I have an RFIC which has an LO input. This LO input has rails 0V and 1V and the LO input should be rail-to-rail. The termination is on-chip and may not be perfect.
The LO comes from an off-board 90 degree hybrid and enters the board via SMC connector and this is the board schematic into the chip:
How do I make sure that the 2.1 GHz LO signal is rail-to-rail at the chip input pins?
I have a GHz DSO and GHz active probe but I think that's still not enough to probe the signal there.
I have the S-parameters of the MABA balun and a transistor level model of the chip input. I hope with these I would be able to create a reasonably accurate model in a circuit simulator (Cadence). Then I would know (< 1dB) which power exactly needs to go through the SMA. If everything would be perfect, that number would be +10 dBm (the MABA balun has a 1:1 impedance ratio, hence the ideal impedance at the differential outputs is 25 Ohm and hence the amplitude is halfed. 1Vpeak at the port input is +10dBm at 50 Ohm).
Now I would have a coupler in front of the SMA input which is connected to an RF power meter.
- Is this the proper way to do it?
- Which coupling ratio is proper for this scenario? I thought about 30 dB ... giving minimum insertion loss but -20dBm should still be nicely detectable
- I have I and Q LO inputs on the board. For matching, I would insert the coupler on both inputs but connect the power meter on just one (since I only have one). Is there anything wrong terminating the unused coupler with a normal Mini-Circuits 50 Ohm terminator?
- How to deal with the fact that the signal path will not have perfect 50 Ohm termination? To my understanding, only the incident RF power would be coupled to the RF power meter and the power meter would not see which part of this is actually absorbed by the chip input (hence giving rise to the desired amplitude level) and which part reflected.