Differential output impedance matching

[veeti]

Hello

I Found really cool project of 6GHz radar https://hforsten.com/third-version-of-homemade-6-ghz-fmcw-radar.html

But i really can't uderstand his IF output solution from the mixer.

-Bias is done via 50ohm resistor, but chip output is 200ohm - doesn't it restrict it?
-If Mixer output impedance seems to be 200 Ohm and ADA4940-02 is set to have 1100 Ohm impedance
-I allso have tried to find filter calculator to verify/understand the filter design, but the only document which i have found is this https://www.analog.com/en/technical-articles/understanding-and-designing-differential-filters-for-communications-systems.html And from there i can't understand terms "normalized" C1 = 1.433, L2 = 1.594, C3 = 1.433

[David Hess]

The datasheet you linked explains exactly what is happening.  The outputs are open collector (1) which means high impedance current outputs.  The output impedance is essentially set by the load resistance.  In a less critical application, simple 50 ohms pull-ups will yield a 100 ohm differential impedance which will work fine.

(1) They could be the collector of a cascode transistor for maximum performance.  If not, an external common base cascode stage could extract better performance.