Recently a customer asked what the settling time of the DC port on our surface mount bias tees was. Settling time is a tricky spec. When you apply a DC voltage to the bias tee, you want it to show up at your device immediately so you can do whatever it is that you need to do. Conversely, you don’t want any high frequency noise from the DC line contaminating your bias, so we try to make the DC to common port isolation high down to low frequencies. Here is the step response of some various bias tees to the following step input:
This is a pretty clean, low rise time input pulse. Here is the response of our most common bias tee, the BT-0034SM. Note that this is the same circuit as the BT-0014SM, BT-0014SMG, BT-0024SM, BT-0024SMG, and BT-0034SMG, they are just tested over a different range:
If we zoom in a little more, we can see the immediate response:
There is a fast rise in response to the applied signal, a brief overshoot, and then it exponentially approaches the steady state value. It is 95% there within about 100 microseconds. Contrast this to the BT-0030SM (or BT-0030SMG):
Despite having a higher RF cutoff frequency, the settling time is 300 microseconds or so.
Now for the connectorized units. Here is the BT-0018:
and the BTN-0040:
This has a faster risetime than the other bias tee design, but also more ringing.
Finally here is the BT-0026, which is not similar to any other connectorized bias tee:
This settling time is pretty close to the BT-0030SM, which makes sense because they are a similar circuit.
There you have the standard bias tees. If you need the settling time of our high power bias tees please feel free to ask, and I’ll add another post with them up here.