--- In LTspice@..., "zeeglen" <glen@...> wrote:
Hi all. I've just uploaded a file "Monostable VCO Schmitt problem.asc". The simulation works when a INV is used, but has problems when a SCHMITT inverter is used instead. Both use default values, and as far as I can tell they are identical other than the SCHMITT is about 4 usec more delay. The SCHMITT has no visible hysteresis when driven from a sine source.
V2 on the far left is the VCO control voltage. Up top there is a choice between INV A1 and SCHMITT A2. As long as INV A1 is in the circuit it oscillates with V2 as low as 0 volt. But if A1 is disconnected and SCHMITT A2 connected in its place the circuit does not oscillate when V2 is below 1 volt. The falling ramp from opamp U2 appears to get too small to cross an assumed lower trip point, yet there doesn't seem to be any difference between trip points; I have not changed Vt and Vh from the SCHMITT default values (not sure how).
I am trying to replicate the Schmitt action of the trigger input of a 74HC221 monostable with the LTC6993-1 and the INV along with Q1. R10 and R11 are there to divide the opamp U2 output by 5 to switch at the A1 A2 threshold of 0.5 volt when U2 is 2.5 volt.
Can anyone explain why the difference between INV and SCHMITT? Maybe I do need to set a value of 0 for Vh. Also tried SCHMTINV (A3), it has the same problem
Thanks in advance.
Hello,
I just tried your example.
Right-mouse-click on the Schmitt-inverter.
SpiceLine: Vt=0.5 Vh=0.1m
I remember a case where I additionally needed a small delay.
SpiceLine: Vt=0.5 Vh=0.1m Td=10n
Best regards,
Helmut
