DAQ card with external clock

[kaptronika]

Hello all!

My first post here, so please be lenient
 
Get to the point. I am working on a project at university which deals with optical fiber sensors but that's irrelevant now. I have a problem with signal processing: we have two photodetectors for two optical signals, where one represents the signal from the optical fiber sensors and the second is from an auxiliary interferometer. The photodetector output is signal with  varying voltage from 0-10V based on the optical power. The thing is: I want to acquire the signal from the sensors BUT it needs to be sampled when the auxiliary signal crosses zero voltage. To be specific, the auxiliary signal is cos signal with frequency around 1MHz which may vary 10% around that frequency. Therefore, I need a DAQ card with external sampling clock for the ADC. However, due to higher frequency and satisfying the Nyquist theorem, USB cards cannot be used because they have limited bandwidth. Also I need to acquire data for about 0.5s.

I was looking for such DAQ cards and they are very expensive like NI-6115 from National Instruments.
There is also Red Pitaya, which has external sampling clock ability according to this link https://redpitaya.readthedocs.io/en/latest/developerGuide/125-14/extADC.html However, the ADCs LTC2145CUP-14 have 1MHz minimum sampling frequency. Don't know what would happen,  if that sampling frequency was lower than 1MHz (from the auxiliary signal).

So question is: Do know any DAQ card which has external clock input? Or any ideas how to overcome this problem?

Thank you.