For example: Agilent E4440A or Anritsu 68369
Frequency Time Standards
Racal Instruments 1990
Universal Counter DC to 1.3 GHz 8-digit IEEE-488. Input C to 1.3 GHz

Manufacturer: Racal Instruments
Keysight / Agilent 58503A
GPS Time and Frequency Reference Receiver

Manufacturer: Keysight / Agilent
Stanford Research Systems FS700
10 MHz LORAN-C Frequency Standard

Manufacturer: Stanford Research Systems
Spectracom GPS-12R
GPS-controlled Rubidium Frequency Reference with 1.544/2.048 MHz 1-pps

Manufacturer: Spectracom