N5511A Phase Noise Test System User’s Guide 87Absolute Measurement FundamentalsCalibrationMeasured Beat NoteThe measured beat note calibration method is the most common usercalibration and is the least complex. This method does not require anadditional source to set a calibration tone and is therefore the best optionwhen hardware is limited. The beat note frequency for each channel is set bythe relative frequency difference between the DUT and the reference in therespective channel. If the DUT and reference are very accurate sources set atthe same frequency, the resulting beat note will be very close to 0 Hz. Theadvantage is this is a simple method of calibration. The disadvantage is itAdvantages— Simple method of calibrationDisadvantages— Requires two RF sources, separated by 0.1 Hz to 50 MHz at the phasedetector. The calibration source output power must be manually adjustedto the same level as the power splitter output it replaces (requires a powermeter).Searching for the beat note will require that you adjust the center frequency ofone of the sources above and below the frequency of the other source until thebeat note appears on the oscilloscope's display. If incrementing the frequencyof one of the sources does not produce a beat note, you will need to verify thepresence of an output signal from each source before proceeding.TheoryRecall the diagram for a single channel setup, Figure 5-24. A "beat note" isestablished by shifting the frequency of the reference by 10% of the peak tunerange. This delta frequency will show up as a spur in the IF at the amplitude ofthe carrier minus any losses.The slope is measured in the linear region of the sinusoid and using the "smallangle" theorem Vpeak is determined. This value is then used to calculate thedBc values when the phase noise is measured.Beat note user calibration works identically in the dual channel setup, with theonly difference being that the calibration is performed separately by theinstrument for each of the phase detector modules.
