Given the margin in the QO-100 receiver’s input, I added an FR13L TV LTE filter to the RX path before the RTL-SDR receiver. Its specified parameters are as follows:
Passband attenuation in the 5–700 MHz band: 0.9 dB
Passband attenuation in the 700–790 MHz band: 3.6 dB
Passband attenuation in the 822–2,050 MHz band: 60 dB
Isolation in the 5–40 MHz band: 20 dB
Isolation in the 40–790 MHz band: 14 dB
What were the results of measuring the LTE filter using a vector analyzer?
For QO-100, it is recommended to place a filter before the SDR receiver to suppress extremely strong signals that the LNB converts to a higher frequency spectrum than 739 MHz, where signals from QO-100 are processed. These extremely strong signals could shift the SDR’s operating point and thus reduce the sensitivity of the receive chain.
Measurements of the FR13L LTE filter showed strong attenuation of signals between 800 and 1300 MHz (-20 to -30 dB), but around 1400 MHz, the attenuation drops to 0 dB. This is followed by a sharp peak around 1800 MHz with attenuation of nearly -50 dB. Higher frequencies are attenuated by only -10 dB. The VNA plot provides further insight:
Measurement of the LTE FR13L Filter
Although a filter such as the LTE FR13L can help, it is not a suitable solution. This is because if interfering signals fall precisely within the range that the filter does not sufficiently attenuate, they will reach the input of the SDR receiver.
