72Measuring NoiseMeasuring Signal-to-NoiseMeasuring Signal-to-NoiseSignal-to-noise is a ratio used in many communication systems as an indication ofnoise in a system. Typically the more signals added to a system adds to the noiselevel, reducing the signal-to-noise ratio making it more difficult for modulatedsignals to be demodulated. This measurement is also referred to as carrier-to-noise insome communication systems.The signal-to-noise measurement procedure below may be adapted to measure anysignal in a system if the signal (carrier) is a discrete tone. If the signal in your systemis modulated, it is necessary to modify the procedure to correctly measure themodulated signal level.In this example the 50 MHz amplitude reference signal is used as the fundamentalsignal. The amplitude reference signal is assumed to be the signal of interest and theinternal noise of the analyzer is measured as the system noise. To do this, you need toset the input attenuator such that both the signal and the noise are well within thecalibrated region of the display.Step Action Notes1 Set the analyzer to theSpectrum Analyzermode.• Press Mode, Spectrum Analyzer. This enables the spectrumanalyzer measurements.2 Preset the analyzer. • Press Mode Preset.3 Enable the internalreference signal.• Press Input/Output, RFCalibrator, 50, MHz.4 Set the center frequency,span, reference level andattenuation.a. Press FREQ Channel, CenterFreq, 50, MHz.b. Press SPAN X Scale, Span, 1,MHz.c. Press AMPTD Y Scale, Ref Level,−10, dBm.d. Press AMPTD Y Scale,Attenuation, 40, dB.5 Place a marker on thepeak of the signal andplace a delta marker inthe noise.• Press Peak Search, Marker Delta,200, kHz.6 Turn on the marker noisefunction.• Press Marker Function, MarkerNoise.This enables you to view thesignal-to-noise measurementresults.
