The IF Filter Board contains two relay switched narrow band 9-MHz crystal filters. The ExpressPCB MiniBoard service provides 3 boards. Two of the boards are used, providing four selectable 2nd IF filter bandwidths of 2100-, 1800- 500- and 250-Hz.
Page 1 of the IF Filter Board schematic shows the relays and associated control circuitry.
Relays RL1 through RL4 select one of the two IF filters on each board. The relays have 12V coils, and are driven by ULN2803A Relay Drivers on the second I2C I/O Board. The ULN2803A Relay Drivers include diode inductive spike protection, as such, protective diodes are not included across the relays on this board.
The relays are TE Connectivity type IM06, high frequency RF non-latching relays. These relays are available from Mouser or Digikey. The version used in the RF Bandpass Filter Board is the IM06TS, through hole mount version. To ensure maximum isolation between the filter inputs and outputs, separate relays are used at the inputs and outputs of the filters, the relay coils are isolated with inductors and bypassed with capacitors, and careful attention paid to the PCB layout, in particular the ground traces.
All capacitors are 1206 style surface mount devices.
L1 – L6 are Murata 1.5uH fixed inductors in 1206 style SMD package, Mouser PN 81-LQH31MN1R5J03L.
Page 2 of the IF Filter Board schematic shows the board populated with the 2100- and 1800- Hz filters.
Page 3 of the IF Filter Board schematic shows the board populated with the 500- and 250-Hz filters.
Note, I did not populate the 27pF capacitors C9, C10, C11 and C12 as the filter passband, skirts and stop band characteristics looked good on the VNA scan. I assume there is sufficient parasitic capacitance without adding additional capacitance.
The crystal filters are International Radio parts as follows:
- 2100-Hz model #2309
- 1800-Hz model #2308
- 500-Hz model #2303
- 250-Hz model #2301
J1, J2, J3 & J4 are female machined pin breakaway headers from Sparkfun, PN PRT- 00743. Refer to the RF Band Pass Filter page for additional information. J1 and J3 are installed on one side of the board, and J2 and J4 on the opposite side of the board to facilitate daisy-chained connections between the boards in the IF strip of the Express Receiver.
Refer to the RF Band Pass Filter page for information on the connector J5.
The broadband ferrite transformers T1 through T4 provide a broadband match between the 50 ohm input / output impedance of the IF Filter Board and the 500Ω input / output impedance of the crystal filters. The broadband match ensures the crystal filters see a 500Ω termination over a broad frequency range, ensuring optimum filter response away from the passband. The broadband transformers are wound on Amidon FT-37-43 ferrite toroids, the number of primary and secondary turns are noted on the schematic. Insertion loss for each filter is noted on the schematic.
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