In Bias Tees

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:

BT-0034SM Rise time

If we zoom in a little more, we can see the immediate response:

BT-0034SM Impulse

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):

BT-0030SM rise time

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:

BT-0018 Rise Time


and the BTN-0040:

BT-0040 Rise TimeThey have about the same settling time (3-400 microseconds), which makes sense because they are the same circuit, as well as the BT-0025, BTN-0050, and BTN-0065.

Here is the BT-0050, which should be the same as the BT-0040 and BT-0065:

BT-0050 rise time

This has a faster risetime than the other bias tee design, but also more ringing.

Here is the BT-0026, which is not similar to any other connectorized bias tee:

BT-0026 Rise Time

This settling time is pretty close to the BT-0030SM, which makes sense because they are a similar circuit.

The following is the step response of one of our new surface mount bias tees, the BTL-0026SM:

It exhibits a 95% settling time of ~300 us and no overshoot or ringing.

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.

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