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[Technical article / FAQ, rfforum Editorial Team]
Bias tees look like a two-port convenience until the DC arm leaks RF, the RF arm leaks supply ripple, or the inductive path resonates inside your band. On the bench, a "good" DUT S11 can be the tee, and a spur can be the regulator through the tee.
1. What the tee actually owns
- Finite RF–DC isolation puts supply switching edges and cable pickup onto the RF line; narrowing RBW will not remove a real conducted spur from the DC arm
- The inductive path and capacitors set a high-pass / low-pass corner; near that corner return loss and group delay move even when the DUT is quiet
- Long DC leads without local bypass turn the tee into an antenna for pickup and into a resonant stub at VHF/UHF
2. Freeze list before you blame the DUT
- Measured isolation (RF port ↔ DC port) across band with the real cable length, not only the datasheet plot
- Local bypass at the tee DC pin and at the DUT bias pin, on the same sheet as S11 and spur plots
- Whether you re-check thru / short residual with the tee in the path after final dress
- Regulator family and switching frequency next to any spur that tracks attenuator toggles oddly
3. Habit
Archive tee PN, isolation plot, and bypass sketch next to every biased S-parameter or spur claim. Swapping a "temporary" tee between cal and DUT is a method change.
Discussion: on your last bias-path spur or S11 surprise, did the fix land in tee isolation, local bypass, or the regulator — and do you require an isolation note before freezing a biased measurement?
— rfforum 编辑部 / Editorial Team
Bias tees look like a two-port convenience until the DC arm leaks RF, the RF arm leaks supply ripple, or the inductive path resonates inside your band. On the bench, a "good" DUT S11 can be the tee, and a spur can be the regulator through the tee.
1. What the tee actually owns
- Finite RF–DC isolation puts supply switching edges and cable pickup onto the RF line; narrowing RBW will not remove a real conducted spur from the DC arm
- The inductive path and capacitors set a high-pass / low-pass corner; near that corner return loss and group delay move even when the DUT is quiet
- Long DC leads without local bypass turn the tee into an antenna for pickup and into a resonant stub at VHF/UHF
2. Freeze list before you blame the DUT
- Measured isolation (RF port ↔ DC port) across band with the real cable length, not only the datasheet plot
- Local bypass at the tee DC pin and at the DUT bias pin, on the same sheet as S11 and spur plots
- Whether you re-check thru / short residual with the tee in the path after final dress
- Regulator family and switching frequency next to any spur that tracks attenuator toggles oddly
3. Habit
Archive tee PN, isolation plot, and bypass sketch next to every biased S-parameter or spur claim. Swapping a "temporary" tee between cal and DUT is a method change.
Discussion: on your last bias-path spur or S11 surprise, did the fix land in tee isolation, local bypass, or the regulator — and do you require an isolation note before freezing a biased measurement?
— rfforum 编辑部 / Editorial Team
