Use DSP to implement fractional N PLL

Hi all,

I have an integer PLL and DSP in my system, I want fine resolution (a 
fraction of the PLL step size). Is possible to just use the DSP to 
adjust the reference frequency instead of using a fractional-N PLL chip?

Thanks
Guang

ghlou wrote:
> Hi all,
>
> I have an integer PLL and DSP in my system, I want fine resolution (a
> fraction of the PLL step size). Is possible to just use the DSP to
> adjust the reference frequency instead of using a fractional-N PLL chip?

you can always have two simultaneous NCOs (Numerically Controlled
Oscillator), one that is in the loop of a regular 1:1 PLL with your
incoming signal and the other which is a "stub" controlled by the
(fractionally scaled) control value of the PLL NCO.

> Thanks

FWIW,

r b-j

Another way to "tweak" the frequency is to use a VCTCXO for a reference to 
your PLL and then control it with a DAC or a PWM. What do you mean by fine 
resolution?

"ghlou" <rftengineering@rftechnology.com.au> wrote in message 
news:e8kt5v$4in$1@news-02.connect.com.au...
> Hi all,
>
> I have an integer PLL and DSP in my system, I want fine resolution (a 
> fraction of the PLL step size). Is possible to just use the DSP to adjust 
> the reference frequency instead of using a fractional-N PLL chip?
>
> Thanks
> Guang