I have just purchased and setup my ADL5920 EVAL board and trying to measure the S11 of some simple loads such as 50 ohm matched load and T-resonator etc.
I am able to setup the board and connect it with quikeval.
I am also able to calibrate the board using the GUI and the measured input power given by GUI is almost correct (with some offset but I am not too concerned about it at the moment).
I have signal source at J1 (with 0dBm power typical) and I have connected load at J2.
My problem is that the power measured at J1 (incident power) is correct but the reflected power at J2 is not true representation of the load.
For example, when I connect a 50 ohm matched load at J2, the reflected power measured at J2 should be very small (in the range of -20,-30 dBm) but the GUI measures the reflected power of almost -3 dBm. The same amount of reflected power (-3 dBm) is measured even if J2 port is left opened. So, I suspect that the reflected power is not being measured correctly.
Can anyone suggest how to debug the problem?
It should be noted here that the incident power measured by GUI follows the exact amount of input power I put using signal source.
Will anyone please help me here?
what is the frequency you are testing at? the directivity of the bridge gets worst at higher frequencies. but you should be able to make reasonable reflected power measurements at lower frequencies below 1Ghz.
I have connected Broadband (0-6GHz) load at J2 and tested the reflection coefficient at multiple frequencies (10MHz, 100MHz, 1GHz, 2GHz, 5GHz etc.) but return loss value does not change a lot rather stays in the range of 2-3 dB, which actually should be in the range of 20-30dB for the matched load.
Similarly I tested a T-resonator with fo1=77.5MHz and tested return loss at multiple frequencies between fo1 and fo2 but return loss does not follow the trend, I get from S11 measured from the VNA.
it would be good check the insertion loss, S21 thru the bridge,from RFIN to RFOUT?
if the insertion is good, that suggest the RF connections are okay. but the reverse channel is not working properly.
have you verified the same load on VNA with the good return loss?
try start testing at 10MHz first, and use 15dBm in input level if available.
I can check the S21 from RFIN to RFOUT to see if there is any insertion loss.
As stated earlier, load tested has been first verified on VNA and is working fine at the test frequencies.
I have already tried testing wide range of power levels ranging from -5dBm to 20 dBm but to no avail.
If there is any other debug step which I can try that.
Another thing is that, I also have ADL5920-EVALZ in addition to EVAL-ADL5920-ARDZ DC2847A (which I am testing right now). Should I also test ADL5920-EVALZ? Since ADL5920-EVALZ does not come with GUI so I should be looking at VDIFFN and VDIFFP and convert it the difference into dBs to find S11?