Variable phase shifter?

• posted

Anyone have any favorite circuits for accomplishing a variable phase shift which is relatively independent of frequency? What I'd like: o Logic level input; logic level output o Input is square wave between about 2MHz and 120MHz o Output is two channels, same frequency as input, whose relative phase may be adjusted over 360 degrees minimum (-180 to +180 OK too; just need 360 span) o Output same logic levels as input, nominally. o Ideally, the phase will be controlled through an analog voltage input, say 0-5V, but could be different.

It's obviously relatively easy to generate variable delays which are essentially independent of frequency, but the resulting phase shifts then depend linearly on frequency. I'd like the phase shift to be nearly independent of frequency. You can also pretty easily make phase shift networks that provide a constant frequency-independent relative phase shift between two channels, but I need the phase shift to be controllable. I have some ideas about how to do this, but figured there may be some standard easy way that I'm missing.

Cheers, Tom

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A simple differential all-pass will give essentially constant phase shift over about an octave... single SINE frequency only.

See papers by Sid Darlington for multi-section analysis.

Your description (above) really isn't a phase shifter, it's a variable delay. IIRC Larkin has the very product you need ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
• posted

Jim Thompson wrote in news: snipped-for-privacy@4ax.com:

I think it really does seem to be a phase shifter-- if you dial in 90 degrees, you'd require a 90 degree shift for any frequency square wave.

I think adding a variable offset to a phase detector in a phase-locked loop might be one approach to run down-- if you're limiting the input to square waves.

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Scott
• posted

Yep, like I said, a constant phase shift over even a decade or more isn't difficult--done all the time for phasing-type single sideband generation; it's the variable that's more fun.

Yes, it's probably a bit misleading to describe phase shifting of a square wave, but if I'd said "delay," that's misleading too since it would be "a time delay that is proportional to the inverse of the fundamental frequency, and also proportional to a control voltage." And I suspect that a "product" isn't what I need--I'm looking for a relatively simple circuit that will fit in maybe a couple square inches of board space.

I should add that milliseconds of settling time between changes in frequency or control voltage is OK.

Cheers, Tom

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OK. Seidman's PLL offset might work. What kind of resolution do you need per adjustment step? Do you really need 120MHz ?:-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
• posted

Tom Bruhns wrote in news:3abc06f4-9a24-4c0d-bcca- snipped-for-privacy@w24g2000prd.googlegroups.com:

Do you really need to take an input square wave and generate a phase- changed output, or can you generate both waves? You can use two DDSs (direct digital synthesizers) with a variable offset between them for phase, and both using the same step size. You change the freq by changing the step size, but the phase between the output waves would be the same, and modifiable through a separate control.

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Scott
• posted

Scott Seidman wrote in news:Xns9AFB88143A713scottseidmanmindspri@130.133.1.4:

Check out the AD9953, for example.

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Scott
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innews: snipped-for-privacy@4ax.com:

Thanks Scott. Yes, that's one approach I've been thinking would work, though I'm expecting to have an input that's very low phase noise, and a 60:1 range VCO in a PLL isn't going to be particularly low phase noise, I'm afraid. I'll give that some more thought, though.

I've been wanting to do this "analog" to avoid having to deal with digital control signals and the implied human interface, but if I were to go digital, then a DDS chip would do everything very nicely. That is, I don't need to limit the solution to using a square wave input from an external source; the circuit could be the source. It's just easier for me to use one of our existing good signal generators as a source and add a circuit to give me the two outputs whose relative phase is adjustable. But if the analog circuit to do it is too complex, it's not worth the effort to go that way.

Cheers, Tom

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Tom Bruhns wrote in news:89f57ce0-a6e2-4d3c-b659- snipped-for-privacy@p10g2000prf.googlegroups.com:

I haven't been deep into the data sheet of the DDS I posted before, but just deep enough to know that there are synchro inputs. You might be able to run the whole thing off of the nice signal generator you have. Probably worth the read.

