What is RFC for in this mixer?

[tony_lth]

Hi, Gurus,
I met this mixer circuit, as the following picture.
I have several questions:
1. What is the RFC for?
2. If for matching the input of diodes, why it is in the root of the pin? And the diodes input impedance should be varied by the freq and input LO power level, right?
Best,
Tony Liu

 

[vfone]

Self-biasing diode in mixer applications

I am working to understand this application's mixer design note. An image of the mixer is shown below. It uses two transistors and a hybrid ring coupler (rat-race coupler). What I am having trouble

electronics.stackexchange.com

 

[tony_lth]

In the attachment report, it showed the conversion loss when IF=1Hz, 100Hz. How can they do so?
I guess they tested in Radar, but how to calibrate the test system?
Best,
Tony Liu

 

[vfone]

The application is a Doppler (CW) Radar, where the speed of interest is shown at the IF output of the mixer as a frequency shift.
To measure independently the mixer conversion loss you can use two signal generators, as Infineon does using an evaluation board (page 7).

https://www.infineon.com/dgdl/Infineon-AN_1808_PL32_1809_130625_The%20Schottky%20Diode%20Mixer%20for%205.8%20GHz%20Radar%20Sensor-AN-v01_00-EN.pdf?fileId=5546d46265f064ff0166440730a11057

 

[tony_lth]

"using two signal generators", that's what I am curious.
It sounds impossible to find IF=1Hz for two signal generators.
So they should test in radar.

 

[vfone]

I don't know where you see 1Hz. Minimum shift frequency in Fig.10 is 10Hz.
Anyway, to test with accuracy in a radar, and to don't do this in a field, they may use a Doppler (velocity) emulator, which are pretty expensive devices.
But two signal generators (one for RF and one for LO) connected both to the same reference (10MHz let's say) may do the job.