[sdiy] BODE Frequency Shifter

David G Dixon dixon at mail.ubc.ca
Wed Oct 31 19:43:23 CET 2018

Here's my two cents on Dome filters, which I call 90-degree phase
displacement networks...

If you have built a 90D PDN with 10 or 12 allpass stages, all individually
tuned with trimmers, then you should have no problem tuning each one.  Just
achieve a perfectly circular Lissajous figure between the fed sine and the
stage output on each one at the desired frequency, and then the whole thing
should give you a 90-degree (+/- 1 or 2 degrees) difference between the two
cascades over the designed range of frequencies.  You should be able to feed
a slow frequency-swept sine wave to the filter and see a more or less
perfectly circular Lissajous figure between the two cascades over the entire
range.  However, the phase shift will be dramatically different once you
exceed the frequency range, on either end.

At least, that's my experience.  I built a 12-stage (6 x 6) filter for a
range of 15 to 15000 Hz using QuadNet, and it worked perfectly.  I did it
two ways:

1) The first one was with all different monolithic ceramic cap values
selected to give the proper frequencies with a resistance of about 32k, and
I used 27k fixed resistors and 10k trimpots and tuned each one individually.

2) The second one was with 102, 103 and 104 film caps (4 of each) and I used
pairs of 1% resistors to get as close to the required time constants as
possible.  For example, for the frequency of 1340Hz, given a 103 (10nF) cap,
the required R is 11875 ohms (the cap values were selected to keep the
resistances mostly between 10k and 100k).  This can be obtained with an
11.8k resistor and a 75R resistor, both standard 1% values.  (My biggest
resistance error was 0.013%.)  I bought a small pile of 102, 103 and 104
film caps, and I made a simple triple relaxation oscillator with 10k, 100k
and 1M resistors that should generate outputs of 455Hz for 104, 103 and 102
caps, respectively.  Then I tested all the caps in my collection and
separated them into bins depending based on 4-Hz ranges.  Hence, all the
caps that gave between 453 and 457Hz went into one bin, between 458 and 462
in another, and between 448 and 452 into a third.  Any that were outside of
those ranges were excluded (there weren't that many -- I find that caps are
generally closer to their stated values than the spec would lead you to
believe).  I didn't select the resistors, since my design suggested that 1
or 2% tolerance on the RC value wouldn't make a significant difference to
the performance of the PDN.  It worked well (in that it gave a more or less
perfectly circular Lissajous figure between 15 and 15000 Hz) and required no
trimming whatsoever.  No trimmers means a much simpler layout and quicker
build.  Testing the caps went quickly, since I could do three at a time (one
of each value), and so within a half hour I could generate enough caps for
half a dozen 90D-PDN circuits.  My spreadsheet analysis suggested that the
PDN would work equally well if all of the caps had the same tolerance.  In
other words, if all the caps were 1% low or 1% high, the PDN would work just
as well as if they were all perfect.

Not sure if any of this helps, but that's my experience.  I've done similar
thinking on all parts of my frequency shifter so that there are no trimmers
at all and it works very well, with little if any carrier bleed.  The key
thing is to AC-couple all of the inputs to the multipliers, since the
majority of carrier bleed is from DC levels on the various signals.

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Dave
> Sent: Wednesday, October 31, 2018 7:02 AM
> To: synth-diy at synth-diy.org; atsushi maeda
> Subject: Re: [sdiy] BODE Frequency Shifter
> On October 30, 2018 3:45:25 AM PDT, atsushi maeda 
> <shine32 at kr.tcp-ip.or.jp> wrote:
> >The current problem occurred when adjusting DOME Filter.
> >It may be adjusted so as to have a phase difference of 90 
> degrees at 40
> >
> >Hz, 170 Hz, 725 Hz, 3080 Hz, 13090 Hz, but if I adjusted it to 90 
> >degrees phase difference at 13090 Hz, the phase difference 
> will be 90 
> >degrees at other frequencies It will not be.
> >No matter how many times you try to adjust it, it will not change.
> >Is my understanding wrong?
> Please post your measurements.  How large are the errors?
> -Dave
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