RTL-SDR is our top budget software-defined radio pick for the year and it is one of the most popular SDRs worldwide. This is the product that most people use when getting started with SDRs.
The RTL-SDR is small and portable and can be carried around easily.
With a frequency range of up to 1.7 GHz along with excellent community support, there’s a lot you can do with this product. 🚀
Some applications however require better sensitivity.
This is where a low noise amplifier helps.
This post answers a few different questions:
- The benefits of using an amplifier
- What amplifier can we use with RTL-SDR?
- What are some of the limitations?
So without further ado, let’s get into the details.
The benefits of using an amplifier
In general an amplifier
- reduces the overall receiver noise figure
- adds gain
and therefore boosts the range of a wireless transmission. Adding an appropriate amplifier to a receiver improves it’s sensitivity.
What amplifier can be used with RTL-SDR?
There are a couple of different options, depending on the gain required. Gain is specified in dB and the two options we present have 20 dB and 30 dB of gain.
20 dB LNA
This LNA is best suited for an operational range of up to 4 GHz. It has a noise figure of 0.8 dB at 1 GHz.
The noise figure of the RTL-SDR is in excess of 5 dB so this LNA can be used to reduce the noise figure to less than 1 dB. This is quite a significant improvement (by 4 dB).
Now before you plug this LNA in here are some important precautions:
The RTL-SDR has an input damage level of +10 dBm. This means that a signal with level +10 dBm or 10 mW will damage the front end of the radio receiver.
So considering the 20 dB gain of this LNA, it is important to ensure that the signals entering the antenna are lower than -10 dBm. Signals of this magnitude are not normally encountered in the real world. There are however a couple of exceptions:
- The output of a signal generator is injected directly into the amplifier
- A wireless transmitter directly input to the amplifier using a cable
The RTL-SDR also has a built-in bias tee that can be enabled. The bias tee is capable of providing 180 mA at +4.5 V.
We explained the benefits of a bias tee in detail here. It allows you to install an LNA close to the antenna for best noise figure performance. This in turn enables greater sensitivity and therefore improved listening range. If you only want to use an amplifier with a built-in bias tee then this LNA is one option.
⚠️ An important precaution with the Bias-T enabled LNA. The bias tee inductor is installed and enabled by default. When powering the device using either the DC header or the USB connector, +5 VDC will appear on the RF output. If your receiver does not have a DC blocking capacitor on its input or some other form of DC voltage protection, this voltage will damage your receiver.
30 dB LNA
This LNA provides a higher level of gain 31 dB at 100 MHz and 19 dB at 2 GHz. That covers the entire range of the RTL-SDR.
Note however that the noise figure of this amplifier is 2.2 dB. So it’s not as sensitive as the 0.8 dB NF amp above but it provides a lot more gain.
What are some of the limitations?
When adding an amplifier to a RTL-SDR to improve reception it is important to understand that an LNA will amplify all signals present at its input. Most importantly it will amplify already strong out-of-band signals that you may not be aware of, or interested in receiving.
These strong signals can either saturate or 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. There’s always the question of where to place the filter.
A FM Notch filter will for example block strong signals in the 88-108 MHz band.
Summary
In this post we have discussed the use of different low noise amplifier options for use with RTL-SDR. We also highlighted some of the precautions that should be taking when using an amplifier.
