[sdiy] 90-degree phase displacement network calculations

David G Dixon dixon at mail.ubc.ca
Sun Jan 24 18:51:39 CET 2021


Hi Tom (and all),

Yes, in a Frequency Shifter, the 15 to 15000 Hz PDN is customary.  I'm using
one of those as well -- a "standard" 12-stage unit -- and it works just
fine.

However, for the next "generation" of my frequency shifter, I'm doing away
with the on-board oscillator, and using a second PDN to convert any incoming
signal to sines and cosines.  This PDN needs to generate accurate quadrature
well into the LFO range so that the shifter will act as a phaser.  It also
needs to cover the audio range.  I'm compromising a bit and cutting it off
at 10000 Hz (I don't hear anything above 12000 Hz anyway, and the shifter is
never used above about 5000 Hz).

Hence, what I'm really asking is, what is the lowest modulation frequency of
use to send to a phaser?  Is 0.1 Hz low enough for the LFO to get a broad
range of phaser effects?  The thing is, once I commit to a frequency range,
I place a big order from Digikey for about two dozen very specific resistor
values, and that's that.  I don't want to do it all again.  I also don't
want to exceed five orders of magnitude with this PDN, because the caps get
too big on the low end.

Of course, I could answer my own question easily by just putting a 0.1 Hz
LFO into my existing frequency shifter and seeing whether the phaser effect
is still meaningful.  I'll do that and report back.

I guess the alternative would be to build two smaller PDNs -- one from 15 to
15000 Hz, and the other from 100 Hz or so down to whatever, and have a
routing switch.  But that's a bit inelegant.

Cheers,
Dave

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Tom
Bugs
Sent: Sunday, January 24, 2021 5:30 AM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] 90-degree phase displacement network calculations

[CAUTION: Non-UBC Email]

While I'm definitely keen to hear & check the more precise & wider range
phase network, I know that you can get amazingly rich phasing feedback with
just a 15-15kHz network.
Probably a case of engineering vs ears - where I'd describe as characterful
to sidestep the less deep engineering approach ;)

As I mentioned on your MW thread, feedback is really important for phasing
at sub-audio rates.

On 24/01/2021 01:57, David G Dixon wrote:
>   
> 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 stages.
>
> 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?
>
> Cheers,
> Dave Dixon
>
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