Four-bank RF filter for amateur bands

When I built my AVALA-01 I wanted to make an multi-band SDR transceiver. Consequently, I did not install the components for the internal 40-meter low-pass filter. This filter was designed to attenuate the harmonics generated by the quadrature sampling mixer. For an multi-band transceiver, I would need a band-pass filter for each amateur radio band I wanted to use. I elected to design and construct a switchable band-pass filter module, so I wouldn't have to manually exchange filters when I wanted to operate on a different band.

For my design I had several design goals in mind. First, I wanted the to cover as many amateur bands as possible for the fewest filters used. This was possible since some amateur radio bands are relatively close in frequency in comparison to the harmonics that needed to be attenuated. Second, I wanted to use latching relays, so that I could set the filter to a particular bank and keep it there without constantly expending current to energize relays. Finally, I wanted to try to use a RadioShack Aluminum Project Enclosure (270-238). They are very inexpensive compared to other enclosures of similar style.

The first goal was met by using the filter designs obtained from Band pass filters for HF contest stations with insertion loss lower than 0.5dB, version 3, which can be obtained from the excellent website YU1LM QRP belonging to Tasić Siniša (a.k.a. Tasa). These filters provide low insertion loss and good attenuation of third-order harmonics. While Tasa's paper presents eight filters, I could only use four for my filter design. I elected to use the filters for 7 MHz (40-meter band), 14 MHz (20-meter band), 18-21 MHz (17-meter and 15-meter bands), and 24-30 MHz (12-meter and 10-meter bands). Covering six amateur radio bands with four filters was quite acceptable for me.

Meeting the second goal was more of a matter of economy, rather than technology. Relays, in general, are not inexpensive. Latching relays are definitely not inexpensive. I was fortunate in sourcing cheaply some Panasonic NAIS DS2E-SL2-DC5V dual-coil latching relays. These relays are particularly useful, as switching involves energizing one coil or the other, not switching the direction of the current flow, as single-coil latching relays require. Ideally, 12-volt relays would have been better for a radio application, but use of an appropriate current-limiting resistor took care of the voltage differential. A schematic was generated in EagleCAD and an Adobe PDF of that schematic is provided at the bottom of the website. Note that no component values for the inductors and capacitors are listed. I never got around to entering in the values. It should be clear from Tasa's document and the filter bank number where the components should be placed.

Accomplishing the final goal was the most challenging. Fitting four filter banks on a single-sided PCB into the RadioShack enclosure required numerous attempts placing components compactly and routing traces without resorting to wire jumpers. I finally hit upon a nice, tight, and symmetrical layout, as shown in the layout illustration below. After fabricating the PCB, I realized that I had forgotten to include the protection diodes for the relays. I ended up tack-soldering them across the coils of the relays. I will revisit the design of the filter bank in the future and add those parts.

Fabrication and assembly of the PCB was very straightforward. The greater challenge was actually wiring the switching diode to the rotary switch. I used 1N914 (1N4148) diodes and soldered them free-form around the rotary switch. Heat shrink tubing was used to insulate the wiring from the case. The photo below shows the finished, not entirely tidy assembly.

The finished four-bank RF filter is shown in the photo below. A bank is selected by turning the knob to the desired band(s) and pressing the "engage" button momentarily. A satisfying clicking of relays announces that the new band has been selected.