Most Software-defined Radios operate over a large frequency range. The RTL-SDR for instance can operate from 500 kHz to 1.7 GHz while the HackRF can operate from 1 MHz to 6 GHz. This is both a blessing and a curse. It’s a blessing because unlike with conventional hardware, you can process many signals with one radio. This is in fact one of the biggest benefits of SDR. It’s a curse because signals that you may not necessarily be interested in can overwhelm or saturate the SDR receiver to a point where it cannot effectively listen to any other signals. This is the reason that it’s always advisable to use a filter that limits the input to the frequencies of interest when using a LNA.
A question that comes up often is “Should I place a LNA before or after the filter?”. While it’s easy to experiment with both configurations it’s worthwhile to understand why one might work better than the other in your particular situation.
Let’s take a look at the two different configurations:
Antenna – Filter – LNA – SDR
In this configuration, the filter blocks out any strong out-of-band signals that might saturate the SDR. On the flip side, every filter attenuates an in-band signal to some extent. Sometimes the attenuation can be as high as 10 dB! This attenuation results in reduced sensitivity which in most situations is a bad thing.
Antenna – LNA – Filter – SDR
In this configuration, the LNA sees all signals received from the antenna including the strong ones. The noise figure of the system is lower than in the first configuration and the sensitivity is maximized. The SDR will be able to process very weak signals and that’s a good thing in most situations.
A filtered LNA for applications like AIS or ADS-B includes a low noise amplifier and a bandpass filter. The LNA in these products precedes the filter so you can get the noise figure improvement and the filtering effect. This is an integrated device.
Use the filter before the LNA if you are operating your SDR in a strong signal environment. One example of this would be in the presence of a strong FM transmitter. In all other situations, use the LNA before the filter.
Read our article on the Best Software-defined Radio.