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SatNOGS now equipped with Diplexer

A project log for SatNOGS - Global Network of Ground Stations

SatNOGS is an open source ground station and network, optimized for modularity, built using affordable tools and resources.

Pierros Papadeas 10/27/2014 at 00:163 Comments

We've been thinking for some time that there is no need to install two separate RTL-SDR dongles to cover both VHF and UHF. It increases power consumption on WR703N without actually having any benefit, as using two dongles simultaneously is limited by the CPU power and network bandwidth.

A diplexer would allow the connection of a single dongle on both antennas. After studying the theory behind filters, we decided to implement a pair of Chebyshev filters. This type of filter has very steep roll-off and the maximum ripple can be a design parameter.

The two filters, a high-pass to attenuate the VHF band and a low-pass to attenuate the UHF band, should be crossing at exactly 288MHz@-3dB. Attenuation on each band should be at least 50dB. We calculated that a 9-pole Chabyshev filter would match these requirements and started designing a prototype on Qucs. Qucs is an open source circuit simulator which helped us verify that our calculations are correct.

LPF

The LPF has a maximum insertion loss of 0.4dB in VHF. The attenuation in UHF is more that 60dB. Some ripple can be observed on the chart, but it should not affect reception of narrow band signals.

HPF

The HPF has a maximum insertion loss of 0.2 dB in UHF. The attenuation in VHF is more that 80dB.

Special care has been taken to ensure minimal cross-talk between the two filters. The bottom layer of the PCB is a complete ground plane and two copper shieldings have been solder on top of each filter.

Please note, that this diplexer can be used only for receiving. For transmission, a higher grade of components (high voltage caps, high current inductors) should be used.

All source files can be found in our github repo here.

More pics from the diplexer can be found here.

Discussions

coflynn wrote 10/27/2014 at 00:34 point
Beautiful, my hand-made shields always look a lot messier than yours, and mine end up with all sorts of finger-cutting sharp edges! Have you seen the VNWA tool too? It's a cheap VNA and can go up to about 1.3 GHz, would be useful for checking how they perform in real-life too, I've found between QUCS + the VNWA makes building/checking/tuning filters pretty quick :)

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Manthos Papamathaiou wrote 10/27/2014 at 17:21 point
Thank you coflynn! We are looking to buy a network analyzer and your suggestion looks interesting. At this time we are using a VNA at the Athens technical university but those days is an national holiday in Greece so we couldn't check our filter. We will come up with the results in sort period of time.

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coflynn wrote 10/27/2014 at 22:01 point
Ah - yeah if you've got access to real equipment, really no need to buy the cheap version! But if you end up with $200k funding then you could buy a few VNWAs without hurting too bad ;-)

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