PTT and band switching

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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.

Band outputs

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.


Band decoder.JPG


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.

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


PTT steering

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

Tx Rx sequencer

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. The sequence is as follows:

To go to Tx:

1) Turn off receiver pre-amp

2) Switch ant c/o relay to transmit

3) Apply power to PA

To go to rx

a) Turn off Tx

b) Switch ant c/o relay to receive

c) Apply power to pre-amp.


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

David M0YDH shows Ian's design in operation:- G8ADE RELAY SEQUENCER VIDEO and an updated version with extra relay for Portsdown PTT band switching.. If you prefer an elegant kit solution then you could try the TROPIC sequencer from Shortwave radio.This is listed at £11.50 but price is available on application. W6PQL and W1GHZ have sequencer designs.

Summary of GPIO connections

As the Portsdown project has expanded in scope, so has demand for the GPIO connections on the RPi. Here is the latest list as of January 2018:

Pin No BCM No Wiring Pi RPi Name Primary Use Secondary Use DTX-1 Use
1 - - 3.3v - - EXT-DVLM
2 - - 5v - - -
3 2 8 SDA 1 - EXT-SD0
4 - - 5v - - -
5 3 9 SCL 1 - EXT-SD1
6 - - 0v - - -
7 4 7 GPIO 7 Keyed Streaming LED Indication Transverter Select EXT-SD2
8 14 15 TxD ADF5355 LE - EXT-SD3
9 - - 0v - - -
10 15 16 RxD Attenuator LE - EXT-SD4
11 17 0 GPIO 0 LCD Display - EXT-SD5
12 18 1 GPIO 1 Keyed Streaming or Keyed TX Switch EXT-SD6
13 27 2 GPIO 2 Shutdown LED EXT-SD7
14 - - 0v - - -
15 22 3 GPIO 3 Shutdown Button EXT_TSCLK
16 23 4 GPIO 4 LCD Display - -
17 - - 3.3v - - -
18 24 5 GPIO 5 LCD Display - EXT-PSYM
19 10 12 MOSI LCD Display - -
20 - - 0v - - -
21 9 13 MISO LCD Display - -
22 25 6 GPIO 6 LCD Display - -
23 11 14 SCLK LCD Display - -
24 8 10 CE 0 LCD Display - -
25 - - 0v - - -
26 7 11 CE1 Touch Sensor - -
27 0 30 SDA 0 ADF4351 LE - -
28 1 31 SCL 0 Band LSB (D0) - -
29 5 21 GPIO 21 ADF4351 CLK Atten and ADF5355 CLK -
30 - - 0v - - -
31 6 22 GPIO 22 ADF4351 DATA Atten and ADF5355 DATA -
32 12 26 GPIO 26 I output "Ugly" and RPITX RF output -
33 13 23 GPIO 23 Q output - -
34 - - 0v - - -
35 19 24 GPIO 24 Band MSB (D1) - -
36 16 27 GPIO 27 IQ Filter LSB (D0) - -
37 26 25 GPIO 25 IQ Filter NSB (D1) - -
38 20 28 GPIO 28 IQ Filter MSB (D2) - -
39 - - 0v - - -
40 21 29 GPIO 29 TX LED and PTT Drive - -

Each pin on the RPi3 GPIO can be referred to in 4 ways: its physical pin number, the Broadcom I/O number, the Wiring Pi number or the name. Where possible, the Portsdown software uses the Wiring Pi driver and hence Wring Pi numbers in the code. The Broadcom numbers are referred to on the Willow Electronics GPIO breakout board.

For reference, here is the original Portsdown GPIO allocation chart, drawn up in early 2017.

GPIO pinout.JPG