90 degree phase shift in stereo box

[theplastictoy]

Hi all!

I'm planning on building a stereo A/B/Y box. Anyone knows how I can shift one of the outputs 90º out of phase with the other? Kind of what the stereo memory man does.

Thanks,
Ricardo

[niftydog]

check out "The Technology of..." series on Phasers and Flangers at //www.geofex.com.

[Paul Perry (Frostwave)]

Here's a precision 90 degree shifter:
http://home.att.net/~wa1sov/technical/allpass/allpass.html

Here's a PCB you can buy for a less precise one (but, no schematic!!)
http://www.cgs.synth.net/modules/cgs45_domefilter.html

A somewhat cryptic schematic & note from Jurgen Habile:
http://www.oldcrows.net/~jhaible/tonline_stuff/hjfs6_df.gif

You find them in ham radio single sideband modulators, as well.

[puretube]

maybe he wants 180 degrees?...

[R.G.]

There is no circuit that will do a fixed phase shift at all frequencies. Mother Nature's math forbids it to the best of my knowledge.

What you can do is create two circuits, one of which has a continuously increasing phase shift with frequency, and the second of which is offset from it so that at any point, the second and first outputs are 90 degrees apart. It's a standard circuit now in what is called I-Q modulators (It think this means "inverted and quadrature").

One first order all-pass filter, familiar to us from phasers, generates a phase shift that starts at some point and shifts more or less linearly for a range of about 5:1 of frequency. The phase in the middle of this slide is 90 degrees compared to where it started.

If you set up two of them so that the second's phase shift starts up where the first one is trailing off, you get a steadily changing phase shift for two decades. A third one gives three decades, and so on. The audio spectrum is 1000:1 wide (20Hz to 20KHz) so you need about seven or eight of these to cover the whole audio spectrum. If you set the frequency points correctly, you can get a phase shift which increases linearly for as much of the audio spectrum as you like.

But that's not a 90 degree shift.

To do that, you use a second chain of phase shift circuits, with its start frequency staggered lower or higher than the first chain. The staggering is set so that when the first circuit hits 90 degrees phase shift, the second one is already at 180 degrees. When this is true, you have a constant (if you did your design well!) phase shift between the two of 90 degrees.

And that's how you do it. This is sometimes called a Dome filter, or a Hilbert filter.

There's lots of fun with Dome filters and IQ modulators Juergen Haible did a frequency shifter which gives analog frequency shifts including through zero shifts using a wideband Dome filter and LM1496 amplitude modulators. Lots of circuitry, but a neat sound.

[Vsat]

I-Q = "in-phase and quadrature" in RF-speak.

If the in-phase output happens to get inverted, the relationship will still be quadrature though. Great fun to watch this on a scope. Just don't leave the scope in x-y mode while the circuit is turned off, or you will burn a little hole in the phosphor :0
Regards, Mike

[puretube]

the closer to 90 degrees, the smaller the hole in the screen...