Freq. Independent Phase Shifter
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From: John Larkin on 9 May 2010 19:16 On Sun, 09 May 2010 16:04:45 -0700, Winston <Winston(a)bigbrother.net> wrote: >On 5/9/2010 2:33 PM, 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? >> >> Is it possible to do same using an off-the-shelf transformer and >> current subtraction? >> >> Any advice would be appreciated. >> >> Bill Murphy > > >I would approach this using DDS (Direct Digital Synthesis). > >http://www.fpga4fun.com/DDS2.html > >A binary counter generates an address for two different memory >devices. The devices are programmed with a binary sine wave. >The second device is exactly like the first except the data are >displaced an equivalent of 120 degrees in the address space. >Each data stream drives a separate DAC. The output of the second >DAC is filtered and amplified to drive your 3rd amplifier which >has exactly the same phase characteristics as your first two >amplifiers. > >Vary the clock speed into the binary counter to vary >the output frequency. > >--Winston The usual DDS technique is to use an accumulator that's clocked at a constant rate, and at every clock add a fractional value to it that determines frequency. The MSBs of the accumulator drive the sine lookup table. So everything happens in one stage. If you wanted a 12-bit binary counter to make a sine wave at 1 MHz, you'd have to clock it at 4 GHz. But a DDS accumulator clocked at 4 MHz can easily make a 1 MHz sine wave. John
From: Joel Koltner on 9 May 2010 19:22 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:03geu592n811uiilhhh4teqone5bf6k2l1(a)4ax.com... > If you wanted a 12-bit binary counter to make a sine wave at 1 MHz, > you'd have to clock it at 4 GHz. But a DDS accumulator clocked at 4 > MHz can easily make a 1 MHz sine wave. ....although you'll need rather more filtering on the output in the later case!
From: Winston on 9 May 2010 19:34 On 5/9/2010 4:16 PM, John Larkin wrote: > On Sun, 09 May 2010 16:04:45 -0700, Winston<Winston(a)bigbrother.net> > wrote: > >> On 5/9/2010 2:33 PM, 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? >>> >>> Is it possible to do same using an off-the-shelf transformer and >>> current subtraction? >>> >>> Any advice would be appreciated. >>> >>> Bill Murphy >> >> >> I would approach this using DDS (Direct Digital Synthesis). >> >> http://www.fpga4fun.com/DDS2.html >> >> A binary counter generates an address for two different memory >> devices. The devices are programmed with a binary sine wave. >> The second device is exactly like the first except the data are >> displaced an equivalent of 120 degrees in the address space. >> Each data stream drives a separate DAC. The output of the second >> DAC is filtered and amplified to drive your 3rd amplifier which >> has exactly the same phase characteristics as your first two >> amplifiers. >> >> Vary the clock speed into the binary counter to vary >> the output frequency. >> >> --Winston > > The usual DDS technique is to use an accumulator that's clocked at a > constant rate, and at every clock add a fractional value to it that > determines frequency. The MSBs of the accumulator drive the sine > lookup table. So everything happens in one stage. > > If you wanted a 12-bit binary counter to make a sine wave at 1 MHz, > you'd have to clock it at 4 GHz. But a DDS accumulator clocked at 4 > MHz can easily make a 1 MHz sine wave. > > John That'll be what I learned today. Sounds like this approach could be coded into a $2.00 PIC using PWM outputs fairly painlessly. Thanks, John! --Winston
From: Jamie on 9 May 2010 20:28 Winston wrote: > On 5/9/2010 2:33 PM, 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? >> >> Is it possible to do same using an off-the-shelf transformer and >> current subtraction? >> >> Any advice would be appreciated. >> >> Bill Murphy > > > > I would approach this using DDS (Direct Digital Synthesis). > > http://www.fpga4fun.com/DDS2.html > > A binary counter generates an address for two different memory > devices. The devices are programmed with a binary sine wave. > The second device is exactly like the first except the data are > displaced an equivalent of 120 degrees in the address space. > Each data stream drives a separate DAC. The output of the second > DAC is filtered and amplified to drive your 3rd amplifier which > has exactly the same phase characteristics as your first two > amplifiers. > > Vary the clock speed into the binary counter to vary > the output frequency. > > --Winston > > > > > Fancy name "DDS" for techniques that have has been around for years. :) How about selecting a Uc (programmable processor) that has 3 DAC's on it. and some input so that you can select the freq you need and generate the output for your 3 phase signal you're creating.. Or, 3 DDS function generators, with their external triggers dasy chained, or 1 Dual unit like mine, and one extra with the external trigger put in play to keep all three at 120 apart. hell, I did a replacement 3 phase generator for an obsolete chip using Buffered PWM output.. Yawn, to many possibilities.
From: Phil Allison on 9 May 2010 22:23
"Bill Murphy" > >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? ** How even is "evenly" ?? How accurate does the 120 degree shift need to be ?? Are the test signals all sine waves or not ?? Is the shifted output level flatness critical or not ?? > Any advice would be appreciated. ** Yep - try posting a question that explains what you REALLY want to do. ..... Phil |