Hi all. I don't have much background in PLL implementation and would appreciate if someone can refer me to basic info. I need to measure the phase of an unmodulated 15 kHz tone down to 0.1 degree if possible. The tone has decent S/N of about 30-40 dB. For acquisition purposes I know its frequency closely within +/- 1 Hz, and I can take up to 10-15 seconds to acquire lock. Could do this in either the analog or digital domain. Is 0.1 deg phase resolution a reasonable goal? Thanks much.
Phase resolution in PLL at audio frequencies
Started by ●July 13, 2009
Reply by ●July 13, 20092009-07-13
PalapaGuy wrote:> Hi all. I don't have much background in PLL implementation and would > appreciate if someone can refer me to basic info. > > I need to measure the phase of an unmodulated 15 kHz tone down to 0.1 > degree if possible. The tone has decent S/N of about 30-40 dB. For > acquisition purposes I know its frequency closely within +/- 1 Hz, and I > can take up to 10-15 seconds to acquire lock. Could do this in either the > analog or digital domain. Is 0.1 deg phase resolution a reasonable goal?Phase is a relative measurement. What is your reference? In other words, phase relative to what? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●July 13, 20092009-07-13
>PalapaGuy wrote: >> Hi all. I don't have much background in PLL implementation and would >> appreciate if someone can refer me to basic info. >> >> I need to measure the phase of an unmodulated 15 kHz tone down to 0.1 >> degree if possible. The tone has decent S/N of about 30-40 dB. For >> acquisition purposes I know its frequency closely within +/- 1 Hz, andI>> can take up to 10-15 seconds to acquire lock. Could do this in eitherthe>> analog or digital domain. Is 0.1 deg phase resolution a reasonablegoal?> >Phase is a relative measurement. What is your reference? In other words,>phase relative to what? > >Jerry >-- >Engineering is the art of making what you want from things you can get. >�����������������������������������������������������������������������Well, the problem I'm dealing with is that the only filtering on the tone so far is a 10 Hz BPF which lets more noise through than I want. Presently the noise-induced phase jitter is on the order of 5-10 degrees. I'm planning to use the PLL to reduce this jitter so that I'm able to accurately measure the zero crossing times down to 0.1 degrees of error.
Reply by ●July 13, 20092009-07-13
On Mon, 13 Jul 2009 10:07:21 -0500, PalapaGuy wrote:> Hi all. I don't have much background in PLL implementation and would > appreciate if someone can refer me to basic info. > > I need to measure the phase of an unmodulated 15 kHz tone down to 0.1 > degree if possible. The tone has decent S/N of about 30-40 dB. For > acquisition purposes I know its frequency closely within +/- 1 Hz, and I > can take up to 10-15 seconds to acquire lock. Could do this in either > the analog or digital domain. Is 0.1 deg phase resolution a reasonable > goal? > > Thanks much.SNR at what bandwidth? Given a level of random noise after filtering your phase measurement noise in radians is roughly equal to 1/2 the SNR; 0.1 degrees is about 0.0017 radians, which implies a SNR of 50dB or so. BUT -- SNR is only meaningful for a defined bandwidth, and if you have ten seconds to make the measurement you could, in theory, achieve a practical noise bandwidth of 0.1Hz, and I doubt that your SNR is measured at your 15kHz +/- 0.05Hz. I suspect that you'd have quite a challenge to do this measurement with analog hardware in the space of 10 seconds, because getting the necessary precision _and_ settling time would be quite a trick (it's probably possible, and I'll bet there were clever systems built for the military in the '70s that did it with analog hardware, but it'll be a lot easier to acquire a vector of samples and chew on them after the fact). -- www.wescottdesign.com
