Can I use 11970 or 11974 Series external mixers with my Keysight 859x Series Spectrum Analyzer?
No, while the 859X products did offer an L.O. output on the rear panel with Option 010, Tracking Generator. There was never a provision for an I.F. Input, which is required for external mixing. Therefore, external mixing cannot be performed using an 859x product.
Individual mixer product pages describe the Keysight spectrum analyzers with which they are compatible.