I was told that a/an can be applied to the sound of the following word, not strictly to the next letter. So, do what sounds right.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Friday, December 5, 2014 6:48:22 AM UTC-8, Phil Hobbs wrote: [about a phase meter]
It's possible, too, to mix down to an 'easy' frequency (maybe 1 MHz) and use software-defined-radio type digitizers. If you had simultaneous mix-down of reference and unknown signals, against the same LO, the phase shift of the IF signals is the same as the RF ones.
Then, it's just a matter of doing a couple of FFTs, and doing a weighted curve fit of the phase (determined from I and Q) with the weight being determined by the amplitude (because phase is indeterminate if sqrt(I**2 + Q**2 is zero. I think the weight is I**2 Q**2 /(I**2 + Q **2)
I/Q at 14 bits requires one of two things: (1) very accurately known waveforms, or (2) a 2-D calibration table. All the upstream signal processing stuff will have some amplitude dependence of its phase shifts, for a start.
A nulling measurement gets rid of both of these requirements, which is why I like it.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
And no typos!
-- www.wescottdesign.com
Doing it that way sounds pretty plausible. I'll think about it, and I'll certainly take you up on your offer of a second set of eyes, thanks.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
One thing I read years ago, was that the English language has fewer, and less rigid grammatical rules and cases than many of the languages to which it is related (e.g. Latin, Greek, French, German) because there was quite a long period of time during which it was essentially a peasant's language - the common tongue of the common folk - and far more a spoken language than a written one.
Scholars and rulers tended to do their business in one of the languages I mentioned above - these were the languages that were written down, preserved, studied, prescribed, and criticized. English? It's what those farmers and woodcutters speak, down at the local pub... not a subject for serious study. So, without school-marms cracking kids across the wrists with their rulers for "mis-pronouncing", pronunciation and spelling did what they did based on what seemed right to the speakers at the time.
I suspect that early English started out almost as a pidgin, mixing French and Scandinavian languages and grammers with Anglic and Saxon, and went through the common process of developing into a creole and then into a more standardized language. I've heard it described as "the language which developed so that the sons of Norsemen who had invaded and then settled down, could make dates with Saxon bar-maids in town."
And, as others have noted, English doesn't just borrow words and meanings from other languages... it sneaks up on those languages in a dark alley, clubs them into unconsciousness, and steals words out of their pockets :-)
So, pretty much by definition, "correct" pronunciation in English is the way that a large fraction of the people pronounce. Pronunciation rules are rather after-the-fact rationalizations.
Hi Phil, I've been sorta half following this thread, and I wonder if you could tell me what a nulling type phase digitizer is? (I "turn" the phase knob of a lockin type mixer/detector till the signal goes to zero?) Maybe just a reference to your instrument paper...?
George H.
Lots of people have elaborated on the point, so I won't recite it.
It's true that choosing tuning words with the lower k (one chooses a suitable value such that nothing is truncated in lookup tables) bits zero will greatly reduce the number of spurs, and get rid of the phase bump when the phase wheel rolls over, but there will still be lots of spurs from the limited width of the lookup tables and DACs.
So, the question is if your application is bothered by a bunch of spurs, some near in, at about -60 dBc. This is the key analysis to perform. If the answer is no problem, then life is simple. If it is a problem, there is a longer discussion in store.
ADI has a very good tutorial on DDS theory, "MT-085: Fundamentals of Direct Digital Synthesis (DDS)". I'd read it.
Joe Gwinn
Actually, *no one* has elaborated on a "phase jump" on the "wheel rolling" over. I have no idea what is being described with this and I think no one else does either. From the references given this is a misunderstanding from the discussion of the spurs and how they relate to the phase step size and the modulus of the counter.
What? First you say to choose a step size (I assume that's what 'k' is) so the lower bits are zero precluding truncation in the lookup table then you say the lookup tables will still give spurs. Do you mean the fact that while the phase is now exact (although the frequency choices are very limited) the resolution of the DAC still creates spurs? Yes, that is pretty obvious actually. The lookup table doesn't need to be a problem as nearly any width DAC can be matched by the lookup table. But even if a DAC had huge resolution, there will always be distortion in any analog component giving rise to harmonics and spurs.
What are your assumptions for this number? The paper you reference below shows examples with spurs below -90 dBc for a 15 bit phase input to the lookup table. The spurs for the amplitude truncation at the output of the table depend on the value chosen for that word width.
I've read this and nowhere does it mention "phase jumps". I've asked more than once for someone to explain what they mean by this and no one has stepped forward. I have to assume it is a term showing a misunderstanding of what is going on.
-- Rick
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ood analog multiplier chips. AD734 and AD834 come to mind.
approximation to a sine wave through an integrator (with the right gain) tu rns it into a straight-line interpolation approximation to a sine wave, whi ch is a lot nicer, (and slightly easier to filter).
good filter suddenly make things better?
An integrator converts the sawtooth error signal implicit in a staircase ap proximation to a sine wave to a series of much smaller of continuous arcs.
You've still got a high frequency error signal to filter out, but pretty mu ch all of the higher frequency content (at multiples of DDS update rate) ha s gone away.
The integrator isn't functioning as a bad filter here - it's a device to im prove the quality of the approximation to the desired sine wave.
-- Bill Sloman, Sydney
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One strategy that can help - and is easy to implement with ECL, which tends off balanced outputs - is to make all your signal chains balanced, so you generate both the in-phase and the 180 degree out of phase version of every clock, and ship them around to balanced lie receivers.
This means much less higher-frequecy return current circulating through you r plane, and, you get the common mode rejection of your line receivers to g et rid of any residual potential differences across your ground plane.
For stubborn problems you can invoke 1:1 transmission line transformers - t he side to side capacitance is always nasty (tens of picofarads) but they c an offer useful extra attentuation.
Watch out for gate-input interaction in gated - AM685 -style comparators
The SPAD guy in Milan used them, but found it necessary to gate them on som e 10nsec early, and delay closing the gate for 10nsec after the - possible
- event.
There wasn't much interaction between the long-tailed pair detecting the si gnal of interest and the gating signal, but there was enough to worry about .
-- Bill Sloman, Sydney
Wrong. Tom Swift mentions "variable modulus DDS chips like the AD9913, AD9914 and AD9915" so he seems to share my opinion that Analog Devices does offer non-binary radix DDS chips.
If the binary-radix DDS chips are the only parts you can buy, you need a better buyer.
-- Bill Sloman, Sydney
If the 'hidden' bits in the phase register are always zero, then the output of the DAC should be strictly periodic at f_out. That means that all, and I mean *all*, of the artifacts will be harmonics of f_out. Isn't that so?
-60 dBc is pretty crappy.
Thanks. I did read it, but it didn't say what you said.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Hi, George,
The idea is to use a phase detector wrapped in a successive approximation loop. Like other SAR ADCs, you run the register to null out the error signal to N bits' accuracy, and read off the value from the DAC control word corresponding to the null. In this case, the 'DAC' is a phase shifter.
I looked around for a copy of the instruments paper, but couldn't find it--it predates my earliest digital archives, having been published in
1987. It's atCheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
I'm not sure that follows. There are still spurs related to the clock rate which may not be a harmonic of F_out.
-- Rick
How do you get the amplitude? Don't you need to work the two together? Or I guess you can get the phase from the DAC setting at null and then calculate the amplitude from the depth of the null?
-- Rick
Okay, maybe slightly overstated--the system is never exactly perfect. However, all the aliased higher-order products that show up as close-in spurs--the ones related to the hidden bits in the phase accumulator not being the same from cycle to cycle--should be gone.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Sure. Absolutely everything repeats at Fout.
Not for DDS at non-trivial frequencies! If the DAC doesn't getcha, the opamps will. But you can bandpass filter.
And I've seen big-name RF signal generators that guarantee -20 dBc harmonics!
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
The amplitude comes from the DLVA output, suitably calibrated. One of my favourite ways to calibrate log detectors is to hit a crystal with a long tone burst at its series resonance, then turn that off and use the ring-down transient. It's very exponential, so the desired log characteristic turns into a straight line. A 10 MHz crystal with a Q of
10**6 decays with a time constant of about 0.12 dB/ms, which is pretty convenient.Most DLVAs work by cascading a bunch of differential pairs, each with a gain of about 10 dB, detecting the output of each stage, and summing the results. Every time one stage saturates, the gain of the whole drops by
10 dB. That means that the output is ideally a piecewise-linear approximation to the logarithm of the input level.The detection is often done by summing the tail currents of all the diff pairs. When a stage is running linear, the junction of the two emitters sits pretty nearly still, but once the stage saturates, it bounces up and down every half cycle, and the average current goes up, making it a reasonably decent amplitude detector.
Have a look at the SA604A datasheet.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
No, that's not correct. All of the phase words will repeat every cycle of Fout only if the modulus is a multiple of the phase step which in the case of a 2^N modulus means the step size is power of 2 as well. Or in other words, the clock rate is a power of 2 harmonic of Fout or octaves.
Having the lower order bits of the phase step be zero is not even necessary for the phase pattern to repeat each cycle of Fout. If the phase step word were all zeros except the largest bit that is truncated is a 1, the phase pattern will repeat each cycle of Fout, but there will be spurs. The same spurs you would have if the clock rate were halved for the same Fout.
This idea of arbitrarily truncating the phase value to suit a lookup table is a bit simplistic. The only real limitation is the DAC resolution. The generation of a sine value to input to the DAC can incorporate a lot of bits from the phase accumulator and does not need to be limited by the size of an actual lookup table.
BTW, has anyone considered the CORDIC algorithm for generating the sine function? I know it is used for DDS of sine waves, but I have not seen an analysis.
-- Rick
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