Do I need to have a Oscilloscope device if I learn how to simulate/analyze circuits in LTspice?!?!

Thread Starter

nornandxor

Joined Dec 11, 2017
148
Hi,

Just as in the title for this thread, do I need to have a Oscilloscope anymore device if I learn how to simulate/analyze circuits in LTspice?!?!

V.R.
nornandxor
 

danadak

Joined Mar 10, 2018
3,628
Yes, to confirm the simulation is correct, accurate. If doing actual circuit
design and bench testing.

Free oscilloscopes (low frequency) -

The following for people w/o scopes, but the software also have sound generators -

If you don't have a scope, but have a PC (limited, will not be able to see high speed
transients, but still somewhat useful) -


You can start with a PC sound card based scope for free. Will give you basically
audio range scope, spectrum analyzer, and function generator all using your
PC sound card.


https://www.zeitnitz.eu/scope_en


http://www.zelscope.com/


http://www.ledametrix.com/oscope/


http://www.virtins.com/downloads.shtml


But first build a simple circuit to protect sound card inputs so you do not
ruin from transients, overvoltage. Google "protect sound card input".


For example http://makezine.com/projects/sound-card-oscilloscope/


Sound card impedance bridge -


http://www.marucchi.it/ZRLC_web/ZRLC/Steber_An_LMS_Impedance_Bridge.pdf


http://www.sillanumsoft.org/ZRLC.htm

Regards, Dana.
 

crutschow

Joined Mar 14, 2008
23,538
A real circuit will generally operate close to the simulation if all parameters are included, such as voltage and power limits (which LTspice ignores and will happily simulate a small transistor dissipating a 1000 watts) along with parasitic capacitance and inductance.
But there will still be differences between the sim and a real circuit.

So you can build circuits without an oscilloscope (and many do), but it can be difficult to troubleshoot problems or discrepancies from the simulation in the real circuit without one.
 

OBW0549

Joined Mar 2, 2015
3,028
Just as in the title for this thread, do I need to have a Oscilloscope anymore device if I learn how to simulate/analyze circuits in LTspice?!?!
Yes. As the saying goes, "stuff happens." And when stuff happens you need an oscilloscope to figure out what went wrong, not Spice.
  • Spice doesn't always simulate accurately
  • Spice can't simulate all circuits
  • Spice can't simulate the effect of parasitic capacitances, inductances and leakage currents, unless you include them in the simulation
  • Spice component models are sometimes not realistic, and often don't model every aspect of component behavior
  • Components sometimes fail
  • Components sometimes do not meet their specifications
  • Wiring errors and bad connections do happen from time to time
  • Sometimes there is electrical interference
  • And all manner of other bad things sometimes happen...
 

danadak

Joined Mar 10, 2018
3,628
One other comment on simulation. Manufacturers do not, in general, give
precise models for their parts because of trade secrets a model could
disclose to a savvy silicon design engineer.

In NSC there was always a raging debate about accuracy of sim vs proto and
measure in the linear group.

In general the trend is getting better for complete models, but still some
way to go. For example, in CMOS R-R opamps there is a technique used
with N & P MOS transistors to effect R-R behavior in the input stage. But
most manufacturers rarely disclose that creates a cross over distortion.
Most applications will not be affected by this, but high res del sig A/Ds, for
example, can see this distortion.

So buyer beware on models, may the force be with you.

Regards, Dana.
 

dendad

Joined Feb 20, 2016
3,084
I hear oscilloscopes refereed to as "truth meters" and that is pretty well it.
Maybe I should have a look at the sim software but I have never in my (long) electronics career used any.
My method is to build it and see. Mostly with an oscilloscope. They are a necessary tool in my opinion. So much can be observed by looking at the wave shape. I have 2 in use almost constantly, and another on the shelf. This is not counting the 2 USB ones, or the other 2 that have died and are waiting for me to get around to fixing them.
 
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ebp

Joined Feb 8, 2018
2,332
I regard an oscilloscope as essential. Far more important than the oscilloscope is the development of the mental skills and physical circuit work abilities essentially to debugging and troubleshooting.

I've done a lot of switchmode power supply design. I hate switchers. I really really hate them. In the short time I've been around AAC, I've seen lots of people undertake various switcher projects. Many of them have done simulations that look good. Then they try to make the circuits work. Translating a working circuit on paper to a real functioning power supply can be a very painful process. An oscilloscope with the right accessories is essential to identifying and correcting all the problems that exist because next to nothing in the circuit is more than a vague approximation of the ideal component you would like it to be. Even a piece of wire or a track on a PCB can be a problem.

Good simulations can be a great help and can relieve the need for some complex and tedious calculations. But to be sure your simulation is not misleading you, you need to develop a "feel" for the circuit and a recognition of how even simple passive components differ from ideal and how those differences influence performance. For example, in a simulation you might place a 10 µF capacitor. Unless you edit the model or add all the "extras" you'll get an ideal capacitor. Then you build the circuit and put in a 10 µF Z5U surface mount ceramic cap. It comes with inductance in series with it, resistance in series and across it, dielectric absorption, a large non-monotonic temperature coefficient of capacitance and a really nasty negative voltage coefficient of capacitance. It sort of resembles a capacitor. If your circuit actually requires 10 microfarads of reasonably nearly ideal capacitance, it just isn't likely to work with that Z5U fugitive from a pottery works. When you go in with a good understanding of how the circuit should work and good troubleshooting skills using decent instruments, you have a chance of at least identifying the fact the cap is a problem. Of course your simulation would have made that plain too, provided the model you used for the capacitor was correct. If you have a model for a cow, don't expect it to tell you how a horse will perform.

Somewhere out there is a story told by cranky ol' Bob Pease, a well-known character in the analog world (author of lots of things well worth the reading if you are interested in analog design). The story is of some young engineer who proudly announced some huge improvement he'd come up with in some fairly standard circuit - I can't remember the details. He did this with simulation. Problem was he had used m instead of M (or vise versa) in the value for some part - he was off by 9 orders of magnitude. Oops.
 

crutschow

Joined Mar 14, 2008
23,538
Maybe I should have a look at the sim software but I have never in my (long) electronics career used any.
My method is to build it and see.
Well, if that works for you, that's great. :cool:
But I never build a circuit without simulating it first. It saves me a lot of time, work, and grief (mistakes in my design).
And it allows me to easily try different variations of the circuit to optimize its operation.
But perhaps I'm more mistake prone in my design process than you are. ;)
 

dendad

Joined Feb 20, 2016
3,084
edp is right. And the skill to understand what it is telling you.
I too have a bit of a dislike for switchmode power supplies. It is quite interesting to see how many newbies try to make one. I have memories if flying hot metal and plastic ;)
They are not as simple as they look.
And a common oops with an oscilloscope (I'm old enough to still call them a "CRO") is putting the ground clip in the wrong place.
Sometimes just noise, sometimes smoke!
Just like any tool, there is a learning curve.
 

dendad

Joined Feb 20, 2016
3,084
Well, if that works for you, that's great. :cool:
But I never build a circuit without simulating it first. It saves me a lot of time, work, and grief (mistakes in my design).
And it allows me to easily try different variations of the circuit to optimize its operation.
But perhaps I'm more mistake prone in my design process than you are. ;)
It is probably a good idea for me to have a go with a simulator. Of course, when I started, they were not around.
Valves were, and transistors were just new ;)
I can remember being REALLY impressed with the 78xx regulators. They made it so easy to build a power supply.
I'll have to hunt for some sim software that runs on a Mac. Or maybe they are web based?
 

crutschow

Joined Mar 14, 2008
23,538
Of course, when I started, they were not around.
Valves were, and transistors were just new ;)
Same here, so we must be contemporaries. :cool:
I'll have to hunt for some sim software that runs on a Mac. Or maybe they are web based?
Several on these forums, including myself, use the free LTspice from Linear Technology/Analog-Devices, which I believe runs on the Mac.
It's one of the best free Spice simulators.
It has a somewhat steep learning curve, as do most Spice simulators, but I think you will find it worth the effort.
 

ebp

Joined Feb 8, 2018
2,332
I used simulation a moderate amount in my final year or two of circuit design. I would have started using it some decades ago, but just about the the time I decided I'd buy, microsim decided that the analog portion of pspice should only be sold bundled with the digital portion & some other stuff and the price went up to, iirc, something around ten thousand dollars. I decided against it.

A friend who has consulted or worked for several different companies over the past couple of decades has repeatedly remarked about people he worked with who were pretty sharp and good designers but who had very limited skills in actually debugging circuitry. There seems to be some sort of barrier that is fairly common. I suspect that most freshly minted engineers simply haven't had enough bench time to develop the skills. I have no idea how well it is taught in universities. Probably, like many things, some people pick it up on their own very well, some never manage to and some benefit from working with people who are both very good at it and very good at teaching it. One of the reasons for my great admiration for Jim Williams was his ability to teach people how to actually do things with his writings - from his little cartoon monkeys to his photos of how to connect your scope probe to a circuit. If ever there was a person who really knew how to use an oscilloscope, it was Mr. Williams.
 

danadak

Joined Mar 10, 2018
3,628
Pease and Williams (more so) were lab guys. Williams was anti s plane analysis and advanced
math as well.

Pease evolved over time and became more tolerant of SPICE, Williams not so much.

Both enormously talented, Pease a heavy touch of scientist, Williams a master of deep
insight and intuitive solutions.

I think it takes people of all stripes to make a profession great, was very fortunate to
have known Pease on a somewhat regular basis, he was a couple of desks away.

The industry needs more transparency on analog, whats so secret these days about
OpAmps, and that should lead to better models for our sims.

Regards, Dana.
 

HW-nut

Joined May 12, 2016
94
I was lucky enough to share a beer with Bob P in the hospitality suit for a analog design conference. I think it was in the early 90’s. I remember Bob asking me about the earliest feedback system’s.

His example was litterly the boy pulling the bull by the nose ring as a speed regulator??
 

Doros

Joined Dec 17, 2013
128
I believe using an oscilloscope is a must for people new in analogue like me.

You understand and learn so many things that simulation never reveals. For example noise from a power supply, which can affect badly your circuit.

There are so many offers for descent oscilloscopes today in the market that you must try and get.

This is the experience from a guy new in the electronics world.

Doros
 

MisterBill2

Joined Jan 23, 2018
4,055
Take a page from Bob Pease's notebook. Always test the real circuit. He would never trust a simulator.
Mr. Chips is totally correct!!! Unless a simulator uses PERFECT equivalents of every device the very best results can be fairly close, or very far off. Simulation is OK, usually, for detecting errors but that does not mean that it can show the more subtle problems such as drift and heating.
 
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