Jan uses a DPX-210-270 diplexer which splits an antenna port into two ports with one port having a 50 – 210 MHz low pass filter and the other having a 270 – 1000 MHz high pass filter. His first test shows that the low pass filter correctly attenuates some TETRA signals at around 390 MHz. In his second test he scans the broadcast FM frequency range and finds that with the low pass filter enabled there was a 3.5 dB increase in signal strength for one station and a 10 dB increase for another.
Most wideband SDR’s do not come with any front-end filtering built in. This limits their ability to receive weak signals in the presence of strong signals. Recently Sivan, a reader of RTL-SDR.com wrote in to let us know about a paper he published through the ARRL detailing how to design a concrete front-end unit for SDR use. A front-end helps to filter out signals that are outside of the desired passband, thus reducing interference from nearby strong signals significantly. Although Sivan uses a USRP with WBX daughtercard in his paper, he writes that the same front-end design principals can be applied to the RTL-SDR as well.
In the paper he designs a 431 – 435 MHz front-end using low cost SAW filters, a low noise amplifier (LNA) and a limiter to protect the radio. He writes that the design could easily be adapted for other bands as well.