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VCA810 impedance matching to 50R

Michal Kopacz85
Prodigy 10 points
Other Parts Discussed in Thread: VCA810

Hello,
I used VCA810 in projectsach as regulator of power level of RF. But sometching is incorect. My RF link contain : DDS(source) -> direfential-to-singe operational amp -> low pass filter -> VCA810(end). But when I adjust VCA gain  in my signal harmonics go to hugh level. I measure witch VNA impedance and there is 3ohm-j23ohm. My RF patch is calculated to 50ohm. Signal have around 13MHz.
Can you help my witch matching output impedance to 50ohm?  Becouse i thing that is the case of bad signal THD

I need topology or meybe corect connection.

  • 0
    TI__Genius 13935 points
    Hi Michal,

    Can you attach a schematic? Is the VCA810 attached directly to the VNA? What supply voltage are you using? What is your expected output signal level?

    If you look at Figure 49 in the datasheet you can see R6 between the VCA810 and the output connector. A 49.9 Ohm resistor loaded in this position will match the amplifier output to a 50 Ohm load.

    I would expect that poor distortion is more likely due to too much gain (very large output signal) rather than bad output matching. You can use an oscilloscope to verify the actual output voltage when the distortion becomes very bad.

    Regards,
    Loren