The brief summary: The new DDS chips (AD9914) normally operate with a 32-bit FTW, which gives them 0.8 Hz resolution in frequency with a 3.5 GHz clock. The chip is capable of operating in a special mode called "programmable modulus mode", which allows considerably finer resolution and also allows for the possibility of exact frequency synthesis (see pp.17-18 of the datasheet for details).
What I am proposing below is that: 1) people in the group do want frequency resolution finer than 0.8 Hz for some applications, so we should enable the programmable modulus mode. 2) we can have ARTIQ use this mode in a very simple way that doesn't allow for exact frequency synthesis but just increases the effective FTW to 63 bits, which gives ~400 pHz (picohertz) frequency resolution. I am imagining that this level of frequency error is acceptable to everyone, right? > -----Original Message----- > From: ARTIQ [mailto:[email protected]] On Behalf Of Sebastien > Bourdeauducq > Sent: Monday, May 18, 2015 10:28 PM > To: [email protected] > Subject: [ARTIQ] AD9914 programming details > > Taking this to the list. > > > -------- Forwarded Message -------- > Subject: RE: [ARTIQ] Today's slides > Date: Mon, 18 May 2015 20:53:39 +0000 > From: Slichter, Daniel H. <[email protected]> > To: Jordens, Robert <[email protected]>, Sébastien Bourdeauducq > <[email protected]>, Britton, Joe <[email protected]> > > > >> 1. Programmable modulus mode. The appropriate modulus should be > > >> calculated by the master in the compilation stage, as it may be > > >> computationally expensive to do so on the SoC. For most > > >> applications, exact frequency synthesis may not be necessary and > > >> simply using programmable modulus simplistically to add effective > > >> bits to the FTW (e.g. setting B=2^31 and varying A for fine > > >> frequency tuning, see AD9914 data sheet) may be sufficient and > > >> simpler to implement. > > I think this simplistic mode of operation, which gives additional effective > bits > in the FTW but does not try for exact frequency synthesis, should meet 99% > of the needs of typical users. I would advocate going ahead and > implementing this simple B=2^31 solution for now. We would need to > determine how exactly to expose the functionality to the user. One option > would be to have a mode for the DDS, like the phase mode, which might be > called "resolution". So then we would have a method set_resolution(self, > resolution), with two options, STANDARD_RESOLUTION and > FINE_RESOLUTION. The first option would just use the usual 32-bit FTW > (profile mode with profile 0), and the second would use programmable > modulus mode, setting B=2^31 to give you an effective 63-bit FTW. If you > leave profile mode enabled, then the amplitude and phase settings for the > current profile determine amplitude and phase of the output, while the > programmable modulus determines frequency. > > If we are not implementing profile mode or digital ramps right now, this > should be a very straightforward addition to make. I think that most users in > our group are interested in the AD9914 at least in part because of the option > of an increased-resolution FTW, so I think we really should have at least this > basic capability enabled. > > > > _______________________________________________ > ARTIQ mailing list > https://ssl.serverraum.org/lists/listinfo/artiq _______________________________________________ ARTIQ mailing list https://ssl.serverraum.org/lists/listinfo/artiq
