Hello!
I think I first have to introduce myself. (And I think I also have to apologize for my bad English.) I'm a 25 years old engineering student, I like ham radio, KiCad and building circuits with vacuum tubes and other weird stuff. I like to do things analog, I can not programming computers. Although I like to play with circuits since I have been 10 years old and studying electrical engineering I'm still not an expert.
A few months ago I found a (theoretically) great VNA schematic from a russian ham radio operator:
http://ra4nal.qrz.ru/vna.shtmlIt uses a AD9851 DDS module to generate a RF signal. A directional coupler measures forward and reflected power and an AD8302 measures magnitude and phase of whatever you put into its inputs. And right there happens weird stuff no one I know can explain. So I think i have to ask some real experts.
I started measuring around this chip over the holidays at my parents home but my measurement equipment there is quite old east german and soviet stuff and would do better in a museum (but hey... It still works somehow after 30 to 40 years...).
So I found out that:
1. Input B of the AD8302 has a DC offset of about 4 volts which should not be there. Input A does not have any DC offset. Weird.
2. No matter what happens on the inputs of the AD8302, the phase output will always be 900mV. Although 900mV means a phase of 90° according to the datasheet the chip has an internal phase shift of 90°, so this should be defined as 0°. But for example with a simple filter connected between DUT and DET you cannot have a phase of 0° for all frequencies...
3. The measurement of the magnitude between A and B seems to work somehow. Actually this circuit measures reflection loss quite good, so I can analyze antennas with vnaJ on my computer. But it cannot measure such things like phases or impedances and all this kind of stuff.
I replaced this Chip several times because I thought it could be broken (first I had some cheap clones from china, 23€ for an original bug is quite expensive for a student). I even later bought an original one from mouser. But it also behaves like the chinese clones. I checked several times if there were shorts or faults on the PCB (I also checked with an eyepiece of an microscope) but there seems to be everything OK.
When I'm back home in Dresden I will measure everything again with
newer better hardware in my hackerspace.
So do you guys have an idea of what possibly could be wrong with my toy?
I put the schematic here for you:
So I thank you in advance for your answers.
Ludwig