Difference between revisions of "PTT and band switching"

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Care should be taken to properly buffer these 3.3v signals from relay transients.
 
Care should be taken to properly buffer these 3.3v signals from relay transients.
  
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 PCB details are included.
+
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 a PCB.
  
 
[[File:4 band decode.JPG|400px]]
 
[[File:4 band decode.JPG|400px]]

Revision as of 10:54, 1 March 2017

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.

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 a PCB.

4 band decode.JPG

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 :-)

Tx switching.JPG


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.

Tx rx timing.JPG

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.