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LMX2492EVM: output impedance lmx2492evm and attenuator

romain fouettilloux
Prodigy 70 points
Part Number: LMX2492EVM
Other Parts Discussed in Thread: LMX2492

Hi, 

can you tell me what is the real and imaginary part of the impedance of the lmx2492evm on the rf output and rf output/2 with the original vco macom ? 

Can you tell me what is the attenuator is used ? If i don't want attenuator, can I put resistance 50ohms (what component references to take?)); 

thank you. 

  • 0
    TI__Guru** 107280 points
    Hi Romain,

    I don't have that data, maybe you'd better asked MACOM. From the datasheet, the return loss is not pretty high, so I assume a resistive 50ohms loading is beneficial to the VCO.
    Let say if you only need RF/2 output, my suggestion is feedback RF output to LMX2492. RF --> 50 ohms to ground --> DC blocking --> Fin. I suggest a 3dB pad at RF/2 output.
  • 0 in reply to Noel Fung
    Prodigy 70 points

    I have the datasheet and the output return loss is 3dB (-3dB maybe ??). I don't understand what is your datasheet where "the return loss is not pretty hight"??  I want to know also

    Moreover, I want to have the 4dBm of the macom vco and I want to replace the red and green components attenuator (R8,R10,R9), etc..(R7,R8). I want to have a vco board 50ohms without atténuator, please.

  • 0 in reply to romain fouettilloux
    TI__Guru** 107280 points
    Hi Romain,

    If the return loss is less than 10dB, we know that the impedance we are seeing is far away from 50Ω.
    If you want 4dBm from RF/2, you cannot add any attenuator, you can remove all the circled resistors.
    You still need a power splitter to feedback some signal back to the PLL. A simple 3-way resistor splitter has an insertion loss of 6dB. As a result, you will only get 1dBm from RF port. If you want more power, you will need to use discrete power splitter from vendors like MiniCircuit.