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LM2907 input frequency with duty cycle >80%

Chandan N
Prodigy 50 points

Hi,

Am using 14 pin version of LM2907 IC. Input frequency is RPM feedback from a tachometer. Maximum input freq expected is 13 kHz. 

Circuit works fine over full range of frequency with square wave input with 50% duty cycle.

Practically, tachometer feedback thru optical pulse pick up then converted to electrical signal fed as input signal to LM2907 which has >80% duty cycle.

Ref Voltage at 11 Pin :1.2 V

Input freq signal amplitude 0 - 3.3V.

Supply Voltage :24V 

C1 :470pF, R1: 10kOhms.

With 80% or more duty cycle, output voltage saturates around 3kHz frq. 

Please help me out to solve this.

Thanks & Regards,

Chandan N