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Part Number: LMH2110
I am monitoring power up to 20W using the LMH2110 from DC to 8GHz. To reduce the power to the detector, I am using a resistor divider connected in parallel to my output RF, the output of the detector drops to zero below 0.1 to 0.2V. I tried different resistor combinations but the linear behavior of the detector with logarithmic power fails below 0.1 to 0.2V output. This phenomenon is giving less dynamic range than 20dB to 25dB depending on frequency. Any explanation for this phenomenon? or a suggestion to get rid of the detector nonlinearity at low power levels?
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The max input power that the LMH2110 is rated for is 5 dBm. Since 20W = 43 dBm, an attenuation of at least 38 dBm is required. More realistically, you will want reasonable dynamic range, so the attenuation needs to be closer to 55 or 60 dBm. This is can pose a problem, depending on how high the target frequency is that you are detecting.
In the datasheet 8.2.2, you can use a single resistor to provide attenuation.
The equation Adb=20Log[1 + R1/Rin] would hold true to indefinitely if it were not for the parasitic capacitance of the resistors (R1 especially). The higher the frequency is, the more effect that the parasitic capacitance has.
Reducing the size of the resistor package can help. Also, placing several resistors in series reduces parasitic capacitance effects (remember caps in series add inversely).
Can you please try both of these approaches, and see if it helps with attenuation?
Any additional information you can provide would be appreciated.
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