When is a fractional-N PLL needed?
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From: Mark on 4 Aug 2010 21:46 On Aug 4, 7:25 pm, Allan Herriman <allanherri...(a)hotmail.com> wrote: > On Wed, 04 Aug 2010 23:19:21 +0000, Allan Herriman wrote: > > On Wed, 04 Aug 2010 12:23:24 -0500, gretzteam wrote: > > >> Hi, > >> I'm trying to understand when a fractional-N PLL is required. I'm doing > >> this in an FPGA so it's an ADPLL, but I think my questions are basic > >> enough that it applies to any PLL. > > >> When using a basic PLL topology: > > >> ref -> PFD -> LPF -> NCO -> output > >> | | > >> |<-------div-N<------| > > >> I can only create output clocks that are at an integer multiple of the > >> reference input. For example, if ref=10kHz, I can create 20, 30, 40.... > > >> Say I would like to be able to generate 52kHz. I see two ways: > > >> 1) Keep the exact same topology, but add a div-M at the output of the > >> system. This way, I could set div-N=52, making the output 520kHz, and > >> div-M=10, essentially getting my back to 52kHz. However, even for > >> trivial output clock value, the N divider can get quite high which > >> makes the whole thing harder to design. > > > Dividers are easy to design and cheap to implement. The real problem is > > that the PFD frequency is lower, which leads to (usually) undesirable > > tradeoffs with the loop filter, phase noise, reference spurs, etc. > > Oops, I see I missed something there. (I thought you were talking about a > divider on the reference input rather than the NCO output.) I hope you > found the rest of the post entertaining though. > > > > > Cheers, > > Allan > > if you need only to generate a fixed frequency there may not be any particular advantage to frac N. If, on the other hand, you need to create a tunable LO that has to tune to a set of channels and the channel spacing is low and you want the reference high for phase noise, that is where a frac N can help. For example if you need to have 1 kHz channel spacing with a int N you need the ref freq to be 1 kHz and this may be too low for good phase nosie. If you use a frac N, the ref can be 100 kHz and you can still get 1 kHz channel spacing Mark
From: gretzteam on 4 Aug 2010 22:52 > >3) lower the reference frequency to 2kHz (or raise it to 13kHz for your >specific example). > >Touching on Steve's comment, for some synthesizers you can get lower >phase noise with the fractional-N division (and proper filtering of the >phase error) than you can by lowering the reference frequency (which is >one other option). > >-- After reading all the posts, I can see that the extra div-M could be also placed at the reference input. This would avoid the need for a very fast NCO, but would have more phase noise. Intuitively, dividing the reference is effectively throwing away information. I guess the sigma-delta controlled div-N is the way to go. Now I'm not too sure about: a)Rate of the sigma-delta modulator. You probably want to update the divider value only every time one full 'division' was done, which would mean it's running at the reference input rate? b)Is it as simple as feeding the wanted 'fractional' value (0.2 in this example) into say a 1st order sigma-delta modulator? Then use the 1-bit output to choose between the two wanted integer (5 and 6 in this example)? Thanks a lot for the comments! Dave
From: robert bristow-johnson on 5 Aug 2010 01:11
On Aug 4, 1:23 pm, "gretzteam" <gretzteam(a)n_o_s_p_a_m.yahoo.com> wrote: > Hi, > I'm trying to understand when a fractional-N PLL is required. I'm doing > this in an FPGA so it's an ADPLL, but I think my questions are basic enough > that it applies to any PLL. > > When using a basic PLL topology: > > ref -> PFD -> LPF -> NCO -> output > | | > |<-------div-N<------| > > I can only create output clocks that are at an integer multiple of the > reference input. For example, if ref=10kHz, I can create 20, 30, 40... > > Say I would like to be able to generate 52kHz. why not just have two NCOs, both based on the signal coming out of the LPF, where the NCO connected in feedback has a div-1 (no frequency divider) and the other NCO is identical but works on a scaled (by a factor of 5.2) version of the output of the NCO? seems to me that it would work. r b-j |