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  1. #1
    Newbie level 3
    Points: 286, Level: 3

    Join Date
    Feb 2018
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    Issue with bias tee in RF noise circuit

    Hi, I am facing the following problem with a bias tee in my RF application.

    I need to measure the noise output from a resistor R ~ 500 ohm (in a tank circuit) at about 120 MHz, while exciting it with a low frequency current. For noise measurement, the output of the tank circuit is fed to the low noise amplifier before being detected by an RF diode detector. The current excitation is done using a lock-in amplifier applying a voltage through a large resistor at 17 Hz. Clearly these two different regimes can only be simultaneously joined through a bias tee for which I am using Minicircuits ZFBT-4R2GW+.

    For my experiment I want to measure the amount of current passing through my resistor while simultaneously measuring RF diode output as well. It turns out that every time I connect the A point of lock-in amplifier to measure the voltage drop across the resistor (thereby measuring the current passing in Figure B), the RF diode output goes completely meaningless. On connecting the spectrum analyzer to see what goes into the RF diode, I find it is littered with junk peaks and dips.

    We tried another bias tee from Minicircuits ZX85-12G-S+, however it is bringing in too much of its own noise in out RF detection. Could you suggest any other high quality bias tees? It's important to have low insertion loss and low noise for our experiment. Also, am I using the bias tee in the right manner?

    Thanks and with best regards

    Bias tee problem.pdf

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  2. #2
    Advanced Member level 5
    Points: 34,278, Level: 45

    Join Date
    Dec 2004
    New England, USA
    1347 / 1347

    Re: Issue with bias tee in RF noise circuit

    Peaks and dips? Sounds like something in your circuit is acting like an antenna?
    OR the source you are driving it with has harmonic RF output, so try a battery driven potentiometer...with heavy capacitive shunt filtering.
    try floating the chassis ground on your spectrum analyzer where it plugs into the wall.

    realize that the KTBF noise of a 500 ohm resistor will be VERY small....so it would be hard to actually see it on a spectrum analyzer with the thousand other noise sources in your lab.

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