Alexiares filter for home made SDR

Publié: 12 novembre 2017 dans Filter mobo, Project, SDR

The original OpenHPSDR Alexiares filter (aka “Alex”)  is particularly well suited for most short wave DDC/DUC software defined radio. This complex front-end is a mix of HPF and  LPF filters. The LPF "cleans” the transmited signals and is always “on”, the HPF is only active when receiving and makes a nice “medium width” and adjustable bandpass filter when combined with the LPF. As narrow bpf are not mandatory for DDC-base SDRs –they even could impair the MDS of the receiver-, this architecture is probably the best of breed  in term of filtering

Choosing the righ LPF/HPF combo is simply made by software, via a serial I2S protocol. This protocol also commands and controls many other gadgets : a low noise amplifier for the 6 meter band, a bypass circuit –filtering could be a handicap when dealing with spread spectrum transmissions for example-, a permanent 60 MHz LPF used as an antialiasing filter, and many antenna and receiver switching to select a set of antennas, use a specific external BPF, amplifier or transverter, , select the first or second ADC for receiving, or use the second ADC when transmitting and using the pre-distorsion (aka “pure signal”) function.

But… oh, wait !… Alex is not sold anymore by the TAPR! The bare pcb is almost impossible to find. And anyway, Alexiares cannot be used with certain configurations. Most ham using a Red Pitaya, for example, are looking for a filter board with an I2C protocol. Guys wishing to build a Hermes Lite prefer to use a simple “parallel” protocole that mimic the “J16” 7 bits output (also known as Hermes/Penelope in PowerSDR’s config screen). And people playing with some “kilowatt SSPA” frankly don’t care about the LPF section of Alex : they only need a way to control le kW-class lpf they intend to use, without compromizing the “Hermes/Alex way of combining LPF and HPF filters”

That the reason why I decided to build a new set of board, fully compatible with Alexiares… but with some differences. The two main boards remains the HPF and LPF filters, both of them beeing built on a 10×10 piece of pcb, both of them needing a simple “positive common/ground switched” logic. Depending on the kind of SDR you intend to build, you can drive these two filter with a “plain old” I2S serial interface, an I2C interface, add an antenna selector, add a preamp/transverter/specific filter crowbar, choose to integrate a Solid State Power Amplifier control unit, and adapt you own antialiasing filter depending on you sampling rate (60 MHz for the Red Pitaya or Hermes/Angelia/Orion board, or 31 MHz for the Hermes Lite V1.0 or V2.0).

In the near future, a multi-purpose Tandem-match 33dB coupler will be added to this set of pcbs (for pure signal and vswr control)

Warning : this project is not yet fully tested. It could include a certain number of errors. As long as a full set of prototype boards haven’t been assembled and tested, use these files with caution (and at your own risk).

All these files are open source (CERN Licence) unless specified by it’s respective authors. All schematics from the OpenHPSDR are protected by the OpenHardware licence. The Mentor board intellectual property belongs to ON7EQ. The whole project has been completed with Kicad EDA, an open source CAD software developped by CERN.

– Alexiares_LPF : 


A LPF board, stritly using the very same set of filters as the original LPF, and able to widstand 80 to 100 W PEP. Toroids are T68, relays are able to widstand a 3 amp current. This board is based on a 4 layer pcb. All caps are 500 V ATC-like components. This board can be shielded in a 10×10 cm metal box.The Kicad schematic, pcb and gerber could be found on github on this link.

– Alexiares_HPF : 

HPF bottomHPF up

A HPF board with a integrated 6m LNA and a bypass (passall) filter section and a low noise amp (MMIC). It’s a simple 2 layer pcb using small signal relays and T50 toroids.This board can be shielded in a 10×10 cm metal box. The Kicad schematic, pcb and gerber could be found on github on this link.

– Alexandrie :

Alexandrie dwnAlexandrie up

A full featured SPI interface fully compatible with the Hermes serial protocol (J6 connector). This board has the same 10×10 form factor than the HPF an LPF pcb’s.The Kicad schematic, pcb and gerber could be found on github on this link.

– Alexi2C :

Alexi2C dwnAlexi2C up

A fork of the DC2PD interface that converts the I2C protocol used by the Red Pitaya (Pavel Denim’s firmware). This board can directly drive the LPF/HPF set of filter as it includes a pair of ULN2803 drivers. This board could also be used without any I2C decoder. In this case, the parallel bus of the SDR is directly plugged on the board and the pair of ULN2803 could be used in “J16” mode. This board is using a ”half size” format (5x10cm) The Kicad schematic, pcb and gerber could be found on github on this link.

– Alexiares_LPF_RX_In :

Alexiares_LPF_RX_In bottomAlexiares_LPF_RX_In

A specific 60 MHz lpf with a 88/108 notch filter and a TX/RX relay. This board is in fact a sub-section of the RX High-pass filter that couldn’t fit on the HPF board. I decided to enhance the characteristics of the original filter, with a 7th order “M-Modified” LPF. The LPF section acts as a strong antialiasing 60MHz filter and the two elliptic rejection frequencies are centered on each side of the FM broadcast band. This way, one could not be disturbed by FM signals located in the 4th Nyquist zone heard on the 21, 24 or 28MHz band. One side of this filter –the output- goes directly to the HPF input, the other side is ended with a relay that isolate this filter from the RX path when transmitting. During TX, a small amount of the transmited signal is so redirected to the second ADC and used by the pre-distorsion (pure signal) control software. This board has a small footprint (5x5cm). The whole board or only the filter itself could be shielded. The first option is highly recommended; The Kicad schematic, pcb and gerber could be found on github on this link.

– Alexiares_Coax_Out :

RX_Ant dwnRX_Ant_up

A set of two 5×10 boards for the RX and the TX path, used to switch antennas (3 max), external filters or low noise amplifiers, transverters, and select the current ADC. Both board can receive a peripheral shielding. The Kicad schematic, pcb and gerber could be found on github on this link.

– Mentor :

Mentor dwnMentor up

A Deluxe Arduino-based control board for SSPA designed by ON7EQ. This board handles information comming from the linear amplifier (voltage, current, temperature, vswr, direct power….) and takes action in case of difficulties (drives the cooling fan, shuts the amp down, and alerts the Hermes main board). The Kicad schematic, pcb and gerber could be found on github on this link.


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