digital-logiclogic-gatesmultiplexerorcadsimulation
After simulating the following logic circuit(in OrCad),the resulting output -'y' is unclear. It seems to me that everything is fine, yet the result is ambiguous.
Can you, please, explain what is the problem here ? Thank you in advance.
The Circuit:
The Stimulus Editor:
The Result of Simulation:
The solver is essentially solving a system of differential equations, and there are various algorithms for doing this, some which work better that others depending on the conditions ("stiffness" of the equation - if you know e.g. Matlab/Scilab/Octave see the various ODE solvers there for different conditions)
Depending on the circuit, the solver may have a hard time coverging, and as the Photon says shortens the time scale until it basically just slows down and stops (sometimes if you leave it long enough it will complete the "difficult" part, but often not).
This often happens when ideal capacitive/inductive elements are present, so it's always a good idea to select a series resistance for an inductor (actually defaults to 1m) and also an ESR for a capacitor. Right click on the component to set these and other values (as you probably know)
One other thing is your voltage source appears to be floating from circuit ground - add a high value resistor across the transformer (e.g. 100Meg) Without a DC path it makes it hard for SPICE to determine the nodes voltage.
The last thing I notice about your circuit is you have not selected a "real" diode - this may cause issues also. Right click and select a diode from the list available, I imagine this combined with setting some reasonable value ESR for the cap (and maybe a little more for the inductors) will make it work for either solver.
The circuit below works fine with either solver (cap has 1m ESR):
Simulation:
Where does 8.28e-19 come from?
Thermal noise of a 50 ohm resistor in a 1 Hz bandwidth at 27 degC is 9.1e-10 volts
To convert this to an equivalent power it needs squaring and this produces a number of 8.28e-19.
The formula also reduces to 20 log\$_{10}(\frac{V_{NOISE}}{9.1e-10}\$) i.e. it compares actual RMS noise against 1Hz-limited voltage noise from a 50 ohm resistor.
Best Answer
The problem is the 'VCC' element.
I intended to use 'VCC' to set input to permanent '1' logic state, but that's not the right way to do this.(Using 'VCC' element, the output of simulation, in my case y, will be two parallel red lines, one for '1' logic state and the other for '0' logic state).
As I already mentioned in this post, "The 'PULLUP_1k' part in 'DIG_MISC' library, will provide 'permanent high to an input '".
So, after I had put 'PULLUP_1k' part instead of 'VCC', the simulation result was correct.