```Iain Hunter  (via Chris.Garnett@dcs.warwick.ac.uk) wrote:

> I would like a phase ADVANCE cct that gives a 90 degree phase
> shift with 0dB gain at 10Hz.  It must be DC coupled.

Iain, you can indeed get a 90 degree phase advance at 10 Hz with
a flat frequency response from DC to ultrasound, using a single
opamp and a few parts.  This circuit gives unity gain, non-nverting
as frequency decreases, reaching 180 degrees (inverting) at DC.
You can set the circuit to get 90 degrees advance wherever you
like.

R2                R3
IN o--+---/\/\/\/---+-------/\/\/\/-------+--------o OUT
|             |                     |
---            |    |\               |
C  ---            +----|-\  U1          |
|                  |  \_____________|
|                  |  /
+------------------|+/   For very low distortion, U1 should
|                  |/    preferably an op amp with good
\                        common mode characteristics.
/                        The TL080 series is good.
R1  \
/        Note that the circuit as shown
\        is using positive and negative
|        power supply rails.
-----
---
-

R2 = R3; their values are not critical, maybe around 10 to 100K,
but for precise results they should be matched.  You get 90 degrees
of shift when X(c) = R1.  In a simple RC circuit, this would
produce 45 degrees of advance, but in this active circuit it
produces 90 degrees of advance from input to output.

The R1 value is also not critical, although you may want to use a
value around 1/2 that of R2 and R3 to balance out offset current.
Then select C according to the value of R1.

R1(in K ohms) * C (in uF) = 15.9 for 90 a degree circuit shift at
at 10 Hz.  So if R1 = 10K, then C = 1.59 uF; and if R1 = 47K, then
C = .338 uF, etc.

You can make R1 variable to set the phase shift to exactly 90
degrees at 10 Hz.

(ctrask@primenet.com) once posted this list of references on one of
the newsgroups:

>    1.   Shirley, Frederick R., "Shift Phase Independent of Fre-
>         quency," Electronic Design, Sept 1, 1970, pp. 62-66.
>
>    2.   Albersheim, Walter J., "Computation Methods for Broad-
>         Band 90 deg. Phase-Difference Networks,"  IEEE TCT, Vol
>         CT-16, May 1969, pp. 189-196.
>
>    3.   Darlington, Sidney, "Realization of a Constant Phase
>         Difference,"  Bell System Technical Journal," Jan 1950,
>        pp. 94-104.

Best of luck, Iain.

Bob Bruhns, WA3WDR, bbruhns@li.net

```

Date: 16 Sep 1996 17:56:41 GMT

Original Subject: Re: Zero dB Phase Advance Cct - Possible?