creating delay with passive rc-network

Very funny.

Since the required delay is a whole period, the answer is yes, there is an optimum schematic, although it is a degenerate case of RC, with R=0, C=0 (inexpensive)

Vin----R--·--Vout | C | gnd

Pere

On a sunny day (Fri, 19 Mar 2010 09:21:13 +0100) it happened oopere wrote in :

Na, C=0 and R=0 is extremely expensive. Impossible to make even. Better just use some meters of coax cable to delay one period :-)

OK that's correct, do you also have an answer in case we want 180ns?

Candide

A passive RC network will not exhibit constant amplification over frequency. This means that you can not say that it is a pure delay. Having said that, a single RC section is able to produce -90 deg of phase shift. Two cascaded RC sections will give you an all pole second order response able to produce up to -180 deg, and so on. If you require

But you need to tell more about your problem. What is the intended application? Do you really need a pure delay? Does it really need to be RC-only? What signal levels are involved?

Pere

But that's not a passive rc circuit ;)

but with an output amplitude of zero.

John

On a sunny day (Fri, 19 Mar 2010 16:20:08 +0100) it happened oopere wrote in :

But it is: a cable has a capacitance and resistance. The inductance comes for free, and makes it possible :-) And the cable method WORKS.

That's just a second order side effect :)

Pere

Depends; do you accept a phase-lead network, or does it have to be phase-lag? A single RC stage can have zero to 90 degrees lag, it'd take four or more to get to the 180 ns mark, and there is quite a lot of attenuation. The 36 degree phase lead is easier.

=3D0,

Ok here's more details, I want the attenuation of the RC-network =3D

I don't think there is any point on the curve where attenuation is that much less than phase shift (in some arbitrary units). For instance, for attenuation > 20dB, you get a phase shift of nearly 90 degrees from one RC. To get more 360 degrees phase shift requires slightly more than four RC stages, and each one must have an impedance about 10 times higher than the previous, otherwise the response gets screwed up more. Accordingly, the attenuation is closer to 80dB, i.e., your signal is gone altogether.

a. Use inductors. b. Use op-amps (all-pass filter).

Tim

At 8nS / foot that's 25 ft which might cause space problems though it would work well. A BAL, Allen Avionics ot Mathey delay line would take a lot less space but cost more and not be as easily adjustable. It still needs an amp to make up the 6+dB insertion loss - or maybe that's acceptable.

G=B2

"Glenn Gundlach" Jan Panteltje

At 8nS / foot

When did the velocity factor of co-ax go down to 12.7 % ??

..... Phil

If I understand you correctly you suggest to use a high pass section, that works indeed with an RC=3D43.8ns I get the required delay and the attenuation is around 1.8dB thats less than 20%. Thanks!