Why do spurious signals appear in my spectrum analyzer span when I apply a clean signal to the RF input?
Overdriving the input mixer of the analyzer may cause the spurs. Most spectrum analyzers, particularly those that use harmonic mixing to extend the tuning range, have diode mixers. Internal distortion is created when the LO used to create the IF signal is combined with the input signal in this diode mixer. Second and third order distortion products are specified for a range of mixer input levels. Refer to the dynamic range curves in the Calibration Guide or Specifications Guide for your spectrum analyzer. Spur-free dynamic range is dependent upon the input level at the mixer. For a deeper discussion of dynamic range, refer to Spectrum Analysis Basics, Application Note 150.
Understanding the dynamic range graph is important, but a simple test can determine if the displayed spurious signal is an internally generated mixing product or part of the input signal: change the input attenuation. The attenuator is the only hardware between the RF input and the first mixer. Place a marker on the spur and increase the input attenuation. If the marker value does not change, the spur is external. If the marker value changes, the signal is internal or the sum of external and internal signals. Continue increasing the attenuation until the marker value does not change any further and make the measurement. This is the point at which the input level to the first mixer is optimized for least internal distortion for this measurement. Generally speaking, the more dynamic range required to make the measurement, the lower the input level to the first mixer should be.
The yellow trace in the screen image below shows an internal distortion product when the input mixer is overdriven. The attenuation is zero. The blue trace shows the reduced level of the spur when the attenuation is set to 10 dB.