Reply by ●July 13, 20092009-07-13
PalapaGuy wrote:>> PalapaGuy wrote: >>> Hi all. I don't have much background in PLL implementation and would >>> appreciate if someone can refer me to basic info. >>> >>> I need to measure the phase of an unmodulated 15 kHz tone down to 0.1 >>> degree if possible. The tone has decent S/N of about 30-40 dB. For >>> acquisition purposes I know its frequency closely within +/- 1 Hz, and > I >>> can take up to 10-15 seconds to acquire lock. Could do this in either > the >>> analog or digital domain. Is 0.1 deg phase resolution a reasonable > goal? >> Phase is a relative measurement. What is your reference? In other words, > >> phase relative to what? >> >> Jerry >> -- >> Engineering is the art of making what you want from things you can get. > > Well, the problem I'm dealing with is that the only filtering on the tone > so far is a 10 Hz BPF which lets more noise through than I want. Presently > the noise-induced phase jitter is on the order of 5-10 degrees. I'm > planning to use the PLL to reduce this jitter so that I'm able to > accurately measure the zero crossing times down to 0.1 degrees of error.So use a 2-Hz bandpass filter and reduce the noise by 5. (Easier to say than to do, but you get the point.) It appears that you don't need to measure the phase, but rather keep jitter within limits. Noise-induced jitter of of .1 radian implies a SNR of about 20 dB. Surely, you can do better than that with a few Hz bandwidth! Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●July 13, 20092009-07-13
>PalapaGuy wrote: >>> PalapaGuy wrote: >>>> Hi all. I don't have much background in PLL implementation andwould>>>> appreciate if someone can refer me to basic info. >>>> >>>> I need to measure the phase of an unmodulated 15 kHz tone down to0.1>>>> degree if possible. The tone has decent S/N of about 30-40 dB. For >>>> acquisition purposes I know its frequency closely within +/- 1 Hz,and>> I >>>> can take up to 10-15 seconds to acquire lock. Could do this ineither>> the >>>> analog or digital domain. Is 0.1 deg phase resolution a reasonable >> goal? >>> Phase is a relative measurement. What is your reference? In otherwords,>> >>> phase relative to what? >>> >>> Jerry >>> -- >>> Engineering is the art of making what you want from things you canget.>> >> Well, the problem I'm dealing with is that the only filtering on thetone>> so far is a 10 Hz BPF which lets more noise through than I want.Presently>> the noise-induced phase jitter is on the order of 5-10 degrees. I'm >> planning to use the PLL to reduce this jitter so that I'm able to >> accurately measure the zero crossing times down to 0.1 degrees oferror.> >So use a 2-Hz bandpass filter and reduce the noise by 5. (Easier to say >than to do, but you get the point.) > >It appears that you don't need to measure the phase, but rather keep >jitter within limits. Noise-induced jitter of of .1 radian implies a SNR>of about 20 dB. Surely, you can do better than that with a few Hzbandwidth!> >Jerry >-- >Engineering is the art of making what you want from things you can get. >�����������������������������������������������������������������������Yes that's what I was trying and failing to say. The phase jitter now is about 0.1 radian after the 10 Hz BPF. I could either narrow the filter which would take some effort to squeeze out a few more dB of S/N, or use a PLL with much narrower BW than the filter could give me, since I have 10 sec or more available for acquisition time. So bottom line: I'm being realistic shooting for 0.1 degrees jitter limit?
