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[Ideas For Design]
An accurate analog delay circuit

Contributing Author  |   ED Online ID #6348  |   December 1, 1997


There are several ways to introduce an analog delay into a signal channel. If you have enough room in your product, an appropriate length of coax cable can be introduced. Inductorcapacitor delay lines can be purchased. My preference is to create the delay with active circuitry, which doesn’t take up much space and can be designed to precisely implement whatever delay time is needed. This approach could even be made voltage-adjustable since there’s only a single resistor and capacitor controlling the delay line.

The Laplace transform of an ideal delay is exponential:

(where π is the desired delay), which requires an infinite number of poles and zeroes to implement. Because the ideal form can’t be implemented (practically), we need to use an approximation.

An accurate, simple approximation to the ideal can be achieved by using a technique known as Padé approximation. The first-order Padé approximation to an ideal delay has the following form:

(where π is the desired delay).

The circuit in Figure 1 has the following transfer function: which has the same form as the first order Padé approximation, with the R*C time constant equaling half the desired delay time.

The circuit was built with 1% resistors and a capacitor with a value measured at 63 pF. This combined with a resistor value of 95.3 Ω gives an R*C value of 5.94 ns. This R*C time constant corresponds to an overall delay time of 11.9 ns (π is 2*R*C).

Measuring the delay time of this circuit with a 5-ns rise-time pulse gave a delay of 12.2 ns, which is a pleasingly accurate result. The accuracy for this short of a delay time can be attributed both to the success of the Padé technique and to the wide bandwidth of the CLC428 op amp used in the circuit. Using a slower amplifier will result in a less accurate delay.

Accurate delay values are easily accomplished using only a first-order approximation. The circuit works effectively, even for short delay times, if a fast enough amplifier is used.


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    Reader Comments

    I need a delay circuit that can delay AM signal (using MC1496)..

    Sal -August 15, 2008

    good fourm

    Anonymous -August 07, 2008

    Yes, you can change the delay to 1.5 us for low frequencies. The delay is however a function of the frequency! The bandwidth where the delay will be 1.5 us "-3dB" may be 200 KHz only.

    William Kuipers -July 20, 2008

    Is this feasible for creating a much larger delay of 1.5µs by altering the R/C values?

    otakon -July 09, 2008

    The formula's contain an error: It is easy to see that for low frequencies, the circuit is inverting. The transfer function lacks a minus sign.

    Formula's also ignore the bandwidth limitation of the op-amp.

    The circuit needs to be driven from 0 Ohm impedance. Perhaps the circuit is not so briljant an accurate as the text shows.

    It is still nice.

    William Kuipers -June 25, 2008

    Hi guys Can someone tell me how to make my time delay to be little bit longer

    kenneth -June 20, 2008

    Hi if there is any one who can hook me up with a phase delay circuit combined with a heterodyne reciever to detect very weak fm signals

    Eliakim Mwajumwa -April 17, 2008

    this is a nice circuit.

    wasif waheed -March 13, 2008

    answer my questions....tnx

    Anonymous -June 24, 2007

    what is the relation of capacitance and resistance in a delay circuit?is there any effect in a delay cicuit?how does the delay occur?

    Anonymous -June 24, 2007

    hi is there someboby who can help me, im looking for a circuit to invert video signal from a receiver. il be using this for a thesis.. thanks! if u have please contact me at email add. "adb_x@yahoo.com"

    Anonymous -June 05, 2007

    oops... i meant H(s) = 1*exp(j*-2*arctan(wRC)) => phase = -2*arctan(wRC) = w*Td ......ok, its not constant, sorry for the screw up.

    Anonymous -March 11, 2007

    isn't the phase delay (=> time delay) constant for all frequencies? --> H(s) = sqrt((1+(RCs)^2)/1+(RCs)^2)*exp(arctan(-wRC)-arctan(wRC)) = 1*exp(-arctan(2wRC)) => phase = 2wRC =>Tdelay = 2RC.............Is this right?

    Anonymous -March 11, 2007

    All my friends

    It can be used in microphone arrays? If u know any time delay circuits, plz advice to me.

    best regard

    thiha -February 27, 2007

    I have a task where I have to introduce two delays Sin() and Cos() of a 5kHz signal. Does somebody know solution satisfy my need? Lahan

    Anonymous -December 04, 2006

    Can y'all speak english?

    Led zeppelin -September 30, 2006

    HI WE ARE LOOKING FOR A CIRCUT TO SOFT START 12 SOCKETS 240 VOLT 5AMP 4 SOCKETS 1 SEC 4 SOCKETS 1SEC 4 SOCKETS THANK YOU

    REGARDS MEL

    RODGER MELVYN THOMPSON -August 24, 2006

    I would be interested in other similar designs. william.kuipers@tiscali.nl Thanks.

    William Kuipers -July 28, 2006

    I would be interested in other similar designs. william.kuipers@tiscali.nl Thanks.

    William Kuipers -July 28, 2006

    am a new user

    onwuka casmir -June 02, 2006

    I'm looking for analog designers. I have alot of activity going on within my office and I'd love to try and tell you what is available. If you have an updated resume, please forward that onto my e-mail address - raqui3co@hotmail.com. I look forward to speaking with you.

    Anonymous -April 05, 2006

    Avtually the circuit is inverting!! Unless you use an inductor instead of the capacitor. Furthermore the circuits needs to be driven from 0 Ohm impedance requiring input buffering! It works well with video signals, but only for relatively small bandwidth/delay products. I use a higher order modified circuit with 1 AD8055 opamp and 90ns delay for 0 to 7 MHz with one inductor and two capacitors.

    William Kuipers -March 08, 2006   (Article Rating: )

    i am a new members so i want to get free design of delay time circuit, because i want to make moving sensor using infra red with delay time. Thank's

    idam cahyo utomo -October 01, 2005

    in every frequncy the ampedanc of RC chang and your time delay will be chang whith frequncy.

    farhan zahiri/ iran -August 31, 2005

    nice, simple, useful.

    robert -June 25, 2005   (Article Rating: )

    thank you for this circute . but in every frequncy the ampedanc of RC chang and your time delay will be chang whith frequncy.I know very good method to creating a fixed time delay for all frequency. my chat ID : vartan_sah@yahoo.com

    syed ali hasani \ IRAN -May 18, 2005

    thank you for this circute . but in every frequncy the ampedanc of RC chang and your time delay will be chang whith frequncy.I know very good method to creating a fixed time delay for all frequency. my chat ID : vartan_sah@yahoo.com

    syed ali hasani \ IRAN -May 18, 2005

    thank you for this circute . but in every frequncy the ampedanc of RC chang and your time delay will be chang whith frequncy.I know very good method to creating a fixed time delay for all frequency. my chat ID : vartan_sah@yahoo.com

    syed ali hasani \ IRAN -May 18, 2005

    The product catalogues are still needed.

    Suresh Kannan -November 02, 2004

    I have a question about this circuit. What is the response of this circuit when the input signal is a video signal? We know that RC resistance will change for different frequencies.

    Khosro P. -October 27, 2004

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