This board contains three relay switched RF band pass filters. The ExpressPCB MiniBoard service provides 3 boards. All three boards are used, three filters per board, providing a total of nine filters, one for each of the nine HF bands.
RF Band Pass Filter Boards Installed in the RF Chain
Relays RL1 through RL6 select one of the three band pass filters on each board. The relays have 12V coils, and are driven by ULN2803A Relay Drivers on the I2C I/O Expansion 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. The through hole version was used on this board to reduce the board space required to mount the relays.
To ensure maximum isolation between 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.
The band pass filter design is taken from William E. Sabin, W0IYH, “Narrow Band-Pass Filters for HF,” QEX, pp. 13-17, SEPT/OCT, 2000. The filters use two resonators in either a bottom-coupled “high side” filter or a top coupled “low side” filter configuration. Bottom coupling is used on the three lower HF bands, 160M, 80M and 40M, to provide better rejection for higher frequencies; while top coupling is used on the six higher HF bands, 30M, 20M, 17M, 15M, 12M and 10M, to provide better rejection for lower frequencies.
The RF Band Pass Filter Board was designed to accommodate one bottom coupled filter and two top coupled filters. In this way, the three boards from the ExpressPCB MiniBoard service accommodate the nine HF bands:
- Board #1 160M, 30M & 15M
- Board #2 80M, 20M & 17M
- Board #3 40M, 12M & 10M
Inductors are wound on Amidon toroids with the number of turns and spacing on the cores as noted in the schematics below. Capacitors are either polystyrene (Xicon polystyrene film capacitors, 5% tolerance) or mica capacitors (CDE mica capacitors, 5% tolerance), as noted in the schematics below. The capacitors where obtained from Mouser, part numbers are listed below. The PCB pads for the capacitors were made to accommodate the different pad sizes of the SMT mica capacitors used in the various filters.
The filters were tuned (center frequency and insertion loss) by varying the spacing of the turns and the location of the turns on the toroids. A Vector Network Analyzer was used to monitor the filter response in real time while the spacing and location of the turns on the toroids were adjusted. The Vector Network Analyzer used was Paul Kiciak’s N2PK Vector Network Analyzer, built on a PCB from Ivan Makarov, VE3IVM, and operated with myVNA software from Dave Roberts, G8KBB.
Capacitors used in the rf band pass filters are as follows, all part numbers are Mouser.
- 6 pF 598-MC08CA060D-F
- 8 pF 598-MC12CD080D-F
- 10 pF 5982-08-100V10-F
- 15 pF 5982-08-100V15-F
- 130 pF 598-MC12FA131J-F
- 150pF 598-MC12FA151J-F
- 180 pF 598-MC12FA181J-F
- 220 pF 5982-12-100V220-F
- 270 pF 598-MC12FA271J-F
- 300 pF 5982-12-100V300-F
- 470 pF 598-MC18FA471J-F
- 560 pF 5982-MC18FA561J-F
- 820 pF 5982-MC18FA821J-F
- 1000 pF 598-MC22FA102J-F
- 1500 pF 598-MC22FA152J-F
- 1200 pF 23PS212
- 1800 pF 23PS218
- 2700 pF 23PS227
In order to successfully build a project such as this, it’s important at the outset to decide on a standard method to make RF and IF interconnections, and control and power connections between the various boards.
RF and IF connections between the boards in the Express Receiver are made using machined pin breakaway headers. This is a technique I found on Jack Smith’s (K8ZOA) Clifton Laboratories pages. I use machined pin male and female headers from Sparkfun, part numbers PRT-00117 and PRT-00743, respectively. It is essential to use high-quality, machined pin headers. This technique provides a high quality, reliable RF/IF connection between the boards at a low cost. The female connectors, consisting of sections of 3 pin headers, are installed on the boards; the middle pin is the signal, and the two outside pins are ground. Inter board jumper cables are made with lengths of RG-174 coax cable terminated with 3 pin sections of the male pin headers; the center conductor of the coax cable is soldered to the center pin of the 3 pin section of male pin header, the coax braid is carefully divided into two and soldered to the two outside pins. The natural springiness of the RG-174 coax is used to keep inter board connections under tension as shown in the photograph below; otherwise, a drop of hot glue can be applied to the assembled male and female pin headers to keep them firmly attached. As shown in the photographs above, the female pin headers are installed on alternate sides of the boards to facilitate daisy chaining of the RF signal between the boards.
In past projects, I have used SMA connectors and commercial preassembled coax jumper cables. This had many draw backs; including, cost, board space, space needed to physically make the SMA screw connections, and reliability of low cost preassembled SMA jumper cables.
Control and power connections to the boards in the Express Receiver are made using TE Connectivity rectangular connector housings and Molex rectangular connector headers, 0.1″pin spacing. The headers have male pins and are mounted on the boards. The housings are used to house the crimp-on female connectors that terminate the interconnect wire. I like this combination of housings and headers as they are easy to engage (and more importantly) disengage. There are other families of rectangular housings – connectors, and some are very hard to disengage; when working in a confined space this becomes very frustrating! Digikey part numbers for the connector receptacles and headers are as follows:
- 2 Pin: Connector Receptacle: A19490-ND
- 3 Pin: Connector Receptacle: A19491-ND
- 4 Pin: Connector Receptacle: A19492-ND
- 5 Pin: Connector Receptacle: A19493-ND
- 6 Pin: Connector Receptacle: A19494-ND
- 2 Pin: Connector Header: WM4200-ND
- 3 Pin: Connector Header: WM4201-ND
- 4 Pin: Connector Header: WM4202-ND
- 5 Pin: Connector Header: WM4203-ND
- 6 Pin: Connector Header: WM4204-ND
I use TE Connectivity crimp connector terminals (Digikey PN A19520-ND) to terminate the wires, which are then pushed into the connector receptacles. You will need to purchase a crimp tool to install the crimp connector terminals on the wire. I paid $40 – $50 for a generic crimp tool that has served me well for many years.
I standardized on a single color of wire (blue) for the Express Receiver to give what I consider a nicer finished look to the project. I use Alpha Wire from Digikey.
- Power: 22 AWG (7/30), Blue, 100ft, Digikey PN 6713 BL005-ND
- Control: 24 AWG (7/32), Blue, 100ft, Digikey PN 6822 BL005-ND
© 2014 – 2021 Rod Gatehouse AD5GH
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