i agree. are the graphs mislabeled?
Sorry, but I didn't understand your set-up. Bias-tee is used to supply some active part, but I don't see any: I think the only reason is that C is a varactor diode, then I suppose the circuit is a VCO.
In any case I see your graphs are labelled as S21, not S11 so it seem you measured a transmission parameter and not a reflection parameter.
You cannot connect the port 1 of a network to terminal C to measure the S11 of the tank circuit.
Please specify a little bit better how did you do to obtain the three graphs (how did you connect the ports of the network, and the content of the "next stage gray box) and what's the purpose of your measurement
First I have to clarify that yes, the graphs are mislabeled. They are all S11. I am very sorry for that mistake.
Second let me explain the purpose. I am interested in measuring the noise in the device under test which has a resistance R. In order to extract it out, I have installed R in an LCR circuit which will have a resonance that can be determined by heterodyne method that is hidden in the 'next stage'. The next stage has an amplifier, then band pass filters, frequency mixer, low pass filter and finally a read-out using a diode to achieve this purpose.
The only option for me to locate the resonance peak is to use the S11 mode of the vector network analyzer (VNA). Each of the graphs shown has been obtained by connecting the port 1 of VNA to that terminal as titled and measuring S11. This has been done in all the terminals labeled and titled in the graphs. Sorry again for the S11 and S21 confusion.
But as part of my design, it is very important that I be able to determine the resonance frequency at all times without dismantling the circuit. Therefore I use a directional coupler. I plan to use the coupler terminal to measure the S11 of the LCR circuit by hooking up the port 1 of VNA to terminal C. This is not working for me. The directional coupler itself has a strange S11 as shown in the top right corner of the page, which is already low in value. I am getting my resonance in terminal A and terminal B so I was expecting the resonance spectrum to superimpose on the S11 of the coupler. But no, there is no resonance there at 92 MHz as I was expecting (see the graph titled Terminal C).
Why am I using the bias tee? That is to satisfy another requirement - be able to measure the device resistance R at all times. The DC part (inductor part) of the bias tee goes to a constant DC current source with a voltmeter. By using the bias tee, I make sure that the DC doesn't go to the RF part of my circuit, but only to my device R.
I hope it is clear what I need. Just to reiterate, I need to see the resonance frequency at 92MHz that corresponds to my tank circuit AT TERMINAL C. Without removing the bias tee.
As you see, Terminal-a and Terminal-B measurements are almost same.So, the problem is between them.
The inductor which is in the Bias-Tee must be decoupled to GND by a low ESR capacitor otherwise ( if it's not well decoupled ) it might show a resonance as seen in your figures.
I am embarrassed to confess that I still don't understand the logic of BigBoss. Since terminal A and terminal B are almost same as I was expecting them to be, then I would think that there is NO problem between them. The resonance in the graph titled Terminal C could be coming because the combination of bias tee with the directional coupler could be leading to a new resonance. But it still doesn't explain why the main resonance at 92 MHz has vanished in the terminal C.
Thanks to all of you!