RF Band Pass Filter Board

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.

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 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.

The ExpressPCB .pcb file for the board is here: RF BPF Board Final.

Receiver Band Pass Filter Page 1
Receiver Band Pass Filter Page 1, Filter Selection Relays

The band pass filter design is taken from William E. Sabin, W0IYH, “Narrow Band-Pass Filters for HF,” QEXpp. 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; and 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.

Receiver Band Pass Filter Board Page 2
Receiver Band Pass Filter Board Page 2, 160M, 30M & 15M Filters
Top view RF Bandpass Filter Board: 160M, 30M & 15M
Top view RF Band Pass Filter Board: 160M, 30M & 15M Filters
Receiver band Pass Filter Board Page 3
Receiver band Pass Filter Board Page 3, 80M, 20M & 17M Filters
Top view RF Bandpass Filter Board: 80M, 20M & 17M
Top view RF Band Pass Filter Board: 80M, 20M & 17M Filters
Receiver Band Pass Filter Page 4
Receiver Band Pass Filter Page 4, 40M, 12M & 10M Filters
Top view RF Bandpass Filter Board: 40M, 12M & 10M
Top view RF Band Pass Filter Board: 40M, 12M & 10M Filters
Bottom view RF Bandpass Filter Board
Bottom view RF Band Pass Filter Board

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 below, 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.

RF Assembly with control cables
Rf Bandpass Filter Boards installed in the RF chain between the RF Attenuator and LPF Board, and the RF Preamp Board

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 – 2015 Rod Gatehouse AD5GH

7 thoughts on “RF Band Pass Filter Board”

  1. I’m building a transceiver that I was planning on having 20 and
    40 meters only , primarily because of the size of the enclosure I
    will be using. Could this be scaled down and if so, how much
    would it cost. Thank you. David Raetz W4VZL

    1. Hi David,

      The Express Receiver could be scaled down to reduce its size in the following ways.
      – Reduce the number of Rx Band Pass Filter boards to one, providing support for 3 bands (or 20 and 40 meters only).
      – Reduce the number of IF Filter Boards to one, supporting 2100- and 500-Hz filters only, for example.
      – Reduce the number of I2C Relay Control Boards to one based on the changes above.
      – Reduce the number of LCDs and Display and Control Boards to one each. This would reduce the simultaneous display/control functions, but is workable if space is restricted.
      – Eliminate the RF Attenuator, Low Pass Filter, and RF Preamplifier Board; however, I recommend the LPF be included ahead of the RX Bandpass Filters.

      I have not totaled the cost for each of the boards, however, it’s a simple matter to compute the cost from online resources such as Digikey, Mouser & etc.

      Best regards,

      Rod

  2. Rod,
    Good to see more information being posted on the receiver.
    I have a question on the band pass filter capacitors. You have mainly used SMT CDE silver mica capacitors from Mouser but have not specified a tolerance for these capacitors. Mouser list capacitors with tolerances ranging from 0.5% through 5%. The tight tolerance capacitors are not stocked and come with high prices, large MOQ’s and long delivery times. What is the highest capacitor tolerance that can be comfortably “tuned” by varying the winding spaces on the inductors? Could I use thru-hole radial silver mica capacitors instead, as these are more readily available.

    Best regards,
    Phil

    1. Hi Phil,

      Thanks for the feedback. I will add the Mouser part numbers for all the capacitors used on the RF Band Pass Filter Board to that page.

      All the mica capacitors are 5% tolerance, and I experienced no problem tuning the filters by adjusting the spacing of the windings on the toroid cores. I prefer to use SMT devices if I can, but I am sure the radial silver mica capacitors would also work fine.

      Best regards,

      Rod

  3. Hi Rod, I am planning a Progressive Rx build as a summer project and have been enjoying reading your Express descriptions. I am sticking to the original design (double-conversion superhet with 3.5MHz tunable IF and 9MHz second IF) for simplicity and will replace all oscillators with DDS. My boards for the RF filters/pre-amplifiers (exactly as per W7ZOI) have miniature relays on each input and output as you have done. However I plan to connect the module input/output to the relay’s movable contact so that un-energised modules have their inputs/outputs earthed. I noticed that you simply leave out-of-circuit BPFs open-circuit. Do you think shorting to ground is unnecessary in a receiver? Comments welcome as I have not etched the boards yet! 73 Paul vk3hn

    1. Hi Paul,
      Thanks for the feedback! Regarding the BPFs, I was concerned about the capacitance between the adjacent contacts on the RF signal relays (~2pF) if the unused contact is earthed. As there are 8 unused BPFs in the 9 band receiver ay any time, there will be 8 small capacitances in parallel to earth across the input and output of the BPF in use. As there is a relatively large capacitance across the BPF inputs and outputs it’s probably not a factor, but that’s why I left the unused contacts floating rather than connect them to earth.

      I see from your QRZ page that you are in Blackburn, I grew up in Forest Hill, graduated from Nunawading High School in 1977. I was also in the first batch of Novice Licensees – VK3NBV. Operated CW on 80m with a single 6146B TX and AR7 RX. Now living in Southern New Hampshire.

      Best regards, Rod

      1. Hi Rod, thanks for your response. It’s probably not a big deal in the scheme of things. I’ve etched the boards now and are earthing unused BPF inputs and outputs. Further to my QRZ.com page, I grew up in Eltham, graduated from the high school in 1978, was in the 2nd batch of novices as VK3NCL and worked 80 meters AM with a single 807 and a little valve modulator. I’ve come back to amateur radio homebrew in the last few years. Your receiver project demonstrates just how much has changed for the better, and what capabilities we have at our disposal. If you are interested, subscribe to https://vk3hn.wordpress.com/ where I will be describing my Progressive receiver, in a few week’s time. It is not in the same league as your Express, but some of the motivations are similar. I am also interested in building a continuous tuning up-converting dual conversion transceiver, based on VU2ESE’s HF1. My work is nowhere near as high end and a lot more scratch-built (artisan as I call it) than yours, but it seems we come from a similar place and time! 73 Paul vk3hn.

Leave a Reply

Your email address will not be published. Required fields are marked *