How to spec jitter for a digital NCO
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From: gretzteam on 31 May 2010 16:37 >Not incorrect -- just in need of clarification. > >The "DDS" style of NCO works well (and is by far what people usually >mean when they say "NCO"), but gains a lot from the sine lookup -- this >smooths out the inherent 'jagginess' from it's input clock, and >essentially lets it interpolate zero crossing points in an analog system. > >When you just use the overflow then you have an unavoidable timing >jitter that usually approaches +/- 1/2 an input clock peak-peak, with an >RMS timing jitter that comes directly from the fact that the timing >error is a sawtooth wave -- if you can stand this in your application, >then you're home free. I see. I was expecting the peak-to-peak jitter would be 1 input clock peak to peak. I don't think there is a way around this if I'm only willing to use 'nice-synthesizable rising-edge only digital logic'. However, if the input clock is fast relative to the generated clock, and the NCO has a good resolution, the RMS jitter can be made quite low.
From: gretzteam on 31 May 2010 16:42 >Form a reference clock signal that is jitter free and of the same >exact frequency and same average phase as the NCO. The RMS value of >the time diffeence of the NCO clock edges relative to the reference >clock, divided by the period, is the RMS jitter as a dimensionless >ratio. > >(If you don't divide it by the period, it's the RMS jitter in >time units.) > >For a given NCO you might be able to compute this analytically >from parameters such as your N and M, if it is important to >do so. > >Steve > This is where I'm struggling. You'd need to calculate the error term for at least on whole period of the NCO (by period I mean the '6 6 6 6 7' in the example above. How can you go from M and N, and calculate a sequence such as '6 6 6 6 7', apart from running the simulation? It's easy enough to write the 'for loop' but I wonder if there is a quicker/nicer way.
From: Steve Pope on 31 May 2010 16:50 In article <D5-dnVlh-YzagpnRnZ2dnUVZ_hOdnZ2d(a)giganews.com>, gretzteam <gretzteam(a)n_o_s_p_a_m.yahoo.com> wrote: >>Form a reference clock signal that is jitter free and of the same >>exact frequency and same average phase as the NCO. The RMS value of >>the time diffeence of the NCO clock edges relative to the reference >>clock, divided by the period, is the RMS jitter as a dimensionless >>ratio. >This is where I'm struggling. You'd need to calculate the error term for at >least on whole period of the NCO (by period I mean the '6 6 6 6 7' in the >example above. >How can you go from M and N, and calculate a sequence such as '6 6 6 6 7', >apart from running the simulation? Nothing wrong with running a sim if an analytical expression does not come immediately to mind. Back to the modulo arithmetic. Fc = system clock MOD = modulo of the MSB's of the phase counter NLSB = number of LSB bits (counting in binary) in the phase counter to the right of the above MSB's Your NCO can then generate any frequency which is a multiple of Fc / (MOD * 2^NLSB) without any intrinsic jitter. If that is good enough for your application then do it, and save yourself the trouble of calculating the jitter in a suboptimal design. Steve
From: dbd on 31 May 2010 17:26 On May 31, 12:08 pm, Eric Jacobsen <eric.jacob...(a)ieee.org> wrote: > ... > > The OP's description of an NCO seems strange, though, and I'm trying to > get clarification to better understand the issue. Usually fout is a > function of the PIR contents and the update frequency only. I'm not > sure how his description of N and M fit, especially since those terms > are typical for synthesizers, not NCOs. > > -- > Eric Jacobsen > Minister of Algorithms > Abineau Communicationshttp://www.abineau.com My guess is that the OP is talking "fractional-N synthesizer" and Tim and Steve are talking "DDC". My post was some fractional-N literature in case my suspicion is correct. Should we start a pool on it? Dale B. Dalrymple
From: Steve Pope on 31 May 2010 17:53
dbd <dbd(a)ieee.org> wrote: >My guess is that the OP is talking "fractional-N synthesizer" and Tim >and Steve are talking "DDC". OP clarified he/she is talking about just the NCO. Steve |