Builder's Notes: Pocket FT8

I am building the "Pocket FT8" transceiver by Charles Hill W5BAA

Project announcement: https://wsjtx.groups.io/g/main/message/28645
Project description and files: https://github.com/Rotron/Pocket-FT8
Project Manual ver.1.0
Hackaday article: The Simplest FT8 Transceiver You’ll Ever Build
Project status: Halted.


Building the "Pocket FT8"

testing display
Fig. 1. Testing the Touch Screen.
Enlarge a picture by clicking on it

Touch Screen
The Adafruit Touch Screen was purchased from Digi-Key (part number 1528-1470-ND). The screen is described by Adafruit.

Testing the screen with UNO
I used Arduino UNO for testing the touch screen. The test procedure is described in Adafruit 3.5" Color 320x480 TFT Touchscreen Breakout pages 23-27. The sketch uploaded to Arduino UNO (graphictest.ino) draws graphics and text on the screen. Figure 1 shows text with different fonts.


test results
Fig. 2. Screen measurements.

Screen parameters

The UNO measures different screen parameters and sends them to the IDE via the serial interface. Figure 2 is a screen-dump from IDE showing the measurement results.

Screen connections
The touch screen is supplied with 3.3 V DC and is connected to Teensy 3.6 and the MCP3422. Please note, that four of the screen wires (CLK, MISO, MOSI, RST) are wrongly connected in the Project Manual 1.0 page 3.

Screen connections are defined in Pocket_FT8_August_2021.ino:
HX8357_t3 tft = HX8357_t3(10, 9, 8, 11, 14, 12);
// Syntax: HX8357_t3(CS, DC, MOSI, SCK, RST, MISO)

The proper connections are shown on the wiring diagram (figure 13).
Chassis screen side
Fig 3. Chassis (screen side).
Chassis
The touch screen and the three circuit boards are mounted on an alu-sheet measuring 127 mm x 100 mm x 2 mm.
 
Figure 3 shows the side where the touch screen will be mounted. The 4 stand-offs under the sheet serve as feet.
Chassis alu
Fig 4. Chassis (board side).
Figure 4: Three circuit boards will be mounted on this side of the sheet. 

Receiver, component placement
Fig. 5.Components on the receiver board.
Receiver board; components
Please refer to circuit diagram in the project manual page 3. The receiver was built on a piece of Veroboard measuring 45 mm x 100 mm. The component names are shown on this sketch which also shows a label change at Teensy pin 39.

Don't forget to cut the traces 19 different places, and drill two holes each 3 mm diameter.

Si3745 comes in a 24-pin SSOP housing (0.65 mm pitch) and is converted to 2.54 mm pitch using an SSOP-24 adapter. My adapter is from RoarKit (sold by sheffield_nikki on ebay.com). Soldering instructions are here. Pin 10 and 11 are grounded as recommended in the data sheet page 21.

MCP3422 in a SOIC-8 house has 1.27 mm pitch and is converted to 2.54 mm pitch using an adapter with 8 pins. Soldering instructions are here.

Si5351 comes with a 2.54 mm pitch break-out board.

The decoupling capacitor C0 100 nF was added.

The round 3 V lithium cell (CR2032) is fitted last. It powers the RTC on Teensy.







RX Veroboard
Fig 6. Receiver board without ICs and battery.
Receiver board; photo
Figure 6 shows the receiver board. The Si3745, Si5351, MCP3422, and the battery are not fitted yet.
Veroboard for Teensy
Fig. 7. Veroboard for Teensy.
Teensy 3.6 board
Teensy 3.6 is fitted on a piece of Veroboard using sockets. The Veroboard measures 68 mm x 30 mm.

Teensy 3.6 product page
Teensy 3.6 pin-out card (front side)
Teensy 3.6 pin-out card (back side)
TX diagram
Fig. 8. Transmitter circuit diagram.
Transmitter circuit diagram
Receive: Q3 is on; Q4 is off. The signal from the antenna runs through the LPF and Q3 and is terminated by R6. The signal goes to the receiver via C7.

Transmit:
Q3 is off; Q4 is on. The transmit signal from GVA-84+ flows through the LPF to the antenna. Q3 is off and prevents the strong RF signal from reaching the receiver. Q4 conducts and puts "RF - Rcvr In" to ground for receiver protection. 
 
I've modified the transmitter as shown in figure 8. Please refer to original diagram in the project manual page 4. R0 was added to allow current into Q2. R2 was added to limit the Q2 base current when RF-PTT is high. The resistor in Q3's gate was removed (it had no function). Same destiny for the resistor in the gate of Q4. 

