MSO5 series spurious tone

I'm trying to use an MSO5 series scope to do some low level measurements and I noticed persistent spurs at integer fractions of the sampling frequency, with significant spurs at 195.3125 MHz, 390.625 MHz and 781.25 MHz.

The level of the measured noise changes with the vertical settings and increases with larger vertical divisions.

I can even pick up a very faint (about -95dBm) 390.625MHz tone when the scope is running with a spectrum analyzer which sits next to it. The tone disappears when the scope is turned off.

Is this normal? I am new to Tektronix scopes.



 

Hello,

Spurs at integer fractions of the sample clock are normal.  These can be slightly reduced by running regularly Signal Path Compensation (SPC) after the scope has warmed up.  For low level measurements we recommend running SPC once a week, or if the ambient temperature has changed by more than 5C.  You can run SPC by pressing Utility > Calibration > Run SPC.  

The other two things are also normal in terms of digital signal processing and how spectrum analyzers work.

Along with this, changing the horizontal/vertical scale on the MSO5 series moves both the waveform and graticule relative to the display position, which may be something you're not used to on other scopes.