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# How to finish this PRF protection circuit

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#### huvarda

##### Junior Member level 3
PRF protection circuit

I have a pulse repetetion frequency protection circuit (PRFPC)project. Actually I designed a five order active filter (by using two LM741) for this work. My purpose is to attenuate the input voltage of the PRFPC with the multiplied of 0.25 (If Vin=4V=> Vout=1V) at 4kHz, which is a low-pass filter.

Although the circuit performance is perfect for high duty cycle operations (>%10), it is not good for low duty cycles (<%1) because of the capacitor characteristic used in the design. Do you have better idea for this purpose?

I think the only way I can solve this problem is to use MCU. I will be grateful if you teach me how to fish.

Thanks

ealkan78@hotmail.com

what is being protected?

What are the limits of the prf detector? If these are logic level pulses or can be converted to logic levels by a comparator you can use a counter which is reset periodically and then examine the count value.

prf protection

flatulent,

The only purpose of the prf protection circuit that I used is to protect an another circuit against higher pulse repetition frequencies independent of the shape and duration of the pulse at its input. Shortly, what I need is that if PRF exceeds 4kHz, I want the circuit give 0 (zero) at its output.

Thanks

simple circuit

It lools like a simple counter circuit will work. Use a simple ripple counter like in the 4000 family and tap off the output you want. Use a 555 timer oscillator to reset the counter. feed the pulses to be kept below 4000/sec to the counter. Have the 555 frequency such that the counter overflows and produces a carry when the prf exceeds your limit.

Another approach is for the case where the time between two pulses cannot be less than a certain value. Use a one shot and an AND gate. Trigger the one shot on the falling edge of the pulse and have it the minimum time between pulses. AND this with the pulses. If one occurs too soon after the first the AND gate will have an output change.

Prf

Do you think this combination works even for very small pulse durations (0.5micro second, %0.13duty cycle) ?

If so, could you please tell me a specific counter part number that works.
Another thing, I couldn't understand the function of 555 in the circuit. How can I used 555 as reset ?

Details; the prf protection circuit is just before a monostable multivibrator (P/N:CD54HC221E) with reset. So I just need a protection circuit if pulse exceeds the pulse requiered(<4kHz) it resets the monostable multivibrator( which is also protection circuit against pulse width).

I see the problem now

I see the problem now. It is not the average pulse rate over a short time period. It is the time between any two pulses. The one shot you use is not retriggerable. This means that the output pulse number will be less than the input number for the high fequencies.

What do you want the circuit to do for higher frequency pulses? Do you want a warning that this situation exists?

If the input pulse train is a constant PRF over a short time interval, you can use a phase-frequency detector such as in the 4046 PLL IC. Put your pulse train in one input and an oscillator (the 555 timer can be wired to be an oscillator) in the other input. Filter the output with a RC network.

Since you do not seem to know about the 555 IC, a design and wiring program is attached.

prf

I just want the circuit to filter PRF higher than 4kHz. Kind of low pass filter.
thanks

fundamental limits

You are up against fundamental limits. The transition band of the filter will not be enough unless you want to threshold on 2k. That way you have an OK signal available by low pass filtering-amplify-compare.

The method I propose with the 555 and the 4064 takes only 2 cheap ICs.

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