Texas Instruments High Performance Frequency Synthesizer System with Integrated VCO LMX25312820EVAL/NOPB LMX25312820EVAL/NOPB Datenbogen

 
 
 

The LMX2531 features an output divider which may divide the VCO frequency by two.  The result is an 
Fout frequency half the VCO frequency.  When this VCO divider is enabled a spur will occur between a 
multiple of the phase detector frequency and the Fout frequency. 
 
In the example below the phase detector frequency (F

PD

) is 10 MHz.  The VCO frequency is 2820.005 

MHz.  The divide by two is enabled and the output frequency (Fout) is 1410.0025 MHz 

1410 MHz = 10 MHz * 141, which is a multiple of the F

PD

. 

1410.0025 MHz is the divided output frequency. 
Therefore a spur will occur at 2.5 kHz offset = (1410.0025 – 1410 MHz) 

 
If the frequency was set to 1409.9975 MHz there would be a spur at 2.5 kHz because: 

1410 MHz = 10 MHz * 141, which is a multiple of the F

PD

. 

1409.9975 MHz is the divided output frequency. 
Therefore a spur will occur at 2.5 kHz offset = (1409.9975 – 1410 MHz) 
 

Technically there are spurs at caused by mixing with all multiples of the phase detector frequency but 
they will be far away from the carrier and the loop filter will eliminate them. 
 
The spur shown below at 2.5 kHz off from 1400.0025 MHz is an example of inter-modulation that 
occurs.  This only happens when the VCO divider is enabled. 
  

 

 

 

 

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