Reply by ●July 13, 20092009-07-13
PalapaGuy wrote:> Hi all. I don't have much background in PLL implementation and would > appreciate if someone can refer me to basic info. > > I need to measure the phase of an unmodulated 15 kHz tone down to 0.1 > degree if possible. The tone has decent S/N of about 30-40 dB. For > acquisition purposes I know its frequency closely within +/- 1 Hz, and I > can take up to 10-15 seconds to acquire lock. Could do this in either the > analog or digital domain. Is 0.1 deg phase resolution a reasonable goal?The goal can or can not be met depending on how do you define the SNR. However the 0.1 deg phase resolution in the audio system is outside of the reason. This corresponds to the time delay about 20ns; the phase drifts are dozens of times higher then that. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●July 13, 20092009-07-13
PalapaGuy wrote: I wrote: ...>> It appears that you don't need to measure the phase, but rather keep >> jitter within limits. Noise-induced jitter of of .1 radian implies a SNR > >> of about 20 dB. Surely, you can do better than that with a few Hz > bandwidth!...> Yes that's what I was trying and failing to say. The phase jitter now is > about 0.1 radian after the 10 Hz BPF. I could either narrow the filter > which would take some effort to squeeze out a few more dB of S/N, or use a > PLL with much narrower BW than the filter could give me, since I have 10 > sec or more available for acquisition time. > > So bottom line: I'm being realistic shooting for 0.1 degrees jitter > limit?In one sense, a PLL is a very narrow band filter, so yes. On the other hand, the more the input signal's phase jitter, the lower the PLL's bandwidth needs to be to reach a given spec, and that makes for longer acquisition and settling times. Why not take a different tack altogether? Count zero crossings for ten seconds, and determine the average frequency by dividing by ten? That's all the information that the signal can yield, so no method can do better. A tighter filter will help the end points, so there's maybe a factor of two there if you need it. Jerry -- Engineering is the art of making what you want from things you can get.
Reply by ●July 13, 20092009-07-13
> > >PalapaGuy wrote: > >> Hi all. I don't have much background in PLL implementation and would >> appreciate if someone can refer me to basic info. >> >> I need to measure the phase of an unmodulated 15 kHz tone down to 0.1 >> degree if possible. The tone has decent S/N of about 30-40 dB. For >> acquisition purposes I know its frequency closely within +/- 1 Hz, andI>> can take up to 10-15 seconds to acquire lock. Could do this in eitherthe>> analog or digital domain. Is 0.1 deg phase resolution a reasonablegoal?> >The goal can or can not be met depending on how do you define the SNR. >However the 0.1 deg phase resolution in the audio system is outside of >the reason. This corresponds to the time delay about 20ns; the phase >drifts are dozens of times higher then that. > >Vladimir Vassilevsky >DSP and Mixed Signal Design Consultant >http://www.abvolt.com >Yes I understand there will be phase drift (although frequency drift will be negligible for PLL lock purposes). The phase drift actually is what I am trying to measure, and I'm trying to achieve 0.1 degree measurement accuracy of that phase. I can't define S/N accurately because it is subject to variation. But loosely speaking I have about 30-40 dB S/N due to jitter and thermal noise combined in the filter's noise BW. My feeling from the very helpful responses above is that I DO have a shot at measuring phase to the accuracy I need. Can't hurt to try. Thanks everyone.
Reply by ●July 13, 20092009-07-13
>PalapaGuy wrote: > >I wrote: > > ... > >>> It appears that you don't need to measure the phase, but rather keep >>> jitter within limits. Noise-induced jitter of of .1 radian implies aSNR>> >>> of about 20 dB. Surely, you can do better than that with a few Hz >> bandwidth! > > ... > >> Yes that's what I was trying and failing to say. The phase jitter nowis>> about 0.1 radian after the 10 Hz BPF. I could either narrow thefilter>> which would take some effort to squeeze out a few more dB of S/N, oruse a>> PLL with much narrower BW than the filter could give me, since I have10>> sec or more available for acquisition time. >> >> So bottom line: I'm being realistic shooting for 0.1 degrees jitter >> limit? > >In one sense, a PLL is a very narrow band filter, so yes. On the other >hand, the more the input signal's phase jitter, the lower the PLL's >bandwidth needs to be to reach a given spec, and that makes for longer >acquisition and settling times. > >Why not take a different tack altogether? Count zero crossings for ten >seconds, and determine the average frequency by dividing by ten? That's >all the information that the signal can yield, so no method can do >better. A tighter filter will help the end points, so there's maybe a >factor of two there if you need it. > >Jerry >-- >Engineering is the art of making what you want from things you can get.:o) I was planning to do something similar. First, get each zero-crossing measurement as close as possible using the PLL as a jitter filter, then measure over a several-second interval and average the results. Although the phase drifts over the interval (but frequency is relatively constant), for my purposes all I need is the average phase over the interval. Thanks again.
