[sdiy] 90-degree phase displacement network calculations

David G Dixon dixon at mail.ubc.ca
Sun Jan 24 02:57:41 CET 2021

Hello SDIYers,

I'm just about to commit to a layout for a new frequency shifter, and I have
to decide whether to build a 16-stage PDN or an 18-stage PDN for shifting
frequencies from 0.1 to 10000 Hz.  If I build 16 stages, the maximum phase
error is about 0.5 degrees.  If I build 18 stages, the maximum phase error
is about 0.3 degrees.

My question is this: Does that extra 0.2 degrees have any impact whatsoever
on the performance of the shifter?

Of course, these calculations assume perfect tolerance of all caps and
resistors.  In real life, I will hand-select the caps, but the resistors
will be subject to 1% tolerances, so the actual phase errors could be as
high as about 0.9 degrees for 16 stages, and about 0.7 degrees for 18

I have formerly built 12-stage PDNs for 15 to 15000 Hz.  The calculated
maximum phase error for those is about 0.2 degrees, but with 1% tolerances
these could be as high as 0.5 or 0.6 degrees.  They have worked perfectly
well.  Hence, I'm tempted to go with the 16-stage version.

Thoughts?  Does anyone have any experience with this particular problem?

Also, one other question: is 0.1 Hz low enough for general phaser duties?
That's one cycle every 10 seconds, and it seems to me that this would be at
or beyond the limit of being able to perceive the phasing effect.  Does
anyone have any thoughts about that?

Dave Dixon

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