Pspice & simulation

[mohaisen]

hiii...
i have a design..that generate a sawtooth and a square wave...but i could not get any output when i simulate the circuit...anybody got some knowledge in Pspice to help?

thanx

 

[mister_e]

make sure you have
1)enabled Transient Analysis
2)use the right final time

 

[Torben]

hiii...
i have a design..that generate a sawtooth and a square wave...but i could not get any output when i simulate the circuit...anybody got some knowledge in Pspice to help?

thanx

I simmed this in LTSpice and found two things: The units for the capacitor are not given in the original schematic; I found that 1uF or 0.1uF worked OK; and 2) I needed to use +15VDC and -15VDC to make it work. However, I was using a TL072 model instead of TL082, but it should otherwise be the same.


Hope that helps,

Torben

 

[mohaisen]

thank you

I simmed this in LTSpice and found two things: The units for the capacitor are not given in the original schematic; I found that 1uF or 0.1uF worked OK; and 2) I needed to use +15VDC and -15VDC to make it work. However, I was using a TL072 model instead of TL082, but it should otherwise be the same.


Hope that helps,

Torben

thanx alot for your help...i iam just wondering if you can send me your schematic that u simmed

 

[Torben]

thanx alot for your help...i iam just wondering if you can send me your schematic that u simmed

No problem. Attached are the schematic (in LTSpice format) and the TL072 library file. Put the library file in your LTSpice lib/sub directory before trying to run the sim.

Note: I've changed R1 and C1 just to get a lower frequency (bigger C1) and sharper sawtooth (R1). Decreasing R1 makes the sawtooth more like a sawtooth but also makes the pulse output shorter; making R1 larger will make the sawtooth look more like a triangle wave and the pulse output will look more like a square wave.

[n.b.--I had to add a '.txt' extension to TL072.301 to make the forum software upload it; remove the '.txt' from the file name when you save it.]


Hope this helps,

Torben

 

[Roff]

If you decrease the value of R4, it will work on lower supply voltages (and raise the frequency).

 

[Torben]

If you decrease the value of R4, it will work on lower supply voltages (and raise the frequency).

Hi Ron,

I'd tried fiddling with that, but found (at least in the sim) that with supplies lower than +-15V, the thing would take several seconds to begin oscillating. I was at first using .tran 5, and with values of, say, 1k for R4 and rails of +-12V, it would appear to not start oscillating at all. Then I changed to .tran 25 and saw that it did oscillate, but not until almost 19 seconds. So I went with the original 100k for R4 and chose +-15V rails to ensure that even a short sim would show what's up.

I am guessing that there is in fact a way to ensure that the circuit will start up properly without the +-15V rails--in fact, my results might just be a simulation error which I don't know enough yet to diagnose--but I haven't yet worked out what that might be and all my books are at home. I get home from vacation on the weekend so I'm stuck 'til then.

I had actually thought about presenting the clever (well, I like it) circuit you presented in this thread but decided to let well enough alone since I don't stand a chance of being able to explain that one just yet. [Edit: another reason I skipped it is that it requires a sine wave as input and I had added a square wave out in my own simulation of it just to see if I could, so it's not really applicable here anyway. Oops.]


Torben

 

[Roff]

One way of getting it to start (usually, anyway) is to make the power supplies PWL sources. I tried it with a 1mS risetime - not so good. I changed it to 1uS and it worked pretty well.

 

[Torben]

One way of getting it to start (usually, anyway) is to make the power supplies PWL sources. I tried it with a 1mS risetime - not so good. I changed it to 1uS and it worked pretty well.

Weird. Never would have guessed that one. I'll give that a try (soon as I get Mom's Leopard MacBook talking to her printer over Samba).


Thanks for the tip!

Torben