buckaroo
Member level 3
Hi guys,
I am measuring a fractional-N PLL based frequency synthsizer these days, and I encounter a tough spur problem as below:
the synthesizer is used in a wireless SoC for RF applications, it is found that there are two -20dBc spur (located@+/-700kHz offset) at the synthesizer output, it is quite large;
when the SAR ADC(Successive Approximation Register ADC) is OFF, the 2 tones disappears, or when the SDM(Sigma Delta Modulator) is OFF, the 2 tones disappears too; the SAR ADC clock is divided down from VCO frequency(which is to say the spur is not due to the intermodulation between harmonic of ADC clock and VCO);
the SAR ADC share the same supply with synthesizer(with RC filtering on each supply side) except VCO, the SDM is in the digital baseband side;
the location of the 2 tone depends on the fractional divide ratio, the smaller fractional ratio, the nearer offset is;
no matter how I increase the XO harmonic power(by heavily increasing the buffer size of XO buffer), the spur doesn't change;
it is not fractional spur;(99% percent)
it is not because of magnetic coupling;(because we have both LC VCO and ring VCO version, both have the 2 spur)
anyone can give me some clue? thanks very much!
Best Regards
Buckaroo
I am measuring a fractional-N PLL based frequency synthsizer these days, and I encounter a tough spur problem as below:
the synthesizer is used in a wireless SoC for RF applications, it is found that there are two -20dBc spur (located@+/-700kHz offset) at the synthesizer output, it is quite large;
when the SAR ADC(Successive Approximation Register ADC) is OFF, the 2 tones disappears, or when the SDM(Sigma Delta Modulator) is OFF, the 2 tones disappears too; the SAR ADC clock is divided down from VCO frequency(which is to say the spur is not due to the intermodulation between harmonic of ADC clock and VCO);
the SAR ADC share the same supply with synthesizer(with RC filtering on each supply side) except VCO, the SDM is in the digital baseband side;
the location of the 2 tone depends on the fractional divide ratio, the smaller fractional ratio, the nearer offset is;
no matter how I increase the XO harmonic power(by heavily increasing the buffer size of XO buffer), the spur doesn't change;
it is not fractional spur;(99% percent)
it is not because of magnetic coupling;(because we have both LC VCO and ring VCO version, both have the 2 spur)
anyone can give me some clue? thanks very much!
Best Regards
Buckaroo
Last edited: