Power Measurement with Spectrum Analyzer

Use the Spectrum Analyzer to measure the signal produced by a signal generator.

Equipment Required For Power Measurement

  • Signal Generator
  • Spectrum Analyzer
  • Variable Attenuator
  • Couple Of Cables For Interconnection

Safety precaution

Before starting remember that overloading a device like the power meter may become cause of damages.  Always estimate the power you are going to apply to the device before connecting it and be sure it fits in the accepted range. If you are not sure then adopt a safe setting.Power Measurement with a Spectrum Analyzer attenuator

Procedure for Power Measurement

There are nine steps to measure the power

  1. Set the signal generator to output a sine wave (carrier) signal with a value of -20 dBm of average power and a frequency of 2.4 GHz. Disable any modulation. If possible, disable the output of the generator (“RF OFF”).
  • Amplitude -20dBm
  • Frequency 4GHz
  • Modulation OFF
  1. Set the variable attenuator to an attenuation of -20 dB.
  2. Check again the output level of the generator (it should be -20 dBm or 10€µW) and the attenuation of the variable attenuator (-20 dB). Estimate the losses of the cables at the working frequency and compute the expected power level at the input of the power meter.
PowerExpected(dBm)=PowerGenerated(dBm)+LossesCable(dB)+LossesConnector(dB)+Attenuation(dB)

In our case the expected value cannot exceed -40 dBm.Power Measurement with a Spectrum Analyzer

  1. Switch on the Spectrum Analyzer and set the input attenuation of the Spectrum Analyzer to 20dB, the center frequency at 2.4 GHz, the frequency span at 100 MHz, the reference level at -40 dBm, RBW at 1 MHz, VBW at 100 kHz.
  2. Connect the signal generator to the Spectrum Analyzer passing through the attenuator. Enable the output of the signal generator if it was disabled.
  3. If everything is correct, the Spectrum Analyzer should display a peak signal centered at 2.4 GHz as shown in figure.

If you don’t see any signal, switch off immediately the generator and check carefully the connections, the setting of the instruments, the calculations and ask for assistance before repeating the measurement.

  1. You can read the power level of the signal directly on the screen of the Spectrum Analyzer using the grid as a reference. For a better reading, you may use the marker facilities provided by some Spectrum Analyzers. Check on the instruction manual the correct procedure.
  2. You may observe a small difference, of the order of a few dB, between the estimated and measured value. This difference is given by the losses of the connectors in the RF chain and eventually by the inaccuracy of the instruments. The signal generator is not supposed to be a reference source of power, if not stated explicitly in the manual. You can therefore use this procedure to estimate roughly the losses of the cables and connectors, but you cannot expect to get the exact value.
  3. Disable the output of the signal generator, if possible, or reduce the power at the minimum level, then switch the generator off. Disconnect the RF cable from the Spectrum Analyzer and switch it off.