Hello Everyone,
Can someone please explain me how the output coming out of PLL or DLL loop filter is utilised by the NCO to generate new frequency using the look-up table.
I have read it in books that NCO has a phase accumulator which coverts the output from the loop filter to some bits, and based on these bits we see the look-up table and find the corresponding value for these bits.
I fail to understand here that how does the phase accumulator converts the output of the loop filter to bits(1s and 0s). I need to know this because I have to use this NCO in a MATLAB program.
Thanks,
Priyanka
Dear Priyanka,
welcome,
The numerically controlled oscillator NCO is a sinewave oscillator whose frequency and phase are controlled( varied) numerically by code word which is binary coded values for addressing the frequency and phase values in a lock up table.
i will give a simple example
It is required to numerically generate a sine wave with frequency f and phase phi, then
we can express this signal by the relation:
x= xm sin (2pi f t +phi)
In order to generate such waveform digitally we have to sample it in time with a sampling frequency fs which is an an integral multiple the sinewave frequency f. So we can express t=nTs, with n is the time index of the samples, Ts is the sampling time which is = 1/fs,
After time disceretization the above expression can be written as:
x[n] = xm sin 2 pi f nTs +phi
we can further express as a programmable value as k delta f, with delta f is a fixed clock frequency and k is an integer value which represents the numerical control of the frequency.
The third parameter is the phase programming where one can express
phi= p delta phi , with delta phi is the phase resolution and p is the phase programming value which is an integer value.
Finally we can express the sine wave as:
x[n]= xm sin { (2 pi kn deltaf/fs) + p delta phi}
one can select a suitable value for n deltaf/fs, then remains k and p to adjust the frequency and phase.
Best wishes
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