Software-defined radio (SDR) can be easily damaged if proper care is not taken during use. In part one of this article series, let’s take a look at the most important RF precautions when using SDRs.
For this we have split the SDR into two main sections: Receiver that takes RF signals In and Transmitter that puts RF signals Out. Many SDRs such as the RTL-SDR, NooElec, AirSpy for instance, do not transmit signals. In these situations consider only the receiver precautions.
The receiver section of all SDRs have a “Maximum RF input Signal” level that can be tolerated before they are damaged. Maximum levels for a few different SDRs are listed in the table below:
|HackRF||– 5 dBm||0.3 milliwatt|
|RTL-SDR||+10 dBm||10 milliwatt|
|ADALM Pluto||+2.5 dBm||1.8 milliwatt|
|BladeRF||+2.5 dBm||1.8 milliwatt|
|SDRplay RSP1A||0 dBm||1 milliwatt|
So the question is – how does a receiver get damaged?
Damage from high power transmitters
Let’s take the example of the HackRF. From the table above, its maximum input level is the lowest, so it’s the most susceptible to damage. How easy is it to damage a SDR in a practical situation? Consider a two way radio with a transmitter power of 8 Watt or +39 dBm. If you’re in the near vicinity of this radio when it’s transmitting signals, it will very likely damage the receiver.
Damage due to too much gain
In many situations, a user might want to improve the sensitivity of a SDR with a Low Noise Amplifier (LNA). The LNA adds gain to the receiver and also reduces its noise figure thereby allowing the detection of weak signals.
The addition of gain means that the maximum RF input level is now reduced by the same amount. You have to be extra-careful with SDRs as they are typically able to receive signals across a wide frequency range. You can amplify a weak signal of interest with a LNA such that it can be processed by the SDR. But what about a strong signal in another band?
This is also amplified by the LNA and can damage the SDR. This is why in many situations it is advisable to use a filter with a LNA as it helps protect the SDR from such conditions.
Damage from DC Signals
Many SDRs do not have the ability to tolerate DC signals at their inputs – even at very small levels. So be careful when you drive a DC signal into the RF input. How easy is it to do this? Very easy when you consider that this can be done with a Bias Tee that is incorrectly inserted before the receiver.
Damage from Electrostatic Discharge
Electrostatic discharge or ESD is the rapid transfer of electricity from one electrically charged object to another. We’ve all likely experienced an ESD shock when touching a metal doorknob for instance. While such shocks are not harmful to human beings, they can damage sensitive electronics very easily. The rapid transfer might occur when touching the RF Input connector of a SDR to connect or disconnect an antenna for instance. Most SDR vendors include built-in ESD protection using Diodes in various configurations. However this protection does not guarantee against all levels of ESD. While it’s next to impossible to guarantee lab conditions everywhere a SDR is operated, here are some general guidelines for ESD protection.
One of the most common reasons for damage to the HackRF is impedance mismatch on the transmitter output. This mismatch occurs when there is either no antenna connected or a poorly matched antenna connected to the RF port. When either of these situations occur, any power transmitted out of the device is reflected back and damages the front-end circuitry of the HackRF. The simplest way to avoid this is to use the recommended antenna and never to transmit without an antenna connected to the RF port.
In this post we discussed the different ways in which SDR hardware can be damaged and the precautions that can be taken to avoid damage. Repairing hardware can be very tricky and generally requires expertise, a soldering iron and in many situations a heat gun or a heat plate to remove the damaged component and then replace it with a working equivalent.
In Part 2 of this article we will discuss Mechanical and Clocking considerations. Taking the above precautions means your SDR will live a long and healthy life.