# Phase shifter question

Discussion in 'Electronic Design' started by alan, Feb 11, 2008.

1. ### alanGuest

The phase shifter described in H&H has a phase that starts at 0 at DC
and then goes to negative 180 at high freq. (excuse me if I got the
definition of phase backwards) Is there a phase shifter that shifts
phase the other direction with respect to freq, i.e. start at -180 at
low freq and increases to 0?

2. ### MooseFETGuest

The phase shifter that exists shifts the phase later with higher
frequencies. Later in phase is minus values of degrees. The phase
goes from 0 to -180 as frequency increases.

The phase shifter you want poses a bit of a problem. It requires that
the phase go earlier with higher frequencies. Over a limited band
this can be done but not without also having a variation in gain.
This is because, in real life, only information from the past can be
used in creation of the output.

3. ### JoergGuest

Then how about those time machines?

4. ### John LarkinGuest

Just invert the output.

John

5. ### Jim ThompsonGuest

-180° is _always_ lagging.

...Jim Thompson

6. ### Tim WescottGuest

Technically that will yield a phase shifter that goes from -180 degrees
to -360 -- the endpoints will be right, but if the OP needs a positive
slope then he needs to find the contact information for Non-Causal
Systems, Inc. to get the parts. (their number is in next year's phone
book).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

7. ### Jim ThompsonGuest

If the OP is referring to H&H figure 2.29, just reverse inputs phi1
and phi2.

Or as I would designate them (as shown in H&H)...

phi1 => +ein

phi2 => -ein

180° phase difference between inputs +ein and -ein.

...Jim Thompson

8. ### ehsjrGuest

Or he could use those dilithium capacitors that JL
figured the guy in the other thread uses.

Ed

9. ### Joel KoltnerGuest

Oh, come oh, the digital guys use non-casual filters all the time.

You just to overlook the input-output latency involved and designs can be as
non-casual as you'd like.

10. ### krwGuest

You'd be lagging at -180° too.

11. ### Guest

Yes and no. Let's say you have a differential signal.

in out
+ ----C-----+
- ----C----- +

+----L------ -
- ----L ---- +

So feed the low frequency with crossed inductors and feed the high
frequency is non-crossed capacitors.

Assuming source and load resistors, this should end up as an all-pass
network with phase shift.