I’m assuming you are using the sweep mode + markers to perform these frequency measurements? This will have limitations, notably the frequency resolution will be limited to your RBW. If you wanted 1Hz resolution on your measurement you would need ~1Hz RBW. At higher RBWs, the freq bins will land on discrete steps that might give you the illusion that one device is more “accurate” than another, in this case it sounds like the BB60D has a frequency bin that lands exactly on 10MHz, and the CW energy happens to be closer to that bin than the adjacent.
We have other methods for measuring the frequency of a CW signal that are preferable. You can use either the frequency difference meter utility, or the analog demod analysis mode, which will provide you a high accuracy frequency measurements of a CW input with faster update rates than a low RBW sweep.
For logging, sweep mode + low RBW might be ideal. There is a sweep recorder which can log sweeps over long periods. Depending on how much time resolution you need, you can add artificial delays in the in the sweep recording toolbar so that you have adequate time and frequency resolution over your 12 hour period without a prohibitively large recording file.
The SDK will allow you to retrieve sweeps and I/Q data. All post processing such as frequency estimation must be performed by your application, but this gives you the most flexibility in how you log your measurements.
Keep in mind, the OCXO you are measuring will have a frequency error. In addition, as you mention, without a known reference, you don’t know what offset either of your SA’s have. A common approach to this is to use an external 10MHz reference, commonly a GPS disciplined one, which has a known frequency accuracy and is generally quite good, and provide this as a reference into the SA’s 10MHz input port. When an external 10MHz reference is connected to the SA’s, you can instruct the device to use it in the “Settings -> Reference” menu.
