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
If you mean un-tweaked, you need to read up on Sidney Darlington's works.
Otherwise a hand-adjusted all-pass will work, change frequency, tweak a pot to get 120°... did this with smear camera motor drivers when I was a student at MIT more than 50 years ago. ...Jim Thompson
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The only thing bipartisan in this country is hypocrisy
Check out Horowitz and Hill's "The art of Electronics". Section 5.16 talks about phase-sequence filters, which give a constant 90 degree shift over a range of frequencies. They consist of strings of equal value resistors with cross-connected capacitors whose values decrease in constant proportion per stage, halving in the example given, which isn't all that practical to set up. The bottom line is that it isn't trivial, and if you need to ask, you probably don't know enough to put together a circuit that will work.
-- Bill Sloman, Nijmegen
A transformer won't do it.
Williams' filter book has some nice opamp-based allpass networks that will give you 90 deg phase shift over a wide range. A little adding-subtracting can get you to 120 from there.
Often it's easier to generate signals with phase shifts than to start with a sine and shift it. I could sell you one of these...
or better yet one of these...
John
I would approach this using DDS (Direct Digital Synthesis).
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
-- I'm already sending extortion money to my state's "Employment Development Department" but I would like to support *nationwide* organized crime directly, particularly for shakedowns of the elderly, subversion of the Democratic process through graft and corruption and perhaps the manufacture and distribution of illegal narcotics. What is the best way to increase money flow to the Mafia? Should I double my monthly payment to EDD or can I simply send checks to the "TRS Recovery" division of "FirstData Corporation"? Does the Mob have a website that accepts Paypal payments?
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
...although you'll need rather more filtering on the output in the later case!
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
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.
"Bill Murphy"
** 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 ??
** Yep - try posting a question that explains what you REALLY want to do..... Phil
Yeah, the MSBs of the phase accumulator can be run through a sine table that makes PWM codes. The other cute thing to do would be to map the sines into delta-sigma codes and output that to a port pin. It sounds like there must be some slick algorithms lurking there somewhere.
NXP makes an ARM that costs under $1. It does screaming-fast 32-bit math and I think it may have a DAC on-chip. That would make a nice audio generator. Single-cycle multiply does amplitude scaling, within limits of dac or pwm resolution.
John
I'd suggest reading up on Hilbert transformers. Once you have an 90 degree phase shift, you have a sine and cosine, hence an orthogonal pair. Then you can make any phase you need from those signals.
In many cases you just need the two signals to be orthogonal, but of no particular phase to the reference sine wave. You can get two filters that will be 90 degrees out of phase from old single sideband radio designs.
In many cases you just need the two signals to be orthogonal, but of no particular phase to the reference sine wave. You can get two filters that will be 90 degrees out of phase from old single sideband radio designs.
** Long as you only need the telephone speech band ( 300 to 3,000 Hz at -6dB points ) and don't mind large amplitude errors..... Phil
Use a microprocessor with 3X DAO
Since the OP only needed frequencies up to 2 kHz and was using COTS amplifiers, a cheap dedicated computer running a multiple (4-6) channel sound card running at 8-48 kHz sampling frequency would do the trick.
Two separate stereo sound cards can not be used, since the cards generate their own sampling frequency from their own crystal, which are not synchronized.
If you only need the 90 degree phase shift without fast frequency swings, the PLL can be used, since the phase detector produces zero output (after averaging), when the VCO is 90 degree out of phase relative to the reference.
Yeah, I've used this circuit. It gives you nice quadrature output sine/cosine waves that can be used to make any particular phase shift. But phase shift relative to the input signal changes with frequency... The phase tends to keep wrapping around. Mixing the two signals with a potentiometer also causes amplitude variations if that would be a problem. That said I used a 10 section filter that has less than 1 degree of pahse ripple from 3 Hz to 3kHz. Caps are standard 1, 2.2, 4.7, 10... values. The ratio between sections is not as important as keeping the same value in each section. You can also change the order of the sections with no change at the output.
If the OP wants only a few known frequincies then he could make a few dedicated RC sections... or some dedicated All-pass opamp filters. and switch each in when testing at that frequency.
George H.
Paul's suggestion with multi-channel sound cards is a good one. But keep in mind that phase shifts in the very low range (tens of Hertzes) can be iffy on some cards, output cap tolerances and all that. Unless you want to go in there with a solder iron.
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. That was different in the 80's.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
On a sunny day (Mon, 10 May 2010 09:19:56 -0700) it happened Joerg wrote in :
It is too much, all you need is 3 EPROMS, a 4040 counter, and a 4046 PLL as VCO, add a pot to set the frequency. Add 3 R2R networks, or 3 cheap DACS. I have made one variable sine wave generator like that in the long ago past. Milliwats, and in a small box. Mine had auto sweep too, so I could test filters. Just an integrator and a FF, and 2 comparators added.
256 point 8 bits sinewaves. After al this is s.e.d. not 'alt.pc.sales' or whatever.
VCO,
It all depends. When you do this for a living you need to weigh the time it takes to whip this up against the cost and installation time of a multi-channel sound card. When you are retired, different thing. But many of us aren't there yet.
[...]-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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