This generator from Rohde & Schwarz works from 100Khz at 1GHz. The generation of signals is made by three VCO whose operating ranges are nested into each other over the entire operating range of the equipment.

This is a trick for the generation of RF signals over a wide operating range but it slightly complicates the repair of a classic failure of this device : the loss of lock on one or more PLL that drive the three VCO.

I had to readjust these VCO twice in 5 years. Indeed, the VCO drift with use to a point where the PLL is no long able to hang. At this time, the generator reports an lock error and does not generate anything (or in some cases a free signal on a different frequency that the one set on the front panel).

To resolve this problem, this is how I did it :

  • Remove the cover of the unit and return it to see the photo below:


  • Note the position of the three VCO as I did then remove the cover of this block.


  • Set the generator to the 25.00001 MHz on the front panel and adjust the VCO1’s capacity to measure a voltage of about 3V on the point surrounded of VCO1.
  • Then set the generator on 40.945690 MHz and check that the voltage at the point of measurement is approximately 18V.
  • Do the same for the setting of VCO2 in use 40.9457 MHz for the minimum and 51.56 MHz to the maximum frequency of the generator’s front panel value.
  • Do the same for the setting of VCO3 in use 51,57 MHz for the minimum and 62 MHz to the maximum frequency of the generator’s front panel value.

In doing so, we get a few years of operation with a good signal generation across the range of use of SMX.


A venir…