Don't do this with your BG7TBL distribution amplifier

[erikka]

Some time ago I purchased a BG7TBL distribution amplifier. It has been working well.
But as it mentioned on the front panel it could also be a 10 MHz frequency standard so I opened it up and was happy to see you only needed to add an OCXO to have an standalone 10MHz standard and distribution amp.
The external input was still operational and after connecting to an external clock the device switches over to the external clock, this is nice behavior when you want to run everything from one better reference.
But something weird showed up.
When ever you connect an external reference some phase shifts started to happen. To investigate I used an external source at 10.000001 MHz, 1 Hz offset and recorded the output phase versus the external source using Timelab.

As you can see in attached picture the output phase shifts with the same period as the difference in frequency. This suggests that the internal OCXO leaks into the external input signal and causes a phase shift depending on the phase of the internal OCXO versus the external input
Something to be aware of and maybe avoid this distribution amplifier with an internal OCXO if you ever need to connect it to an external reference.

[NiHaoMike]

Wouldn't an easy fix be to add a circuit to disable the internal OCXO when external input is connected? Or if the failsafe behavior is wanted, have the OCXO be phase locked to the input.

[erikka]

Yes,
Depending on your phase accuracy requirements sometimes you have to do a bit more work.