Builder's Notes: Pocket FT8

OZ1BXM builds the "Pocket FT8" transceiver by Charles Hill W5BAA
Project announcement: https://wsjtx.groups.io/g/main/message/28645
Project description: https://github.com/chillmf/Pocket-FT8
Project Manual: https://github.com/chillmf/Pocket-FT8/blob/main/Project%20Manual_1.0.pdf
Hackaday article: The Simplest FT8 Transceiver You’ll Ever Build

Touch Screen 

testing display
Fig. 1. Testing the Touch Screen.
Any picture can be enlarged by clicking on it

Alu-sheet
The screen and the boards are mounted on an alu-sheet measuring 127 mm x 100 mm x 2 mm. The sheet shown in figure 1 is too small and was replaced later.

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

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

 

test results
Fig. 2. Screen measurements.

Screen measurements

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

Circuit diagram TX
Fig. 3. The modified transmitter.

Transmitter circuit diagram
I've modified the transmitter. Please refer to original diagram in the project manual page 4.  

R0 was added to let current flow into Q2. R2 was added to limit the Q2 base current when RF-PTT is high. The 5k1 resistor in Q3's gate was removed (it had no function). Same destiny for the resistor in gate of Q4. 

The input low-pass filter (L1, L2, C1, C2, C3) was removed. The low-pass filter following GVA-84+ should be sufficient for removing transmitter harmonics. 

R3, R4, R5 was added. They attenuate the clock signal by 12 dB.

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 photo) which can carry more current than other types.

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 


block diagram
Fig. 4. Gain distribution (transmitter).
Gain distribution
The gain distribution in the transmitter is shown in figure 4.
Veroboard TX
Fig. 5. Component placement, transmitter.
Transmitter lay-out
The transmitter was buildt on a piece of Veroboard. It measures 74 mm x 30 mm and is shown in figure 5.
Measure freq response
Fig. 6. Measuring LPF frequency response.
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 my 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

QRX
I am waiting for the amplifier GVA84+ to arrive.
 
Receiver - Pocket FT8
Fig. 7.Component placement, receiver.
Receiver
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.

Si3745 comes in a 24-pin SSOP housing (0.65 mm pitch) and is converted to 2.54 mm pitch using an SSOP adapter with 28 pins. Soldering instructions are here.

MCP3422 in SOIC-8 housing 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 round 3 V lithium cell is CR2032. It powers the RTC on Teensy.





Written by OZ1BXM Lars Petersen oz1bxm@pobox.com 13-Oct-2021. 

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