The input low-pass filter was removed. The low-pass filter following GVA-84+ should be sufficient for removing transmitter harmonics. The output transformer T1 was removed and replaced by the application circuit from the GVA-84+ data sheet page 4. RFC was changed from a core-wound choke to a standard choke. It must be the upright type (see this photo) as this type can carry more current. R7 and LED are optional. I've added the LED to indicate when PTT is activated.

Component values for 18 MHz
L3, L4: 0.5 uH. Make 10 turns 0.5 mm wire on Amidon T37-2 core. Picture of this inductor.
C4, C6: 75 pF
C5: 242 pF
TX veroboard
Fig. 9. Component placement, transmitter.
Transmitter board lay-out
The transmitter is buildt on a piece of Veroboard measuring 74 mm x 30 mm (figure 9).


SMD soldering
Fig. 10. Soldering GVA-84+ on the copper side.
GVA-84+
The amplifier GVA-84+ is a surface mount component. It is placed on the copper side of the board and soldered 4 places.
TX board
Fig. 11. Transmitter board.
Transmitter board
RF output from the transmitter was measured using a low-price meter from China. The output was 50 mW at 18 MHz. 
Measure freq response
Fig. 12. Measuring the Low Pass Filter.
Measuring the Low Pass Filter
The low pass filter comprises L3, L4, C4, C5, and C6. I measured the frequency response of the filter using NanoVNA.

Start freq: 1 MHz; Stop freq: 100 MHz
Gain at 18 MHz: -0.78 dB
Gain at 36 MHz: -17.7 dB
Gain at 54 MHz: -33.6 dB
Gain at 72 MHz: -47.6 dB
wiring diagram
Fig. 13. Wiring diagram.
Wiring diagram
The boards are wired together using insolated wire with different colors as shown in figure 13.
screen-dump
Fig. 14. Pdb.h file.
Load Teensy software into Arduino IDE
Follow the Project Manual page 5 (Building the Firmware).

The instructions https://www.pjrc.com/teensy/td_download.html will guide you, and soon Teensyduino is installed with a lot of new libraries. Win 10 advice: Run the installation file as Administrator.

The file pdb.h in the "utility" folder (high-lighted in figure 14) must be replaced by the one in the "Pocket_FT8_Publish" folder.
screen-dump
Fig. 15. The new pdb.h file.
The new pdb.h is shown in figure 15. The new size is 5 kb and the date is 23-Oct-2020.
screen-dump
Fig. 16. Teensy 3.6 is detected by Arduino IDE.
Arduino IDE detects Teensy 3.6
Figure 16 shows Arduino IDE > Tools when Teensy 3.6 is connected to the PC and properly detected.

If the Board string is different from "Teensy 3.6", you can select it here:
Tools > Board > Teensyduino > Teensy 3.6
screen-dump
Fig. 17. New libraries.
Install remaining libraries
How to install additional libraries: https://www.arduino.cc/en/guide/libraries

Disconnect Teensy from the PC. Close Arduino IDE and restart the PC. Open Arduino IDE and install the remaining libraries:
  • Etherkit_Si5351
  • MCP342X
  • HX8357_t3-master
  • Si4735-master
The new libraries should be installed in the sketchbook folder. Open this folder by selecting:
Arduino IDE > Sketch > Show sketch folder.

Copy the four new library folders to the "libraries" folder inside your sketchbook as shown in figure 17.

Verify that the new libraries are available in the list that pops up when you select Sketch > Include library.
The Etherkit_Si5351 library is displayed as "Etherkit Si5351"
The MCP342X library is displayed as "MCP342x"
The HX8357_t3-master is displayed as "HX8357_t3"
The Si4735-master library is displayed as "PU2CLR SI4735"

The new libraries can also be verified in the Library Manager (select Type = Installed).
screen-dump
Fig. 18. This folder contains the main software.
Compile and upload 
Copy the folder "Pocket_FT8_August_2021" to the sketchbook folder as shown in figure 18.

Open the file Pocket_FT8_August_2021.ino and insert your ham call and your locator:
char Station_Call[] = "OZ1BXM"; //six character call sign + /0
char Locator[] = "JO46"; // four character locator  + /0

Save the file and press Verify to compile. Observe, that IDE selects the proper library. If IDE selects a wrong library, it must be moved outside the library folder, or renamed.

Upload the compiled file to Teensy 3.6. 



Current status
Touch Screen has background light, but no text/graphics is shown on screen.
No serial output from Teensy.
Project halted. 
To do: Make the Touch Screen work with Teensy.

     

Written by OZ1BXM Lars Petersen oz1bxm@pobox.com and revised on 27-Nov-2021. 

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