A "band decoder" is a device that reads the frequency/band information from a ham radio transceiver, typically in order to perform automatic switching of antennas, filters or amplifiers.
There are several ways to read the band information from a radio. Most radios have some sort of serial interface (CAT/CI-V), where an external device can (among other things) query the current frequency from the transceiver. ICOM radios also provide a voltage which corresponds to the current operating band. The voltage ranges are tabulated below (taken from AB4OJ's website
| Band / MHz | Voltage / V |
|---|---|
| 10 | 0 |
| 50 | 1 .. 2 |
| 28/24.9 | 2 .. 3 |
| 21/18 | 3 .. 4 |
| 14 | 4 .. 5 |
| 7 | 5 .. 6 |
| 3.5 | 6 .. 7 |
| 1.8 | 7 .. 8 |
Using this band voltage comes with the limitation that 12m and 17m are combined with 10m and 15m respectively.
The TCA965 is a vintage "Window Comparator" IC by Siemens. I recently acquired a large quantity of TCA965, so I was looking for a suitable use for them.
It takes an input voltage, compares it against a window between two reference voltages, and has four open collector outputs: Below the window, in the window, above the window, outside the window.
Using this circuit, a band decoder is limited to four outputs. The idea is not new - a band decoder circuit by DL5DSH from September 2000 was used as a reference for this project.
The requirements for this project were simple, though, only switching between high bands (10-20m) and low bands (40m-160m) should be realized, without the need for WARC bands.
The circuit diagram is available in PDF format. It closely follows the data sheet and DL5DSH's examples. Any general purpose NPN transistor can be used for Q1/Q2, the 2N2905 was chosen because they were available in large quantities.
A simple two sided PCB was designed to fit into a small Hammond extruded alumnium case (1455C802). A few pictures are shown below.
The full KiCad project can be found at https://git.fkurz.net/dj1yfk/tca965-band-decoder/.
Hover over the pictures to enlarge.