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ADF4106 input inpedance matching to 50 OHm (?)

Question asked by stefano.piovan on Nov 4, 2011
Latest reply on Nov 30, 2011 by icollins

Dear RF forum,


I'm a RF experimenter and I'm working on a 2.4GHz LO in a wireless video application. I had wanted to base my project on a ADF4113 but unfortunately the information on the S11 parameter in the datasheet is limited up to 1.8 GHz.


Therefore I've moved to the ADF4106 but I got stucked when I tried to understand the impedance matching of the VCO output to the synthetizer input REFIN pin.


There an ANALOG DIALOGUE article "Design a Direct 6-GHz Local Oscillator", 35-6 (dated back in 2001) where in Fig. 3 the output from the "tee (18-18-18 OHm) splitter" is connected to the ADF4106 via a simple 100pF capacitor.


In others application schematics like in Figure 22 - Local Oscillator for LMDS Base Station in the ADF4106 DATASHEET where the output of the tee is connected to a 51 OHm resistor before going into the input RFIN of the ADF4106.


is such 51 OHm resistor showed in the datasheet figure necessary ?


Maybe not because:


according to ADF 4106 datasheet the parameter S11 is equal to (Mag = 0.55932,  Phase= –82.8246) which in the Smith Chart correspond to a 50*(0.55 -j0.95) OHm and consequently the |Zin| = |27.5 -j47.5| = 54 OHm @2400MHz.


In other words RFIN impedance of the synth is already very close to the 50 OHm required to match the tee impedance.


I'll be glad to share these my few elementary consideration but so important for me.


Thank you very much in advance


Stefano Piovan
Padova (I)
T.  +39 347 3054025