Hello,
I’m completely new to KiCad and was trying to design a ring oscillator using a CD4007UBE CMOS dual inverter. However, I realized that there isn’t a SPICE model available for it, and I wanted to be able to run simulations.
I found an easier approach using three CD4049UB hex buffer inverters, but I’m unsure how to properly connect VDD and GND. I assume I need to set up a virtual VDD and GND, but I’m not sure how to wire them correctly. Any guidance would be greatly appreciated!
You need to learn about symbol units. In your schematic you have actually placed 3 separate ICs on the schematic. You would use a single IC in real life. The power unit is the one you connect the power to.
First, you need to use just one CD4049 and place all seven (!) units on your schematic sheet.
The units will be named U1A, U1B, etc. Use units A, B, C for your oscillator and park units D, E, F somewhere in a corner of your sheet and ground their inputs.
The seventh unit G is for attaching power. For that you’ll need a Simulation_SPICE VDC source from the library.
That should get you started. Report back if you need more info.
I do think it is also OK to make a ring oscillator using 5 of the inverters. I am not sure that there is any advantage to doing that, other than the waveform would probably be more of a square wave and less of a triangle or sinusoid. But as long as the hex inverter is going to drive itself crazy, you might as well get as many of the inverters as possible in on the act, right?
The other challenge is to make an oscillator with only one inverter. It can work but it probably produces electrical overstress so is particularly not recommended if you want the IC to last.
I wound some turns of wire around a pencil and connected that air core inductor from input to output. The capacitors are not much more precise than the inductor: Connect maybe 1 nF from output to ground and 3.3 nF from input to ground. Voila! A lousy and totally illegal resonant oscillator using a logic inverter. I think I did it many years ago with schottky TTL.
Loop delay goes up, frequency goes down.
Use one remaining inverter to square up the waveform.
Yup. But is that better? You get to invert two more times…
Depends on what frequency you want. Usually ROs are used to get an idea of the propagation delay of a logic family. But ring oscillators are affected too much by the environment. If it’s stabilty you want, crystals are cheap enough. I did some experiments here with an obsolete logic family: DTL. I also tried with LSTTL and got 3x the frequency.
Not sure why you would want to simulate a RO though. It’s pretty certain that it will work in real-life. Also buffered vs umbuffered will affect the freuquency.
Sorry, this isn’t much to do with KiCad now. Let’s leave it here.
What happens in simulation depends on the model. A purely logic model will just oscillate at a frequency determined by propagation delays.
An analogue amplifier model, as you might use with the CD4007, takes ages to start as only noise triggers the simulation and takes a large number of cycles to build up. Then you usually find that the model does not include the input protection diodes and gives the wrong result.
The CMOS inverter as an amplifier trick used to also be common in CD players to amplify the photo diode outputs.
I’m very grateful for the help. Based on what I’ve learned here and the information I’ve gathered, I’ve managed to come up with a partial solution. However, I’m having trouble with the simulation. When I try to run it, nothing really happens, and I’m not sure if it’s due to the surrounding circuitry not being properly connected. Is there a way to isolate the ring oscillator and simulate just that?
Did you attach a model to the symbols?
Search the manual for PWR_FLAG.
Right. And while I would think that the CD4049 ought to work, it looks to me like that is intended to drive TTL or DTL. (Gosh I was an engineering intern in 1971 and started out with 7400 series TTL illogic. I have never seen a DTL gate.)
Anyway…what happens if you use a CD4069 model?
CD4069 is a hex inverter. If you have a hex,
the CD4069 will invert it for you.
It’s a bit confusing. I’ve used Multisim to simulate circuits before, and it’s more straightforward since you don’t have to import SPICE models manually. I believe I’ve resolved the ERC issues, but the simulation still won’t run, and I’m struggling to understand what the problem is.
I’ll repeat my question: did you find a model for the CD4049 (you’ll need to search the web) and did you attach it to the symbol?
And now this has absolutely nothing to do with Kicad, in fact apart for a tenuous link to ‘simulation’ it never has. I think that the category ‘Projects’ needs to be defined more clearly as I for one don’t mind answering post like this if I choose to and are able. The current situation is confused and the resident Pussycat of leisure pointed it out ages ago.
I disagree. The questions are mostly to the point, but not all answers are, eg, infantile references to hex bolts and circuit suggestions. But they didn’t come from the OP.
I found the SPICE model for the CD4049. The issue was actually related to the oscilloscope I was using to analyze the oscillator’s transients. Apparently, it doesn’t work properly in KiCad because the software recognized it as a component rather than an instrument. Now, my main challenge is setting up a SPICE directive, but I’m running into issues with the simulation crashing. I’m also not sure if my SPICE netlist is correct.
Error from simulation - No. of Data Rows : 0
Operating point simulation skipped by ‘uic’,
now using transient initial conditions.
No. of Data Rows : 0
doAnalyses: TRAN: Timestep too small; time = 1e-21, timestep = 1.25e-22: trouble with xu5.xu1:d1-instance d.xu5.xu1.da2
run simulation(s) aborted
Spice netlist -
save all
.probe alli
.probe p(V1)
.probe p(XU5)
.ic V(Out)=0.5V
.tran 0.1n 100u uic
.options reltol=0.01 gmin=1e-12 abstol=1e-9 vntol=1e-6
.plot V(Out) V(A)
.OPTIONS numdgt=7
.save all
.tran 1n 100u 0 1u
V1 Net-U5G-VCC GND DC 15
XU5 Net-U5G-VCC Net-U5-Pad2 /Out Net-U5-Pad4 CD4049UB
.end
First, try with this schematic. It has 0 ERC errors and has everything you need for simulation:
You may have a look at Simulation examples for KiCad/Eeschema/ngspice - #4 by holger (scrolling down a bit, made with KiCad 6).
