Freq. Independent Phase Shifter
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From: whit3rd on 11 May 2010 14:09 On May 11, 12:21 am, billmur...(a)protech.com (Bill Murphy) wrote: > On Mon, 10 May 2010 18:24:52 -0700 (PDT), MooseFET > > <kensm...(a)rahul.net> wrote: > >What are the two other channels making? > > Same signal, all separated by 120 degrees. Done in CoolEdit. Gack! That's MUCH easier. if A = sin(wt) and B = sin(wt + 120 degrees) then A-B = sin(wt + 240 degrees) So, get a 1:1 audio coupling transformer, wire the primary to the A and B terminals, and ground one end of the secondary.
From: whit3rd on 11 May 2010 14:20 On May 11, 11:09 am, whit3rd <whit...(a)gmail.com> wrote: > On May 11, 12:21 am, billmur...(a)protech.com (Bill Murphy) wrote: > > > On Mon, 10 May 2010 18:24:52 -0700 (PDT), MooseFET > > > <kensm...(a)rahul.net> wrote: > > >What are the two other channels making? > > > Same signal, all separated by 120 degrees. Done in CoolEdit. > > Gack! That's MUCH easier. Drat, my first reply was bogus. If A = sin(wt ), and B= sin(wt + 120 degrees), and you want to make C = sin(wt + 240 degrees), just note that these add to zero A+B+C = 0 So, C = -(A+B) If you have audio coupling transformers handy, that's a good way to do the summation (no power needed, low noise, high reliability).
From: Joe on 11 May 2010 14:31 On Mon, 10 May 2010 15:10:01 -0700, miso(a)sushi.com wrote: > [someone wrote] >> >Bill Murphy wrote: >> >> I am using a commercial stereo amp to output continuous wave test >> >> signals in the low audio range, up to about 2KHz. However, I need a >> >> third channel with a 120 degree phase shift. Is there a circuit that >> >> will do this evenly across this entire frequency range? .... > If you are going to a digital approach, programming a cordic would be my > advice. [snip reasons] > Sitting over in Terman is a really great Phd dissertation on the cordic, > better than any book I every read regarding the algorithm. My > recollection is the author's name is Ahmed. Searching Stanford's > Socrates doesn't seem to dig it up though. Maybe "Signal Processing Algorithms and Architectures", Hassan Masud Ahmed, PhD thesis, Stanford University, June 1982. (1981?) It is frequently cited* but might not be freely available online. *Eg in <http://www.computer.org/portal/web/csdl/doi/10.1109/12.403715> and <http://www.eecs.berkeley.edu/~newton/Classes/EE290sp99/lectures/ee290aSp996_1/cordic_chap24.pdf>
From: miso on 11 May 2010 14:48 On May 11, 11:31 am, Joe <j...(a)somewhere.org> wrote: > On Mon, 10 May 2010 15:10:01 -0700, m...(a)sushi.com wrote: > > [someone wrote] > >> >Bill Murphy wrote: > >> >> I am using a commercial stereo amp to output continuous wave test > >> >> signals in the low audio range, up to about 2KHz. However, I need a > >> >> third channel with a 120 degree phase shift. Is there a circuit that > >> >> will do this evenly across this entire frequency range? > ... > > If you are going to a digital approach, programming a cordic would be my > > advice. [snip reasons] > > Sitting over in Terman is a really great Phd dissertation on the cordic, > > better than any book I every read regarding the algorithm. My > > recollection is the author's name is Ahmed. Searching Stanford's > > Socrates doesn't seem to dig it up though. > > Maybe "Signal Processing Algorithms and Architectures", > Hassan Masud Ahmed, PhD thesis, Stanford University, June 1982. (1981?) > > It is frequently cited* but might not be freely available online. > *Eg in <http://www.computer.org/portal/web/csdl/doi/10.1109/12.403715> and > <http://www.eecs.berkeley.edu/~newton/Classes/EE290sp99/lectures/ee290...> You are probably right. I always thought of Ahmed as a first name, and found it odd to be this guys last name. Not that I know much about the middle east, but that oddity made the name stick. You need to run the look up table for the coordic a few more bits than word size for the final result if you want the answer good to the last bit. Still, the efficiency of the algorithm is so much better than a simple sine look up table that going a few extra bits is worth it.
From: miso on 11 May 2010 14:57
On May 11, 3:56 am, Spehro Pefhany <speffS...(a)interlogDOTyou.knowwhat> wrote: > On Tue, 11 May 2010 17:25:16 +1000, the renowned "Phil Allison" > > > > <phi...(a)tpg.com.au> wrote: > > >"Joerg" > > >> A Hilbert shifter works well, depends on the precision and how many > >> octaves you want. Also, you'd need to get hold of 0.5% or better film > >> capacitors which is not easy anymore these days. > > >** The use of 0.5% tolerance caps implies a phase ripple or error of better > >than 1 degree max. > > >Using 1% tolerance or 1% values selected from 5% stock, the max phase error > >is not more than 2 degrees. > > >There is no problem designing a Hilbert phase shift network that covers from > >22 Hz to 20 kHz using only 10nF and 1nF polystyrene caps of nominal 1% > >tolerance, 1% MF resistors and a few FET op-amps. > > 0.1% resistors are cheap these days. Where can you buy PS caps of any > tolerance ? NPO ceramic parts are available, but $$$$. > > Best regards, > Spehro Pefhany > -- > "it's the network..." "The Journey is the reward" > sp...(a)interlog.com Info for manufacturers:http://www.trexon.com > Embedded software/hardware/analog Info for designers: http://www.speff.com If you are making ONE box AND you have the right topology, you can just measure your standard tolerance capacitor to a high degree of accuracy, then buy the proper high accuracy resistors. These filter designs usually use more op amp, but have greater flexibility. Leapfrog ladder filters generally can be built in this manner. To be a clearer here, the filter can't depend on ratios of capacitors. Obviously for a production unit, it would be cumbersome to pick all the resistors based on measured capacitors. But to build a few circuits on the cheap (if you don't values your labor!), the scheme works. |