We are using E5052B to measure the output phase noise of a DDS module. At 300.01MHz output the measured Phase Noise at 10 KHz offset is -142dBc/Hz. However, when I configure the DDS module for 300MHz Output, phase noise will increase to -130dBc/Hz (12dB Difference). The only difference between these two tones is several near-carrier spurs (less than “100Hz” offset) with level of -80dBc at 300MHz signal spectrum.
Is it possible that E5052B, not being able to resolve this spurs, so shows them as a spread out noise spectrum close to the carrier at 300 MHz (and causing 12dB increase in phase noise at 10 KHz offset)?