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. These outputs are used to switch the LO filter and can be used to switch external filter and amplifiers.

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 must be taken to properly buffer these 3.3v signals from relay transients.

GPIO band decoder

As we only have 2 GPIO pins to provide 4 outputs, these outputs need to be decoded to provide an output for each band.

The Portsdown team have designed a simple board which can be either constructed on 0.1” perforated board (veroboard) or on a PCB - this can be home etched using the details below or blank PCBs are available in the BATC shop.

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

These band outputs can then used to "steer" the PTT line to the correct Power Amplifier using this simple relay and diode circuit - no PCB needed :-)

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.

Summary of GPIO connections

The BCM numbers in this table correspond to the connector labelling on the Willow Electronics GPIO breakout board. Two of the connections are not broken out and need to be directly connected to the GPIO pins. More details to follow.