There were two responses to Dave's question (TCXO and OCXO) and perhaps neither is correct. I don't have a 8720D VNA myself and Dave wisely doesn't want to spoil his calibration seals by opening up his instrument to take a look inside. So here's my guess based on the clues.
"Agilent 8719D, 8720D, and 8722D Network Analyzers Data Sheet" http://literature.cdn.keysight.com/litweb/pdf/5964-9133E.pdf Base spec: Stability 0 °C to 55 °C ±7.5 ppm C Per year (aging) ±3 ppm C Option 1D5: Stability 0 °C to 55 °C ±0.05 ppm C Per year (aging) ±0.5 ppm C "Installation Note 8719D, 8720D, and 8722D Network Analyzer Option 1D5 High Stability Frequency Reference Upgrade Kit" http://literature.cdn.keysight.com/litweb/pdf/08720-90318.pdf High-stability frequency reference output (Option 1D5) Frequency: 10.0000 MHz Frequency stability (0 °C to 55 °C): ±0.05 ppm Daily aging rate (after 30 days): <3 x 10–9/day Yearly aging rate: 0.5 ppm/year Output: 0 dBm minimum Nominal output impedance: 50 ohms To me, these specs are a bit better than all XO and most TCXO I've seen used in test equipment. But the specs are a bit worse than most OCXO I've run across. So which is it, TCXO or OCXO? The next clue is the attached photo found at: "Agilent 08753-60158 Opt. 1D5 for 8753D/E/ES" http://www.ebay.com/itm/151256172424 So we're looking for the specs for a Corning oscillator, P/N MC599X4. I didn't find much on the web (one link called it a "controlled oscillator"). Does anyone have information on this? It looks too large for a XO or TCXO. MC might be McCoy. Or, if MC indicates MCXO (Microcomputer Compensated crystal Oscillator) then that would nicely explain the shape/size and why the specs are in between a really good TCXO and a not so good OCXO. Some photos of the inside of a MCXO (not necessarily the one in question) here: http://design.ecs.psu.edu/2.0/Design/design_images/MCXO1.jpg http://design.ecs.psu.edu/2.0/Design/projects2.html (scroll down to MCXO) /tvb
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