Mitigating modulated singal delay in CMOS multi-vibrator.

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Alex_Khan

Joined May 27, 2020
44
I am doing PVT analysis of an RC-based multi-vibrator. Simulations under varying temperature (0-100 C) and process corners (FF, FS, SF, SS) show signal delay in the modulated signals. This delay is due to the effect of temp and process on R1, C1, and MOSFET's threshold voltage.

-I used PVT independent Schmitt trigger (ST) with fully adjustable threshold voltage to mitigate the modulated signal delay, but the ST does not work in delay mitigation.

Please comment: Is PVT-independent ST a wise choice to mitigate signal delay? How can o get PVT-independent modulated signal?
Note: I implemented the circuit in CMOS 0.35 technology.

Thanks.
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crutschow

Joined Mar 14, 2008
27,190
You likely need to go with a circuit that is less affected by temperature, such as one using a low-offset analog differential comparator rather than a logic gate.
 

AnalogKid

Joined Aug 1, 2013
9,248
Schmitt trigger logic gates are extremely convenient - right up to the point where they suck. OK, that's a little strong, but a long term problem with them is that the two input transition levels are not tightly controlled during manufacturing. There can be significant variability among the individual gates in a single chip, and wide variations among manufacturers and lots; and that's before temperature changes cause them to wander around.

I'm not a huge 555 fan, but the genius of its design is that the effective Schmitt trigger function is created with two analog comparators and a three-resistor string. The absolute values of the resistors can vary a lot, but their *ratios* track very tightly with manufacturing, temperature, etc. As Wally suggests, a discrete version of this approach is a better way to go if you need any kind of stability. At 1.4 MHZ, the pool of acceptable components will be relatively small, but you will get better results.

ak
 
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