Operating Manual R&S SMIQ - Volume 1 - Rohde & Schwarz

SMIQTest Procedures5.3.5.5 SSB Phase NoiseTest assemblyTest methodMeasurementEvaluationSee section "Test Assemblies"The two signal generators are set to the test frequency andsynchronized with a phase offset of 90° (phase quadrature). Mixing to0 Hz suppresses the RF carrier, and due to the phase quadrature themixer supplies a voltage corresponding to the phase differencebetween the input signals. This is measured by the LF spectrumanalyzer and can be converted into SSB phase noise. Set the levels of the two signals generators in accordance with thespecifications of the mixer used (unmodulated or vector modulationwith max. level). For calibration, reduce the level of the DUT by 40 dB and detuneone signal generator by 20 kHz. Check the signal for harmonics; the2nd and 3rd harmonic should be more than 30 dB below thefundamental. Measure the reference value at 20 kHz at the analyzerand note it down. Set the detuned signal generator to the previous frequency and setthe signal generators for phase quadrature. To this end, raise thelevel of the DUT to the previous level and call PHASE in theUTILITIES menu. Observe the output voltage of the mixer on theoscilloscope and vary the phase until the voltage becomes 0. Read the noise voltage, normalized to a bandwidth of 1 Hz (noiselevel) from the analyzer. Determine the difference relative to the reference level and add tothe value found 6 dB for the second sideband measured(correlated) and 40 dB for level switchover.If the S/N ratio of the second signal generator is not at least 10 dBbetter than that of the DUT, the noise component of the referencesignal generator must be determined and subtracted as well. The corrected S/N ratio is the wanted measured value.Example:The measured reference level is assumed to be 12 dBm. Anoise level of -78 dBm (1 Hz) is determined at 20 kHz. Thedifference is 90 dB, plus the correction for the secondsideband (6 dB) and level switchover (40 dB), yielding anS/N ratio of -136 dB or a noise level of -136 dBc (dB referredto the carrier power). If two identical signal generators havebeen used, the result must be reduced by 3 dB for the(uncorrelated) noise power of the reference signal generator.The final result is then -139 dBc.1125.5555.03 5.23E-9