Chaining Software-defined Radio accessories with a Bias Tee

Introduction

The use of a bias tee was discussed in a couple of blog posts previously. The primary use of a bias tee in an SDR is to power a remote low noise amplifier (LNA). But what if you want to power two or more LNAs? And what if you want to add a filter to reject interference? How would this work with a Bias Tee?

Example setups

Let’s take a look at the receiver chain in the following block diagram.

The SDR with built-in Bias Tee powers the LNA located close to the antenna. The LNA module either has a Bias Tee built into it as shown in the figure or requires an external Bias Tee.

If you want to add another LNA to the chain downstream from the one you are powering, you will need to power it using one of two methods:

1. Select an LNA with a DC path from the output to input and add it between the first LNA and the SDR as shown in the block diagram below.

Here is one such LNA that has a bias tee at each its input and output.

2. Add another bias tee to the chain. You will need to connect the DC terminal of  bias tees together as shown in the block diagram.

The first method is a lot ‘cleaner’, with lower cost and easier to implement. In both situations, you have to ensure that either the SDR or the DC source powering the bias tee is capable of sourcing the total current required by both LNAs.

In the above examples a filter can be added right after the antenna and before the first LNA. In some situations however, you may want to add a filter after the first LNA in the receive chain. This needs careful consideration. High pass and band pass filters will both block the DC signal from the SDR from reaching the LNA (in the first block diagram) and therefore cannot be used. In fact you can damage some filters with DC. Additionally while in theory, low pass filters and notch filters should pass DC, in practice only some low pass filters or notch filters will allow the passage of DC. For instance, this low pass filter from Mini-circuits blocks DC while this one does not. Always check the data sheet to see what’s possible.

Summary

In many setups with SDRs, you might require additional LNAs to add gain and filters to mitigate the effect of interference. In this post we discussed the addition of such modules in SDR setups that utilize bias tees and the different considerations and precautions.