic" Rigol ds1054z 4ch budget scope for $375 today. The price difference between that scope and any other digital 2ch budget scope is minuscule. Siglent also produces the SDS1104X-E for about $500 with MSO capabilities and "super phospher" display. The cheapest comparable 2ch scopes I can find run about $300, please do yourself the favor and spend the extra $75 for 4 channels.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: maginnovision on March 29, 2019, 07:27:11 pm
If you can afford it, go 4 channel. If you can't afford it, just get 2.

A 4CH scope is only US$375

I know, but there are those penny Pinchers out there.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: jackbob on March 29, 2019, 07:40:51 pm
I believe there are some threads floating around on the forum with discount codes for places like TEquipment and other popular equipment vendors. I have heard of people picking up 4ch scopes for under $350. You might even be able to score a used 1054z for under $300 if you are vigilant  on eBay.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: maginnovision on March 29, 2019, 07:47:39 pm

I believe there are some threads floating around on the forum with discount codes for places like TEquipment and other popular equipment vendors. I have heard of people picking up 4ch scopes for under $350. You might even be able to score a used 1054z for under $300 if you are vigilant  on eBay.

That's true. You can also pick up other used scopes for less. My cheapest scope was $1000 in 2005 (that just changed yesterday, 800$ 2019) so I'm not arguing for me, but those who want their pennies. I still strongly recommend 4 channels but if you "can't" do it then don't. I personally prefer 4 + 16 MSO for a primary scope.
Title: Re: Buying a 2CH vs 4CH Scope
Post by: NivagSwerdna on March 29, 2019, 09:14:38 pm
My 2c worth...

A few years ago I bought a Rigol 1052E which I think appeared about the time that these devices had a step change in what you got for your money.  Prior to that I used digital sampling and/or ADC with a micro controller to get some idea of what was going on but it was always a mystery and anything over trivial frequencies was a black box.  Going from zero channels to two channels was a game changer and opened up so many more projects (or actually getting existing projects to work properly).  I have never owned an analogue scope, nor would I want to since having triggers and looking around trigger points is where I tend to go. So... firstly DO get a digital scope, it will make your hobby so much more fun.

The Rigol 1052E was brilliant bang for buck and is two channels.  With two channels you can do a lot of things... specifically you can use one channel as a reference and then vary the second channel across signals.  It can be time consuming (especially if say you are decoding a 20 channel LCD driver) but it is do-able and data can be extracted, put on a USB and later combined into a multi-trace format on the computer.

So.. why not stick with 2CH... 

.. well you can but the incremental cost for a 4CH scope isn't huge now and the 1054Z was the next step change... although you do get extra channels so you can now have one reference and three others (i.e. x3 productivity) you also get extra goodies such as data decoding. As soon as you are looking at a bus which has a clock then you only get one data channel and no spares with a 2CH; with 4CH you get CLK, data and up to two interesting things... game changer.

Although I love my 1052E, I do have 1054Z envy and now it's too late for me as I have invested in my 1052E...

.. so if you can afford it buy a 4CH scope, if you cannot afford it get an extra job and save up for a 4CH and if you cannot get an extra job, buy a 2CH *MODERN* *DIGITAL* scope.... it will make your hobby so much more fun.

PS
If it was my day job then it would be 4+16 MSO all the way