# how to realize a variable delay element

#### shiva kumar

Joined Jun 27, 2008
11
Hi all,,
I need a "variable" delay circuit,,the delay should vary according to the pulse width of input wave,,for example,if the frequency of input wave is 1Khz,and the pulse width is 500us,the output waveform should be delayed by 250us (half of the pulse width),so the output waveform should delay the input by half of its pulse width,,i know that delay element can be realized by cmos inverters,but i have no idea how to incorporate this "variable" idea into this.
Any suggestions????

#### shiva kumar

Joined Jun 27, 2008
11
i need this for my project,there are many delay line configurations out there on the net,i am very perplexed ,i dont know which one to choose and how to make it "variable",,

#### CDRIVE

Joined Jul 1, 2008
2,219
Just so we're not chasing our tail please describe what the overall circuit does and why a variable delay is required. Many times a member thinks he knows what he wants or needs but as the thread progresses we find that the OP is barking up the wrong tree.

#### praondevou

Joined Jul 9, 2011
2,942
As CDRIVE said, we need more details. In order to help please provide the following information:

What's the application?
Will the duty cycle always be 50%?
If not shall the delay be half of the High-pulse or the Low-pulse?
What voltage are you working with?
Single supply or dual supply?
What's the frequency range at the input?

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#### shiva kumar

Joined Jun 27, 2008
11
My apologies,i am new to this forum,i should have provided complete details,we are trying to make a frequency multiplier(frequency doubler to be precise),we will make use of a schmitt trigger to generate a square wave from the input signal,this would be one of the inputs to Xor gate,the delayed square wave(delayed by half of the pulse width) will be the other input,the output of the xor gate should be a higher frequency wave but with a reduced pulse width.

The supply voltage is around 3V.
As far as the frequency range is concerned,we want to make it variable,it should work till 800Mhz,is it possible to design a delay circuit which provides the delay of exactly half the pulse width irrespective of the input frequency??

#### SgtWookie

Joined Jul 17, 2007
22,210
800 MHz? And you want a frequency doubled square wave out?
Do you know that you'd need at least 8GHz bandwidth to get something vaguely resembling a square wave out of it?
Are you certain you didn't mean 800kHz instead?

#### shiva kumar

Joined Jun 27, 2008
11
frankly speaking,,i didn't know that,,,actually i have simulated this on cadence spectre tools,,,i mean,,i applied two signals to the xor gate,one of the signals was delayed by half of the pulse width,the output frequency was doubled with reduced pulse width,,it worked for higher frequencies also,,,i didn't understand when you said i would need 8Ghz bandwidth,,could you please elaborate???

#### praondevou

Joined Jul 9, 2011
2,942
frankly speaking,,i didn't know that,,,actually i have simulated this on cadence spectre tools,,,i mean,,i applied two signals to the xor gate,one of the signals was delayed by half of the pulse width,the output frequency was doubled with reduced pulse width,,it worked for higher frequencies also,,,i didn't understand when you said i would need 8Ghz bandwidth,,could you please elaborate???