help for duty cycle modulation

[mmajid]

I have a 30MHZ clock oscillation feeded to a 74HC4017 decade

counter in order to obtain some lower frequency pulses like

15,10,5,and 3 MHZ square waves.

But the problem is that in this process 4017 also lowers the

duty cycle by the same factor( 1/2, 1/3 or 1/6 for 15, 10 and 5MHZ outputs).

How can I keep the duty cycle at the 50% level or in other words,

how can I controll the duty cycle of the output pulse at the desired level.

 

[akg]

u can use a D Flip Flop, to get 50% duty cycle, but the freq will be halved.

 

[ljcox]

The 74HC4017 has decoded outputs and is not suitable for your task.

You need circuits configured to divide by 2, 3, 6, & 10.

However, if you divide by 3, it will not be 50/50. But the others will.

Search this forum for "Flip Flops" & "counters".

The 74HC390 is a dual decade counter that can be configured to divide by 2, 5, 10, 20, 25, 50 or 100.

To divide by 3, you need a dual JK Flip Flop.

 

[mmajid]

Thank you very much for your reply. I am not an electrical

or electronic engineer,and unfortunately my electronic

knowledge is very limited.


I need a circuit capable of producing square wave signals

(0 to +5V pulses) with some fixed frequencies like 30,20,15,10,5,1 MHZ

(the exact value of them is not so important, for example 9 ~ 11 MHZ

is acceptable for 10MHZ ).


I found the attached schematic on the web.the circuit

produces square wave pulses by means of a 1MHZ crystal oscillator

based on 4584(IC1), and then successively divides the frequency by

10 by means of 4017(IC2~8), and obtains 100k,10k,1k,100,10,1Hz square pulses.


I had tried to use this circuit for my purpose, but the problem was

that the maximum input frequency for 4584 is about 2 MHZ.


because 74HC4017 supports high frequencies( >50MHZ ),can I replace

4584 based crystal oscillator with a single 33MHZ oscillator and

then feed the oscillator signal to 4017 to obtain square wave

pulses whith frequcies like 33/2, 33/3,...33/10 MHZ ?


Why you said me that 74Hc4017 is not suitable for this purpose?

I dont need hex or binary outputs,I want to obtain analog

outputs from 4017 (I think decoded means analog in this situation,

is it correct or not).

I will be very very pleased if you can help me in this way.

 

[ljcox]

You wrote that you want the output to provide a 50% duty cycle.

The 74HC4107 will only provide a 50% ds for divide by 2.

Here is a divide by 3 counter with a 50% duty cycle output.

You can use a D type Flip Flop to divide by 2. Connect Qbar to D.

Or you can use a JK flip flop - connect the J & K inputs to +5V.

The set and reset inputs must be connected to +5V.

 

[audioguru]

Hi Len,
The %10 OUT pin 12 of a 4017 divides the clock by 10 and makes a perfect square wave.

 

[ljcox]

Hi Len,
The %10 OUT pin 12 of a 4017 divides the clock by 10 and makes a perfect square wave.

G'day Audio,
That's true, but he wants more than just modulo 10.

 

[mmajid]

some other questions

Hi dear ljox

thank you very much for your reply and schematic.

but you dont said me that if I can use a DIP 33MHZ oscillator(instead of a crystal based

RC osillator+ 4584) and a 4017 to obtain the desired frequencies or not?

The other question is that what can I do for 1/4, 1/6, 1/8 outputs.How can I turn back

the duty cycle of them to the 50% level?

It has been mensioned on the phillips 74HC4017 datasheet (the attached paragraph)

that the output pulse withs (duty cycle) can be increased by inserting a RC circuit

on the MR input(PIN 15) of the 4017.How it can do this job, and what specifications

it must have? Can you draw the schematic of such a circuit?

On this schematic we can see that the MR pin is connected to a ouput pin.I think

that this will reset the counters successively.what will be the effect of it

on the output?

 

[ljcox]

Re: some other questions

Hi dear ljox

thank you very much for your reply and schematic.

but you dont said me that if I can use a DIP 33MHZ oscillator(instead of a crystal based You need 30 MHz, not 33.

RC osillator+ 4584) and a 4017 to obtain the desired frequencies or not?

The other question is that what can I do for 1/4, 1/6, 1/8 outputs.How can I turn back

the duty cycle of them to the 50% level?

It has been mensioned on the phillips 74HC4017 datasheet (the attached paragraph)

that the output pulse withs (duty cycle) can be increased by inserting a RC circuit This would not be very accurate.
And you would have to have a different delay for each frequency.
on the MR input(PIN 15) of the 4017.How it can do this job, and what specifications

it must have? Can you draw the schematic of such a circuit?

On this schematic we can see that the MR pin is connected to a ouput pin.I think

that this will reset the counters successively.what will be the effect of it

on the output?

The 74HC4017 is the wrong IC for your purpose.
You can obtain a 50% ds with the 74HC390 that I mentioned.
For the modulo 3 & 6 cases, you can use the modulo 3 counter I posted in my previous post.