For the baseline measurements, the Rohde SMA100B supplies the LO signal to the mixer. It is critical that the LO signal be low phase noise with very good low frequency offset performance. Poor phase noise performance in the LO translates directly to the synthesizer output. With the external PFD configuration, the phase noise of the reference is less critical; for these experiments the Agilent PSG signal generator supplies the reference signal.

The LMX2820 output frequency is 9375 MHz. The N-divider is 25. For internal VCO calibration purposes, the internal N-divider must still be set up properly. Switch to the external PFD mode and toggle the frequency calibration bit. The device must first calibrate the internal VCO with the internal dividers before switching to the external PFD input. The LO frequency is 9000 MHz. This sets the PFD frequency to 375 MHz. Figure 5-3 shows the phase noise results with the external PFD compared to the external VCO response. Notice the improved intermediate frequency offset performance due to the effective low N-divider setting and the normal internal VCO performance at the high frequency offsets.

Figure 5-3 LMX2820 External PFD Phase Noise Performance