pulse delay problem between 2 schmidt trigger segments

[Dialtone]

Can anyone steer me in the right direction on why I am having problems delaying a pulse using r/c configuration between two 74ls14 schmidt trigger elements? Typical pulse width is ~1 us and I need a ~250ns delay. I have tried using 100pf cap and 2.2K resistor but can see only small offset from input signal. Stepping resistor to 4.7k produces no output at all so I assume I am dropping voltage too much to trigger second stage. Also tried varying the cap but see no noticeable improvement either. Should I just drop the schmidts and replace IC's with standard hex inverters to get the desired results? Any help would be appreciated.
Dialtone

 

[laroche73]

schmitt trigger delay

You may have better luck using an CMOS or HCMOS version of the '14. The input impedance of a 74LS14 is fairly low (< 10K). A 74HC14 (or 74C14/CD4584) has a much higher input impedance, and the nominal switching threshold is also closer to one RC time constant (3.25V/5V = 0.65; 1 TC = 0.632).

Note that the 74HCT14 has TTL level input thresholds, so the low-high switching point would be closer to 1/2 time constant (1.8V/5V = 0.36; 0.5 TC = 0.393). I'd use the HC version for the extra range.

 

[Dialtone]

Thanks for the suggestion. After studying data sheet for the 74ls14 closer, I see the it has some fairly tight switching thresholds and my delay calculations were based on a 1 time constant value which will not work for the desired time needed.
Dialtone