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Forum: µC & Digital Electronics Lowpass + JK-FlipFlop


Author: naz (Guest)
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Hello,

is anyone of you experienced at using a JK flip flop after a lowpass 
filter?
(output signal of the lowpass filter connected to clock input of a JK 
flip flop)

I have to filter a 1µs - pulse out of a signal that consists of 10ms - 
pulses, and the filtered signal has to be applied to the clock input of 
a JK flip flop. But as soon as a capacitor is connected at the clock 
input, the flip flop isn't working anymore and is submitting a strange 
output signal.
I tried several filter configurations (active and passive, first-, 
second- and third-order) and three different flip flop ICs (74LS73, 
74LS107, 74LS112), but nothing worked properly.

Are there additional conditions that I have to mind when using a JK flip 
flop after a lowpass filter?


Thanks

Author: Uwe (Guest)
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If you mix Analog and Digital Components you have to be sure not to use 
the Digital parts in the Analog range. That means you have to use an 
Shmitt-Trigger like the 7414 or a Comperator. But I think you should 
explain what you want to do.

Author: naz (Guest)
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I am using the JK flip flop as a frequency divider, and have to use a 
lowpass filter to remove unwanted interference pulses, because the 
voltage of this pulses is high enough to trigger the flip flop.

I tried to replace the JK flip flop with a D flip flop (74LS74), and it 
seems like that this flip flop doesn't show the strange behaviour!

Author: Abdul K. (ehydra)
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Some FFs have Schmitt-Trigger CLK input, e.g. 74HC74. They work after a 
LP filter in a real circuit and in simulation - definitely!
With LS devices I see problems because the input is not so good 
decoupled from the internal next stages as in CMOS.
Try to reduce the impedance of the LP filter. This may help to force and 
hold the input voltage for the FF.

Ground bounce noise can be a problem with very fast logic on breadboard 
or very poor routing.

Author: naz (Guest)
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Thanks for your responses, I will use 74LS74, there are no problems with 
this IC.

Author: Abdul K. (ehydra)
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Next time use CMOS. It is much easier to handle. Have fun.

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