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Scott
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innews:Xns9AFB88143A713scottseidmanmindspri@130.133.1.4:

Yep, DDS is definitely an option, but a bit more complicated to hack than I was hoping for. ;-) It does give me nice low phase noise, and can obviously be programmed to whatever phase offset I want. Well-- actually as I think about it, the phase offset comes in pretty big increments if the output frequency is high. If the DDS clock is

400MHz, for example, at 120MHz output the phase would adjust in 108 degree steps. Maybe DDS isn`t the right answer.

Cheers, Tom

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innews:Xns9AFB88143A713scottseidmanmindspri@130.133.1.4:

Duh...OK, I know what I was THINKING when I wrote that, but it is obviously not correct. The DDS has a phase accumulator. In the example, the phase accumulator increments by 108 degrees per 400MHz clock, but it may be initialized to anything you want, down to the same very fine increment independent of frequency. Sorry about that brain fart.

Cheers, Tom

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Tom Bruhns wrote in news: snipped-for-privacy@v13g2000pro.googlegroups.com:

You could take the clock to a Gig on some of the newer chips.

How about this approach (not quite well worked out, but getting there-- just brainstorming). Take one channel of a summer, and put a sine in, in the second channel, modulate the cosine with your phase signal. You pass the input sine and the output results through comparators, and wallah!. This would be OK for 0- near 90degrees (the range is the bit not worked out yet).

So now, you're on the hook to generate quadrature, and can vary the phase from there.

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Scott
• posted

Since there's no difference between phi and phi + N cycles, a DDS that's running at a small offset from an exact subharmonic of the clock will effectively give you fine phase resolution.

Cheers,

Phil Hobbs

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innews:Xns9AFB88143A713scottseidmanmindspri@130.133.1.4:

Phase offset is (POW/2^14) * 360° ~=> 0.022° resolution.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com```
• posted

innews:Xns9AFB88143A713scottseidmanmindspri@130.133.1.4:

Maybe the following would work, use 2 the same oscillators, but one controlled in phase by some means, then from both generate a pulse each zero crossing. From the first oscillator's pulses generate a ramp, first pulse up (0-180 degrees), then down (180 - 360 degrees). Use the second oscillator's pulse output as sample pulse in this ramp, it will be at half ramp voltage at 90 degrees shift in lock. It will always tend to 90 degrees, but if you add some DC offset in the ramp, then you should be able to shift phase. Not sure this reasoning is correct, I have done something similar at a few MHz, never at 120 MHz though.

• posted

Yes, that sort of thing is another one I have thought about. Actually, you can use trig identities to do even better: sin(wt

+phase) = sin(wt)*cos(phase)+cos(wt)*sin(phase). So if you generate the sine and cosine of the phase control signal and use those as inputs to multipliers fed with quadrature RF signals and sum the outputs of the multipliers, you get a phase-adjustable signal. That works over 360 degrees of phase, and in fact if you make two branches, one controlled to +phase and one to -phase, the difference can be swept through 720 degrees. Analog Devices has a nice little analog multiplier that works up to GHz frequencies, and for that matter, it can be done with just a double-balanced mixer, too, though you have to watch out for what is happening with the harmonics that will be present in such a mixer...

Well, as I said in the base note, I have some ideas about how it can be done, I just keep hoping it can somehow be super simple. I really do appreciate your ideas--they have gotten me re-thinking some things.

Cheers, Tom

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For generating fixed phase angles... e.g. every 60°, I have used a shift register "ring-around-the-rosy oscillator" clocked at a multiple of the desired effective frequency, with taps at the appropriate points for the different phases (GSM telephone).

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
• posted

We use 10GHz capable devices at work. They are "Phase Shifter / Attenuators" which allow control over those two elements of a signal.

They ain't cheap though. Made by General Microwave, a Herley company.

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innews:Xns9AFB88143A713scottseidmanmindspri@130.133.1.4:

True, but a DDS with that coarse of step will not have low phase noise. Or a good waveform, huge THD and amplitude errors. You really need to keep maximum step size down around 30 degrees for really good results.

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JosephKK wrote in news: snipped-for-privacy@4ax.com:

And you'd need pretty well-matched reconstruction filters for what you're trying to do.

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Scott