Some time ago I started building my own scalar network analyzer. I already got some great help from Bruce_H on this forum so far for which I am very greatful! Do note that I am not a professional electrical engineer when answering/suggesting, but a chemist-turned-electronics-hobbyist, haha.
The main idea of my board is as follows:
AD9851 DDS chip outputs a sine wave with programmable frequency (5 - 60 MHz). This is passed through and LPF to a buffer amplifier which in turn sends it to a quartz crystal to assess its resonance frequency. I measure the envelope of the sine wave that passes through the quartz crystal using an AD8310 in single-ended configuration. The output is again buffered by an opamp to be sent to the microcontroller ADC to measure the voltage. I use python as my DAQ platform and to control the frequency programming over usb.
Thus far I can see that it works mostly as expected. However, I still have some issues related to noise. When I try to pinpoint the origin of the noise I can at least see that something strange is happening at the AD8310 output. During my frequency sweeps for some frequencies the AD8310 output voltage suddenly drops seemingly out of nowhere.. I know this is not much to go on, considering the full scope of the project but I was wondering if someone has had any similar issues? Most of the time it works just fine, but every once in a while it seems ot run into this instability. Could it be the AD8310 or do you suspect something else?
I posted an image of what it looks like in my DAQ software below. I would really appreciate some help/input.
If you require additional information or have ideas of what I could probe with my scope, please let me know!