In principle DDS spurs due to phase truncation can be largely suppressed using well known techniques when an FPGA is used just leaving spurs due to the DAC.
Bruce > On 25 June 2018 at 06:00 Attila Kinali <[email protected] > mailto:[email protected] > wrote: > > > On Sun, 24 Jun 2018 17:29:43 +0000 > "Poul-Henning Kamp" <[email protected] mailto:[email protected] .dk> wrote: > > > > My plan is to replace the entire chain up to the 60 MHz with that > > DDS, and do the modulation digitally, then put one of those > > "geophone/optical" 24bit ADC's on the photo-sensor and close the > > loop. > > > > > I recently thought about that problem and came up with a solution > that might replace more than just A1: > > The idea is to use an FPGA (can be a relatively small one) as an I/Q DDS, > then use an I/Q modulator shift the frequency into the right spot > and supressing the mirror. The advantage of using an FPGA instead of > an off the shelf DDS comes from that it makes it possible to connect > an ADC to it, which samples the photo diode voltage and do the > demodulation > and integration in the digital domain, thus incuring less noise and zero > drift. Additionally, adding another DAC to generate the 10MHz or any > other frequency directly, gives the possibility to tune the Rb output > in a very wide range without incuring any problems with synthesis chain > and decoupling the reference from the Rb (ie the reference doesn't even > need to be tunable). > > Attached is the page of my notebook with the schematic of the idea. > (sorry for the bad handwriting). The components that are not labled > are: PLL: any that can work with the Ref input, e.g. ADF4001. > The VCXO is an ABLJO or any equivalent with 155.52MHz or 156.25MHz. > The divider is a D-FF, either an 74LVC74A or better NB7V52M. > The output frequency of the DDS is in the range of 11-18MHz, > depending on the output frequency and the VCXO frequency. > > > Attila Kinali > -- > It is upon moral qualities that a society is ultimately founded. All > the prosperity and technological sophistication in the world is of no > use without that foundation. > -- Miss Matheson, The Diamond Age, Neil Stephenson > _______________________________________________ > time-nuts mailing list -- [email protected] > mailto:[email protected] .com > To unsubscribe, go to > https://lists.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://lists.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
