[time-nuts] FE-5680A FAQ update: question about frequency synthesizer architecture

[Graham / KE9H]

On 1/27/2012 12:27 PM, beale wrote:
> I added a bit to the "electronics" section of the FE-5680A FAQ as below.
> http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:fe5680a_faq#electronic
>
> (Note- until today, I had the 8 and 6 digits transposed, calling it the fe5860a. But no one noticed :-)
>
> The updated section is below. I measured the 20 MHz input and 5.3 MHz output of the DDS, but I'm puzzled by how the tuning resolution (4.6 mHz) of the DDS output is divided by such a large factor to achieve 0.18 uHz resolution at the final 10 MHz output. Can any frequency synthesizer gurus explain how this is done?
>
>

The AD9832 is an Analog Devices DDS which has a 32 bit tuning word.

The way a DDS generates the output, is that it (effectively) has a cosine
wave look-up table, with 2^32 entries that comprise a single cosine wave 
cycle.

The tuning word tells it how many entries the DDS should advance every
reference input clock cycle, then it pushes that amplitude value in the 
look-up table
to the output D->A converter.

So, if the input reference is 20 MHz, then the DDS can generate frequencies
with a resolution step of

      Vref/2^32  =  20,000,000 / 4,294,967,296  =  0.0046566 Hz.

The DDS output frequency is    (tuning word /2^32) times Vref.

In the actual implementation, rather than a 4 billion entry look-up table,
I am sure they have some algorithm that calculates the amplitude of
a cosine wave, or a much smaller table with a sophisticated interpolation
routine.

--- Graham / KE9H

==