RTL-SDR Filter Cards 2017-10-17T09:31:59+00:00

Band and Notch Filters Cards

Different filters, providing more favourable working conditions in a particular operating range, are connected between the antenna (50 Om) and dongle RF input (75 Om). For example, it is necessary to use a LPF and Multi-Band Switched Filter Bank 1…30 MHz for operation in the HF range. The usage of the Band Filter 118…137 MHz can improve the reception in the aero range (listening to aircraft traffic control conversations). In the case of the DX, it is often useful to use a notch bandpass filter 87…108 MHz for suppression of powerful local FM stations. The presented list of filters can be expanded, depending on the reception conditions and the specific user requirements.

As mentioned earlier, it frequently occurs that the wave resistance of the dongle and the antenna feeder do not match: 75 Om and 50 Om. There is some impedance mismatch at the dongle RF input. As a result of this mismatch, in practice there is a distortion of the characteristic form of the filter’s amplitude frequency. The reduction of the impact of complex loads on the amplitude frequency filters can be achieved either by including special correction units or, as a cheaper solution, with the implementation of -0.5…-3 dB 50 Om resistive attenuators at the input and output of the filter. Better matching of the wave resistance and the reduction of complex loads can be achieved using these attenuators. Please see the Cauer Notch Filter of 5-th Order for a range of 87 … 108 MHz simulation as an example. This simulation was prepared using the program AADE Filter Design and Analysis:

Low loss helical bandpass filter

Low Pass Filter 30 MHz Cards

All developers of the convertors below 30 MHz are using multipolar LPF with a frequency cuto-ff at 30 MHz, e.g. HF Converter V3.2 from JaniLab, www.janielectronics.com. The discrete SMD elements (capacitors and inductors with a 1% accuracy) are used in order to build the qualitative LPFs, which leads to a higher price of the final device. At the same time, the TDK company produces 3rd order LPF with a cut-off frequency of 35 MHz, which can be used to construct a LPF to convert the HF band signals in order to receive them on the RTL2832.

We have evaluated the possibility to use the MEM2012SC type chips as an LPF for the RTL2832 converters using the Spectrum Analyser DSA-815. The measurements insertion loss vs. frequency characteristics are provided in the graphics below:

3rd Order LPF 30MHz
6th Order LPF 30MHz

The load of 50 Om should be ensured in the filter input and output. This makes it evident that it is possible to develop a filter using only 2 chips with the suppression over 60dB beginning from the frequencies of 60 MHz and above. The limited list of cut-off frequencies and reduced level of suppression in the VHF and UHF ranges are the disadvantages of the above mentioned filters.

Cut-off Frequency, MHz
TDK MEM Series 25 35 50 100 200
Murata NFL/NFW Series 10 20 50 70 100 150 200 300 400

Multi Band Switched Filter Bank 1…30 MHz

Multi-Band Switched Filter Bank 1…30 MHz development board

The RMS of the interference/noise reception by the device can be calculated as follows:

formel-1

EN = (f) df , where
EN – RMS interference / noise;
e(f) –  the spectral power of the interference / noise;
fl – the lower frequency range;
fh – upper frequency range.

This explains the desire to limit the spectrum of the received signal by the useful components in order to achieve the best signal/noise ratio. In practice, this is achieved by using a bandpass filter, which  suppresses the undesired out of band interferences.

There are many publications regarding the design of the bandpass filters, e.g. Eric T. Red. HF-Arbeitsbuch. Daten. HF-Grundschaltungen. 50-Ohm-Technik, ISBN 3-88976-137-2, 1993.

Usually, such filters consist of LC and commutation components  (Low-voltage high-frequency relays, electronic switches on the CMOS structures, PIN diode). The latest trend is the use of  SMD components instead of non-mechanical relays in favour of electronic switches, PIN diodes and MEMS switches (expensive).
The usage of electronic keys is justified to frequencies 2…3 MHz due to the presence of the interference capacities; PIN diodes are more favourable on the higher frequencies.
The proposed Multi Band Switched Filter Bank 1…30 MHz for a HF SDR receiver is built on these principals and has the following characteristics:

  • LW: 148…283 KHz
  • MW1: 520…943 KHz
  • MW2: 943…1710 KHz
  • SW1: 3.2…4.8 MHz
  • SW2: 4.8…6.9 MHz
  • SW3: 6.9…10 MHz
  • SW4: 10…14,5 MHz
  • SW5: 14.5…20.8 MHz
  • SW6: 20.8…30 MHz

Switches FSA3157 are used  for the lower frequencies. PIN diodes BA891 (Diode capacitance CDmax = 1.05 pF) and low diode forward resistance rD = 0.42 Ω, IF = 3 mA. ) are used for the higher frequency ranges.  It enables the management of  these diodes directly from the logical elements using  Ucc =+5V.

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