Digital phase locked loop circuits with multiple digital feedback loops
Designs of devices having digital phase locked loop (DPLL) circuits that include multiple digital feedback loops to generate high frequency clock signals by a digitally controlled oscillator (DCO). A time-to-digital converter (TDC) module is provided in such a DPLL circuit to receive an input reference clock signal and a first feedback clock signal from a first digital feedback loop and produces a digital TDC output indicative of a first phase error caused by a difference in time between the input reference clock signal and the first feedback clock signal. A second digital feedback loop is provided to generate a second digital feedback signal indicative of a second phase error caused by a difference in frequency between a desired clock signal and a generated clock signal generated by the DCO. The first and second digital feedback loops are coupled to the DCO to generate the high frequency clock signals.
1. A digital phase locked loop circuit comprising:
a time-to-digital converter (TDC) configured to receive an input reference clock signal and a feedback clock signal and produce a first digital feedback signal;
an adder configured to add said first digital feedback signal from a first feedback loop and a second digital feedback signal from a second feedback loop and produce a digital adder output;
a digitally controlled oscillator (DCO) configured to receive said digital adder output and produce a DCO output clock signal, said DCO output clock signal being locked in phase to said input reference clock signal.
2. The digital phase locked loop circuit of claim 1 , further comprising a calibration circuit coupled to at least one of said first and second feedback loops and configured to calibrate a TDC signal provided by said TDC.
3. The digital phase locked loop circuit of claim 1 , further comprising a calibration circuit coupled to a conversion circuit to cause a first phase error from said first feedback loop and a second phase error from said second feedback to have a same unit.
4. The digital phase locked loop circuit of claim 3 , wherein said conversion circuit includes a multiplication circuit and is coupled to said second feedback loop.
5. The digital phase locked loop circuit of claim 3 , wherein said conversion circuit includes a division circuit and is coupled to said first feedback loop.
6. The digital phase locked loop circuit of claim 1 , wherein said first and second feedback loops share a sampling circuit configured to sample said input reference clock signal at a clock rate of said DCO output clock signal.
7. The digital phase locked loop circuit of claim 6 , further comprising a supervisor circuit configured to adjust a timing of a sampling at said sampling circuit.
8. The digital phase locked loop circuit of claim 1 , wherein said second feedback loop includes a digital comparison circuit configured to produce a frequency error signal indicative of a frequency difference between a desired clock signal and said DCO output clock signal.
9. The digital phase locked loop circuit of claim 1 , further comprising a digital integrator configured to convert a frequency difference between a desired frequency and a generated frequency of said DCO output clock signal into said second digital feedback signal.
10. The digital phase locked loop circuit of claim 1 , further comprising a loop filter and a dither circuit coupled between said adder and said DCO.