PTT and band switching
In order to make the Portsdown transmitter integrated with transmit and receive systems, Pin 40 of the GPIO goes high when transmit is selected and the signals on pins 28 and 35 can be used for band switching:
| Frequency Mhz | ATV Band | Pin 28 | Pin 35 |
|---|---|---|---|
| < 100 MHz | 71 MHz | Lo | Lo |
| 100 - 250 MHz | 146 MHz | Lo | Hi |
| 250 - 950 MHz | 437 MHz | Hi | Lo |
| 950 - 4400 MHz | 23cms | Hi | Hi |
Care should be taken to properly buffer these 3.3v signals from relay transients.
The band outputs can be decoded and then used to "steer" the PTT line to the correct Power Amplifier. The Portsdown team have designed a simple board which can be either constructed on 0.1” perforated board (veroboard) or PCB details are included.
File:DECODE 4 Band instructions (2).pdf
File:Decode 4 Band Schematic.pdf
File:Decode 4 Band Perf Bd wiring.pdf
File:DECODE 4 Band Mirrored w SS.pdf
File:DECODE 4 Band Mirrored.pdf
File:DECODE 4 Band PCB Layout w SS.pdf
We also recommend the use of a Tx / Rx sequencer to ensure your mast head relay has changed over and your expensive pre-amp is de-powered before you start transmitting. Many circuits and pre-built designs are available including this simple single chip design by Ian Waters G3KKD published in CQ-TV 125.
Also rather than use these pins to switch an expensive RF relay on the output of Portsdown, take a look at this simple idea to use a duplexer / triplexer.