NIST Technical Note 1337: Characterization of Clocks and Oscillators

7. PHASE NOISE AND AM NOISE MEASUREMENTS 273about I kHz. For Fourier frequencies closer to the carrier, the slope is f - 3.i.e.. the sum of the f - 2 slope of Eq. (87) and the f - I flicker noise.Phase-locked sources have phase-noise characteristics that cannot bemeasured at close-in Fourier frequencies using this basic system. Therelative sensitivity of the system can be improved by using a dual (twochannel)delay-line system and performing cross-spectrum analysis, whichwill be presented in this chapter.Labaar (1982) developed the delay-line rf bridge configuration shown inFig. 18. At microwave frequencies where a high-gain amplifier is available.suppression of the carrier by the rf bridge allows amplification of the noisegoing into the mixer. A relative sensitivity improvement of 35 dB has beenobtained without difficulty. The limitations of the technique depend on theavailable rf power and the carrier suppression by the bridge. Naturally, ifthe rf input to the bridge is high one must use the technique with adequateprecautions to prevent mixer damage that can occur by an accidental bridgeunbalance. Labaar (1982) indicated the added advantage of using the rfbridge carrier-suppression technique when attempting to measure phasenoise close to the carrier when AM noise is present. Figure 19 shows thePHASESHIFTERDELAY-LINErfBRIDGEDELAY LINE--------------------­POWERSPLITTERMIXERPHASESHIFTERFIG. 18 Carrier suppression using an rf bridge to increase relative sensitivity. (CourtesyInstrument Society of America.)IN-224