[sdiy] 90-degree phase displacement network calculations

[Tom Bugs]

Ah, I get you!
I've always used quadrature VCO for that side of things & haven't overly 
thought on the +/- scope of using different waveforms - though think 
people did try using the quadrature outputs of the SynthTech E350 on my 
old FS module.
I guess it depends what happens beyond the 0.1Hz bound - this frequency 
does strike me as a bit of a constraint if you want to feed any 
wideranging VC(LF)O in.
Slower rates than cycle every 10 seconds are useful, especially combined 
with other modulation sources.

On 24/01/2021 17:51, David G Dixon wrote:

> 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
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at synth-diy.org
>> http://synth-diy.org/mailman/listinfo/synth-diy
>> Selling or trading? Use marketplace at synth-diy.org
> --
> BugBrand LTD
> UK company No. 07199808
> VAT No. GB 988 2629 57
> 1 Ninetree Hill
> BRISTOL BS1 3SB
> United Kingdom
> www.bugbrand.co.uk
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org
>
-- 
BugBrand LTD
UK company No. 07199808
VAT No. GB 988 2629 57
1 Ninetree Hill
BRISTOL BS1 3SB
United Kingdom
www.bugbrand.co.uk