[time-nuts] HP 5065A A1 replacement with DDS

[Bruce Griffiths]

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 <attila at kinali.ch mailto:attila at kinali.ch > wrote:
> 
> 
>     On Sun, 24 Jun 2018 17:29:43 +0000
>     "Poul-Henning Kamp" <phk at phk.freebsd mailto:phk at phk.freebsd .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
>     --
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