For example: Agilent E4440A or Anritsu 68369
Rohde and Schwarz Calibrators
Rohde and Schwarz XSRM-Z
XSRM Rubidium Oscillator with Frequency Converter
Manufacturer: Rohde and Schwarz
Rohde and Schwarz ZV-Z32
PC 3.5 calibration kit with fixed matches 0 - 26.5 GHz, incl. DCV
Manufacturer: Rohde and Schwarz