Other Parts Discussed in Thread: LMX2594, LMK61E2
We have been designing a downconverter using the LMX2592. Following the lineguide provided in the datasheet, we send the command "Calibrate" (writeR0[3]= 1) after every change in one of the frequency related registers: the command calibrate works fine. However, it happens quite often that a residual (symmetric) spur is present and, until a new calibrate command is sent, the spur is steady. We have tried disabling Autocal after the chip has done the calibration but nothing changed. In other words the spur appears and disappears following the calibrate command (writeR0[3]= 1). The frequency of the spur is ~5 kHz offset from the VCO and it is -60 dBc. The REF_IN is 156.250 MHz, the divider =2 thus the PFD is 78.125 MHz. The frequency of the spur is not related to the final VCO frequency as it still remains about at 5 kHz.
Thank you for support,