[hpsdr] QSD gain and noise.

[Philip Covington]

On 9/24/07, Phil Harman <phil at pharman.org> wrote:
> Hi Phil.
>
> Just use the sign bit from the NCO  and use this to generate a square wave.
> I think that Johan has this working already.
>
> Phil...
>

Hi Phil,

I don't think this will work well unless the output frequency is a sub
multiple of the clock frequency.   Otherwise the MSB has 1 clock
period of jitter which would be terrible at a clock of 125 MHz = 8ps.
Even at 1 GHz clock the jitter is still unacceptable = 1ps.

When the phase accumulator rolls over to the MSB, there is an
undetermined amount of residual fractional phase step left in the
accumulator.  This translates to the uncertainty of up to one clock
period in jitter.

It is easy to get tripped up on this - at one time I convinced myself
that this would work until I realized the the DAC has a very important
interpolation function in the DDS.

73 Phil N8VB

 1190640749.0