Software-defined Radios and Bias Tees

We wrote about Bias Tees and their applications within the context of Software-defined Radios in a previous post. The primary use of a bias tee in SDR is to enable the powering of a remote amplifier (a low noise amplifier in the post) using DC over the RF cable connecting the SDR to the LNA. A bias tee can also be used for other applications in SDR – for example to send low speed control signals to an antenna switch for instance.

In this post we will discuss SDRs with built-in bias tees, the main considerations and how each of them compare. Popular SDRs that have built-in Bias Tees are HackRF, RTL-SDR and RSPdx. Below is a table that compares these.

SDRBias Tee Voltage (V)Bias Tee Current (mA)Over-current Protection
RTL-SDR4.5 180Yes
HackRF3.350?
SDRPlay RSPdx4.7100Yes

Voltage and Current Specifications

Note that each SDR bias tee has a voltage and current specification. Typically the DC supply associated with the internal bias tee is derived from the main USB power of +5 VDC and is almost always a lower voltage. The current is also a fraction of what can be provided by the USB port – 500 mA in the case of USB 2.0. The higher the complexity and current consumption of the main SDR circuit, the less that’s available to the bias tee.

Having a higher voltage and current specification is better for the following reasons:

A wider range of front-end amplifier choices

The HackRF provides +3.3 VDC @ 50 mA and that is adequate to power only a few different LNAs when compared with +4.5 VDC @ 180 mA for the RTL-SDR or 4.7 VDC @ 100 mA for the SDRPlay.

As an example of this consider the PGA-103+ LNA from Mini-circuits. It is a very popular LNA in the Ham Radio world and the component inside many pre-amplifier modules. The PGA-103+ has high (22 dB) gain. low noise figure (0.5 dB) and excellent linearity. The LNA typically requires 100 mA when operating at +5 VDC where it’s most linear. It requires 60 mA at +3 VDC where it’s about 5 dB less linear. This LNA for example could not be powered by the HackRF bias tee. The RSP1a while providing an operating voltage of +4.7 VDC is marginal from a current standpoint. The RTL-SDR’s bias tee would work best in this situation.

Linear operation

For many LNAs including the PGA-103+ above, the higher the supplied voltage (not exceeding maximum), the higher the linearity. Linearity is a key specification as it enables the LNA to withstand stronger input signals without saturating.

Over-Current Protection

Two of the three SDRs above have some form of built-in over-current protection, however the RSP1a can disable the circuit to prevent damage. This is a key differentiator of this product. Along the same lines, it is important to note that both the HackRF and RTL-SDR SDR will be damaged if a DC short is created at the RF port when the bias tee is enabled. A quick test to prevent this from happening is to use your Multimeter to test the DC resistance across the antenna or LNA prior to connecting it to the SDR input.

SDRs like ADALM Pluto and LimeSDR do not have built-in bias tees and in this situation it becomes necessary to provide an external one in front-of the SDR.

Summary

Bias Tees are a very useful feature in SDRs as they enable the powering of remote LNAs connected close to the antenna. This in turn minimizes noise figure and increases sensitivity – a very useful attribute of a receiver. In our next post we will talk about the use of external bias tees for Software-defined Radios.

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