I have to make measurements of the RF power which a circuit is delivering, and I am using an RF spectrum analyzer for that purpose. I am a beginner in this topic, so I apologize if the question is too silly.

So, I am using a 30dB attenuator before the Spectrum Analyzer input in order to ensure that the power levels entering the instrument is well within the design limits of the device.

When I obtained the RF power spectrum, I noticed that there was a setting in the Spectrum analyzer called 'Attenuation', which I could change by using the various knobs provided in the control. When I made the measurement, the 'Attenuation' setting was on 10dB.

So, my question is, have I done a mistake by using the 10dB attenuation setting when I was actually using a 30dB attenuator externally? Or does the attenuation setting mean that an additional 10dB of attenuation is provided inside the spectrum analyzer?

To sum it up: So, using my current setup, with a 30dB external attenuator and a 10dB 'Attenuation' setting in the oscilloscope, if I read a power level of 'x' at a frequency of interest, what is the actual absolute power which is being measured? Is it x+20dB? Or x+40dB?

Also, can you please explain what the attenuation setting in a Spectrum Analyzer does?

  • 2
    \$\begingroup\$ The att. setting is the same as you adding an attenuator at the input but if you add an attenuator of 10 dB you will read 10 dB less power on the SA. If you set the SA to attenuate 10 dB more then it will compensate the reading. So you don't have to subtract the 10 dB, the SA does it for you. Only if you have a very large signal, larger than the SA can handle (like more than +30 dBm) then you need an external attenuator to bring your signal down to below +30 dBm. Otherwise (your signal is less than +30 dBm), you do not need an external attenuator. \$\endgroup\$ – Bimpelrekkie Dec 17 '17 at 21:39
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    \$\begingroup\$ What you should do: connect a signal generator ouput directly to the SA input and set the generator power to 0 dBm. Now play with the SA att. setting. Observe the signal power: it remains 0 dBm whatever the att. setting. Now add a 10 dB attenuator at the SA input and play again. Now you get - 10 dBm. Observe and learn. It's not hard. \$\endgroup\$ – Bimpelrekkie Dec 17 '17 at 21:41
  • \$\begingroup\$ Thank you for your replies. I don't have a reliable way to take the function generator output to the SA, hence I asked the question. Shall I close the question? \$\endgroup\$ – Harsha Dec 18 '17 at 3:22
  • \$\begingroup\$ @Bimpelrekkie: If you're going to write an answer -- especially one that spans multiple comments -- please do it in the "Your Answer" section below. \$\endgroup\$ – Dave Tweed Dec 19 '17 at 0:48

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