4 Band Bandpass Filter Board

The relays are double pole double throw in a standard 18x9mm package size. The transistors are anything you have in CBE footprint, I used 2n3904 and the resistor is there for current limiting, just pick a typical value, i think i used 4.8K. The trimmer caps I use are 3 leg 100pf max value to tune and align each filter.

SMA edge connectors bring RF in on the left side of the board. By default all relays are in the OFF position and no RF can pass through. The 6 pin header brings control signals and power to the board. To turn a relay on, a 5v control control signal is placed on one of the first 4 pins to turn 1 pair of relays on. The last 2 pins are ground and VCC of the relays you used.

Aligning each filter can be a bit tricky, it is best to use a spectrum analyzer and tracking generator if you have one, or a noise source and spectrum analyzer or even in a pinch if you slow down the seconds/division on the oscilloscope so you can see the filter shape. The reason being, if your filters are not quite tuned right, you will end up with 2 peaks and some attenuation in the bandpass you just do not want.

The filters themselves are Double Tuned circuits. To design the values needed for each band I used some software called DTC.exe which can be found on a CD in the book Experimental Methods In RF Design. I have linked it here for download, but I am not sure if I am allowed to and i will pull it down if asked by the authors: DTC DOWNLOAD

The gerber files for the board can be downloaded from here: Bandpass Filters

Here is a schematic of the filters with values for 80, 40, 20 and 15m.


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2 thoughts on “4 Band Bandpass Filter Board

  1. Hello Rob,
    Nice blog !

    Seems to be some mistakes in your schematic and implementation for these Bandpass Filters.
    Input and output capacitive dividers are not connected the right way, check again on DTC software how it should be done. e.g. for 20m Filter C47 pin2 should be connected to C35 pin1 and C48 pin1 should be connected to C37 pin2. Same error on all Filters.

    73 Olivier F1ETJ

    1. Yeah you are correct. It does work this way as its using stray board capacitance with the input capacitor as the capacitive divider, but its not the proper way of doing it. I have since redesigned the board and changed the layout to be correct as per how DTC should be used. Good pickup BTW, no one else had noticed it.

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