N5511A Phase Noise Test System User’s Guide 65Absolute Measurement FundamentalsSelecting a Reference (Single Channel)Selecting a Reference (Single Channel)Selecting an appropriate reference source is critical when you are making aphase noise measurement using the phase lock loop technique. The key toselecting a reference source is to compare the noise level of the reference withthe expected noise level of the DUT. In general, the lower the referencesource’s noise level is below the expected noise level of the DUT the better.(Keep in mind that you only need to be concerned about the reference source’snoise level within the frequency offset range over which you plan to measurethe DUT.)As shown by the graph in Figure 5-9, the further the reference source’s noiselevel is below the noise level of the DUT, the less the reference source’s noisecontributes to the measurement results.Figure 5-9 DUT noise approaches reference noiseUsing a Similar DeviceThe test system performs best when you are able to use a device similar to theDUT as the reference source for your PLL measurement. Of course one of thedevices must be capable of being voltage tuned by the system to do this.To select a similar device for use as the reference source, you must establishthat the noise level of the reference source device is adequate to measure yourDUT. The Three Source Comparison technique enables you to establish theactual noise levels of three comparable devices when two devices are availablein addition to the DUT.If only one device is available in addition to the DUT, you can perform thePhase Noise Using a Phase Locked Loop Measurement using these twodevices and know that the noise level of each of the devices is at least as goodas the measured results. (The measured results represent the sum of the noiseof both devices.)Using a Signal GeneratorWhen using a signal generator as a reference source, it is important that thegenerator’s noise characteristics are adequate for measuring your device.
