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Using a Programmable Input Multiplier to Minimize Integer Boundary Spurs

Using a Programmable Input Multiplier to Minimize Integer Boundary Spurs

The concept of using a programmable input multiplier, also nicknamed Spur-b-Gone, allows the spurs of a PLL to be drastically improved, especially for the worst case VCO frequencies. This can be done without changing the input frequency, as this is purely implemented inside the PLL. This feature is included on some TI PLLs, such as the LMX2571.

The PLL (Phased Locked Loop) starts with a fixed frequency (fOSC) and may divide it down by an integer R divider value ® to get the phase detector frequency (fPD). The VCO frequency (fVCO) is divided down by the N divider value (N) to also get the phase detector frequency. The N divider may be a fraction. The phase detector compares the N and R divider outputs and puts out correction pulses to the loop filter, which in turn converts this to a voltage. This voltage is converted to a frequency by the Voltage Controlled Oscillator (VCO).

 

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