Simulation does not match real world - Page 1
Forum section: electronics / beginners

author: techninja80
author_rank: Regular Contributor
timestamp: September 05, 2022, 02:27:53 pm
content: I know I am doing something wrong just need help pointing me in the right direction. I am using microcap to experiment, and learn, and I came across a problem.The very simple below circuit does not match real world. Below I am simulating a ceramic capacitor under a signal source. When I probe the node I named "Why" in the simulation I actually get a signal. But when I do a real world test I get a flat 0 volts. Why is the simulation not matching real world. Please help as I really need to be able to depend on simulations, and I know you guys would not stand for such a discrepancy with your simulation tools

author: iMo
author_rank: Super Contributor
timestamp: September 05, 2022, 02:39:31 pm
content: What is the capacitance of your capacitor?Also your source has to have some internal impedance, like 50ohm for example.What is your probe?

author: wraper
author_rank: Unknown
timestamp: September 05, 2022, 02:43:14 pm
content: Quote from: techninja80 on September 05, 2022, 02:27:53 pmWhat capacitance? With what you make the measurement? Usual multimeter will not do the job at 1MHz frequency. Did you measure signal source directly?

author: techninja80
author_rank: Regular Contributor
timestamp: September 05, 2022, 02:47:39 pm
content: The cap is 100Pf, also the simulator says the source impedance is 1m, as for the probes I am using. I assume you mean real world then they are really cheap no name probes with 1x, and 10x. I am currently checking on 10x. Thanks for the response.

author: techninja80
author_rank: Regular Contributor
timestamp: September 05, 2022, 02:49:09 pm
content: 100pf, I used the Scope to measure it. I measured the source, as well as the point, on the circuit for "why". I have a two channel so I was able to see the input/output at the same time.

author: iMo
author_rank: Super Contributor
timestamp: September 05, 2022, 02:51:26 pm
content: The impedance of the 100pF capacitor is 1600ohm at 1MHz, so you cannot get the same voltage on the output as it is on input.

author: abquke
author_rank: Regular Contributor
timestamp: September 05, 2022, 02:51:55 pm
content: "What is real? How do you define real?"You're not modeling the output impedance of the signal generator, the load of the o-scope probe, or any of a dozen parasitic impedances.

author: techninja80
author_rank: Regular Contributor
timestamp: September 05, 2022, 02:56:50 pm
content: That is exactly why I am saying this simulation is very wrong, and I would like to know why it is soo wrong. Everyone seems to be saying that its the impedance that is not being modeled. I guess my question is, will adding a resistor for equivalent resistance will help. The model of the signal source shows that the source resistance is 1m. I assume based on this that it is modeling some at least. I guess the question is do I need more.

author: iMo
author_rank: Super Contributor
timestamp: September 05, 2022, 03:00:40 pm
content: See above my simulation.In real world you have to see the same as in the simulation.First you have to fix your simulation, then start to think about your real world.Also do switch to the LTSpice, it is free and all people here are using it without any problems..

author: techninja80
author_rank: Regular Contributor
timestamp: September 05, 2022, 03:01:25 pm
content: Thanks for the suggestion, my simulator shows the following. Should I be pumping that number up, if so by how much?

author: iMo
author_rank: Super Contributor
timestamp: September 05, 2022, 03:10:46 pm
content: RS=50and doublecheck your capacitor value - yours is most probably 100 or 100F and it should be 100p or 100pF.

author: techninja80
author_rank: Regular Contributor
timestamp: September 05, 2022, 03:32:25 pm
content: Thanks alot spot on mate. This fixed a few discrepancies with real world. Had the cap value wrong as well as the resistance. The only thing that is left is the amplitude of the Output node is still a bit of on both our simulations. Can you shed some light into why this might be. See below.

author: techninja80
author_rank: Regular Contributor
timestamp: September 05, 2022, 03:41:18 pm
content: Quote from: imo on September 05, 2022, 03:00:40 pmI have tried it, however after trying microcap I am never going back. Vastly superior especially if you want to learn, as I can literally simulate every second of the circuit and rewind time, change the circuits while live, and watch as things happen. I totally see why it use to cost thousands of dollars for a license, but now its free. So I would be crazy not to take advantage. Again thanks for all the help.

author: iMo
author_rank: Super Contributor
timestamp: September 05, 2022, 03:42:40 pm
content: Your simulation looks ok now (do compare it with mine).IN real world there are a lot of parasitic R L C everywhere in your circuits, cables, probes etc.The higher the frequency you mess with the more influence of those parasitic RLC to the measurement.Your solder-less breadboard has about 3.5pF between the rows, for example, long cables have RLC.Also all your components - even that C and R you are using - consist of many parasitic R L C.For example 100mm of a straight wire is about 0.1uH inductance..You learn to use your simulator - no problem for you to add those parasitics into your schematics - and watch what happens then..

author: capt bullshot
author_rank: Super Contributor
timestamp: September 05, 2022, 03:44:40 pm
content: This is because of the input impedance of the probe.Look up what it is supposed to be (some capacitance in parallel to 1M for a 1:1 probe, somewhat less capacitance in parallel to 10M for a 10:1 probe, but still in the singe to two digit pF ballpark).Then add an capacitor of this value in parallel to the resistor in your simulation - you should get nearer then.As 1M or 10M is large enough vs. your resistor in the circuit, you can skip the resistive part of the probe impedance, but the capacitance of the probe isn't small enough vs. your 100pF capacitor.Edit: as said by imo, there'a lot of parasitic impedances with your test jig, that'll give different results than the simulation, too.

author: iMo
author_rank: Super Contributor
timestamp: September 05, 2022, 04:02:05 pm
content: Your circuit with C1=100pF and resistor R1=1k looks in the "real world" as follows (an example only):

author: MrAl
author_rank: Super Contributor
timestamp: September 05, 2022, 08:15:17 pm
content: Quote from: techninja80 on September 05, 2022, 02:27:53 pmYou'll see some differences between spice simulations and real world measurements for a number of reasons.One is just tolerance of the parts.The other is that spice uses 'models' that mimic the real world parts and that means they are not real world parts they are a mock up of the main behavior of the parts.There are different levels of models also. Some are developed from the physics behind the part and others are just developed from a general behavior of the part. The previous are the best but the behavioral models are very useful anyway especially in theoretical settings where the general operation is being studied.

author: Infraviolet
author_rank: Super Contributor
timestamp: September 22, 2022, 01:21:57 am
content: Simulations not matching the real world, remember: Theory and practice are the same thing, in theory. In practice, less so.