Optional additional filters can be connected via the expansion pins of the FiFi SDR. The required frequency range is communicated to the filter via these pins so that automatic switching is possible. Suggestions for preselectors are listed on this page. They can sometimes greatly improve the reception quality, and in particular the reception on harmonics of the chosen freqeuency can be reduced.
As well as the received signal, the SDR also presents control signals at the expansion interface to automatically switch a preselector. Pins 5-8 of interface X6 can be given any chosen bit pattern, according to frequency. Programming of the pin output can be simply achieved using the configuration tool from PE0FKO. Another possibility is to use the rockprog command line program. The programmed bit patterns can either be directly evaluated or using binary decoders such as the 74HC138 or 74HC154 and used for switching.
Preselector from DK5DN
The preselector from Günter (DK5DN) was presented as a construction project at the Fichten Field Day in 2011. The consideration was that frequencies above 3fe must be sufficiently suppressed. To keep the effort to a minimum, Günter decided on a combination of seven Cauer low-pass filters with a trap frequency as from 1.2fg with trap attenuation as = 60dB. This is achieved in each case with a 7-pole filter with three SMD inductors. A high-pass filter forms the eighth filter. The circuit diagram can be downloaded as a PDF and an Eagle-sch.
The speciality of this 'offical' FIFi SDR presselector is that the circuit board also fits in the original housing without any modification being necessary. It simply slides into the spare slot in the lid. The electrical connection is made via the plug/socket X3/X6 and therefore there is no tangle of wires. On the SDR itself however, the standard low-pass filter may have to be removed (desolder 4 SMD components).
Here is a summary of the eight filters:
- TP fg=30 MHz; fs=36 MHz
- TP fg=12 MHz; fs=14,4 MHz
- TP fg=4,8 MHz; fs=5,76 MHz
- TP fg=1,92 MHz; fs=2,304 MHz
- TP fg=768 kHz; fs=921,6 kHz
- TP fg=307 kHz; fs=368,4 kHz
- HP fg=30 MHz
Construction and modification instructions from the SVN (only in german): http://o28.sischa.net/fifisdr/trac/browser/trunk/Dokumentation/Preselektor.pdf.