[time-nuts] FE-5650A option 58 tuning word for 10 MHz output

Bob kb8tq kb8tq at n1k.org
Mon Jan 9 12:17:00 EST 2017


In most Rb’s (including the FE 56xx’s) the DDS is mixed with a fixed microwave 
frequency signal. The DDS only has to make up “part” of the total offset. You get
roughly a three orders of magnitude improvement because of this. Rick has gone
into all the gory details of why it gets done this way in talking about the 5071. It
is the same thing on an Rb. 

So, your basic math is correct about a normal DDS. In this case you are in the
PPT rather than PPB range due to the multiplication. 


> On Jan 9, 2017, at 10:40 AM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> A 32-bit DDS synthesizing at 1/5 Fs, yields a tuning resolution of ~ 1 ppb.
> So, I would imagine a slightly lower frequency is programmed into the DDS
> and the c-field is trimmed to yield a higher precision. If the new
> synthesized tone you wish to generate is an integer number of DDS codes you
> could start by assuming the c-field is trimmed to be on frequency, but if
> the new tone is a fractional number of 32-bit DDS codes you will have to
> manually trim if you want higher precision.
> On Mon, Jan 9, 2017 at 4:48 AM, wb6bnq <wb6bnq at cox.net> wrote:

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