Alexaires (or Alex for short) is a set of RF Bandpass filters for use with Mercury or any other SDR. The project leader for Alex is Graham, KE9H, who is also laying out the PCB.
Alex is a ham band RF preselector intended to reduce the level of out-of-band signals at the input of a receiver.
It also has provision for two user defined band pass filters as well as a by-pass facility. The insertion loss of each filter has been chosen so that the narrow bandwidth lower frequency filters have higher loss than the higher frequency filters. This is to keep the overall receiver noise figure low on the higher bands.
Protection from potentially damaging high signal levels is provided in the form of a transient suppressor (details TBD) at the antenna socket.
Alex has provision to be switched between two paths; either in the path from the antenna to the receiver, or to filter a low level transmitter signal.
A CPLD is used to interface the filter selection and routing to the Atlas bus to provide maximum flexibility in Alex’s configuration.
A block diagram of Alex is shown below.
The filter sections in Alex are based on the RF filters used in the PIC-A-STAR tranceiver designed by Peter Rhodes, G3XJP, first published by the RSGB in RadCom Aug 2002 to March 2004. The filter sections were originally designed by Harold, W4ZCB using ELSIE. The original design used a number of Toko inductors that have since become obsolete. The circuit was modified by Steve Drury, G6ALU, to use surface mount capacitors and CoilCraft "Slot-10" coils http://www.coilcraft.com/slot10.cfm. These filter section designs have been used by Phil, VK6APH, in his SDRs and have been modifed to use alternative solid state switching and cover additional bands.
Why Alexaires? Alexaires and his twin brother Anicetus - were the Greek Gods of Defense, in particular the defense of fortified towns and citadels.
Both were sons of Herakles (Hercules), born after the hero's ascension to Olympus and his marriage to the goddess Hebe. The pair likely served as the gatekeepers of Olympus, assisting their father in a role which was commonly assigned to Herakles.
If you want to look at the proposed filter set, you can download the following files. The "READ ME" file gives values for all bands, the "Sample schematic" shows the topology, and Plots of transmission loss and VSWR for each band follow. If you look at the bottom of each plot, more details are available for each of the marker frequencies. The last file is a sample 40 percent wide filter, such as might be used for multiple overlapping continuous coverage.
All these plots, plus the original design of these filters were done in ELSIE. The FREE "student version" of ELSIE is sufficient to design and analyze this order of filter.