Here is the
simple band pass filter designed for the central frequency 137 MHz in order to
filter the APT weather satellites reception and protect the receiver front-end
from the strong signals nearby the central frequency. The biggest problems are
coming from the close WBFM radio transmitters located on the 88-108 MHz. They
can be successfully attenuated using the notch filter but still the rest of the
possible „blockers“ may create a lot of problems in reception. The bandpass
filter is the optimum solution where most of the problems should be solved. The
broadcast TV towers, GSM/cell towers, Wi-Fi signals, radars are kept well
bellow 35db. Even the aircraft AM transmissions are attenuated in a descent
manner but not so effectively as other mentioned signals. The pass-band
attenuation (insertion loss) is kept under 2dB and this value directly affect
the overall receiving system noise figure. This should not be a problem as the
man made and surrounding noise on that band is even higher . Even your LNA
should be placed after the filter if you are using one. This should protect
your system from IMD products that may come also from the wide opened LNA
input.
Of course, it
will be nice to have a small size filter, with low insertion losses, high out
of the band attenuation and low cost at the end. As usual compromises are
required, even here. I decided to give a try to a DIY project characterized by
the small size, low cost, simple to design and fairly descent filter shape /
performance. Small Butterworth bandpass filter can do the job. The calculations
gave us a bit different part values and this should be corrected in manner to
meet the standard capacitor and coils values so you can copy the design using
the of the shelf parts. At the same time I try not to spoil the nice filter
design. Here is the scheme using the standard capacitors 1pF, 4.7pF, 15pF and
68nH inductance.
The filter is designed for the 50 ohms input/output impedance.
If you plan to use the wire wound SMD coils than this will end as the no tune
project. All you can do is test the filter and hope for the best, that your pass-band is really working on 137MHz as
calculated. If you make the coils by your self using the speaker wire, then you
will be able to tune a bit the filter to the desired frequency. Using the one
of the many on line inductance calculator make the 68nH air coil where you will
have a possibility to stretch it in order to obtain the required pass-band. The
tuning can be done using even the simple noise generator and the SDR dongle
itself. You should reach the values where the WBFM is at least 35dB down at the
end of the band (108MHz) and 50dB at the beginning of the band (88MHz). If this
is not enough, you can combine the FM notch filter in series with the proposed
BPF. Such a setup should result of 60dB plus attenuation of the WBFM signals
which should be more than enough for the majority of RTL dongle users.
Attenuation above 137 MHz is 35-40dB up to 2 GHz. Shielding the filter is
always good approach but this is pain in the ass for the most of the builders.
Yes, the filter will work even without the shielding using just the input /
output connectors but the attenuation of the unwanted signal may suffer because
of that. The strong signal may penetrate through the unshielded filter
structure. Advice – use the shield for the best performance. That's it, check
the video and everything should be clear, even on your spectrum :-)
