Help needed with circuit

[fxdif]

Hi,

I need some help regarding a circuit operation. I will try to describe it as best I can.

Input is a 1:1 pulse train of 200-10Khz. This is fed into one input of a NOR gate. Output goes through a 10nF cap to a pullup resistor of 10K. This also goes through a 10K resistor to another NOR (other inout is tied to gnd). Output is fed back to first NOR gate. I dont understand the purpose of the feedback. Any help would be greatly appreciated.

                     _|_pull up
                     | |
        _____        |_|  _____     _____
input___) nor\0___||__|___|10k |____)nor \0____output
     ___)____/    ||      |____|  __)____/     |
    |              10nf           |gnd         |
    |__________________________________________|

 

[Skogsgurra]

It looks like this circuit produces pulses in the 100 microseconds range. The feed-back is there in case the input signal is shorter than the output pulse width. The 10 kohm resistor is to prevent input in NOR gate #2 from affecting timing too much. It also prevents unwanted possible latch-up of the #2 gate. That happens if the gate is an old CD4xxx CMOS gate.

Gunnar Englund

http://www.gke.org

 

[fxdif]

thanks for your reply,

so basically it generates a 100us pulse at 200-10Khz depending on input frequency?

 

[Skogsgurra]

Yes. Probably a little less than 100 us width. Period time at 10 kHz is exactly 100 us. So it cannot output 100 us there.

Gunnar Englund

http://www.gke.org

 

[melone]

Where did you get this circuit?

 

[fxdif]

it is part of a circuit we use at work for a frequency to voltage converter

 

[Skogsgurra]

OK. Makes a lot of sense. Putting an RC filter after the output will give you an analogue voltage that is proportional to frequency.

Gunnar Englund

http://www.gke.org