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| can low pass filter change signal width? |
| (1/3) > >> |
| elcrni:
hi all, So i have a device that should serve a purpose of making a radio signal more resilient to all sorts of noise but after a lot of tests i've discovered that my output signal is "wider" that it should be. How it works, i have a DCF77 (atomic clock time transmission) receiver that connects to this sort of filter and amplifier to remove any noise and to make a stable signal output. DCF77 is a 1Hz signal and every second 100ms and 200ms pulses should be received. This is of course never exactly 100 and 200 ms but should be very close. when i connect DCF receiver alone i get pulses 95-205ms and 195-205ms and this helps my MCU easier to recognize and decode BUT, then when i connect the filter device, my output is on average 130ms and 230ms, so the signal is now wider.. very stable but too wide. I am trying to figure out what is making this signal wider, not so much to save my project but i really want to know how this works and why. Anyone has any clues? Schematic attached. many thanks, Alek |
| fourfathom:
What does your filter circuit look like? If you run a pulse through a low-pass filter you will reduce the amplitude and stretch out the duty-cycle. With enough filtering you will transform a narrow pulse into a sine wave with 50% duty cycle. |
| elcrni:
thanks fourfathom! --- Quote from: fourfathom on April 25, 2021, 06:15:56 pm ---What does your filter circuit look like? --- End quote --- on above attached schematic, filter is R6 and C5, both seem rather high in values, to my untrained eye... many thanks, Alek |
| bsfeechannel:
A low pass filter will invariably widen your pulses if the decision point on the pulse regeneration is closer to ground (as in the case of the figure below). R7 is pulling up the negative inputs of IC1(pins 2 or 6), which will shift the decision point away from 1/2 UB+ provided by R5 and R4. |
| fourfathom:
--- Quote from: bsfeechannel on April 25, 2021, 07:44:44 pm ---R7 is pulling up the negative inputs of IC1(pins 2 or 6), which will shift the decision point away from 1/2 UB+ provided by R5 and R4. --- End quote --- R7 may have that effect, or it may have no effect at all, it depends on the characteristics of the pulse source. Your 100K / 0.1uF filter gives the 100ms and 200ms pulses enough settling time to where the distortion with a 50% comparator threshold should be minimal. But this will only be the case if the input signal is a 0V / 5V signal. If it is anything else then your 2.5V threshold will give you pulse shrinkage or expansion. |
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