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Raspberry PI hat for radio amateurs

History

I've been doing a lot of SOTA activations with Raspberry PI like SBCs, and there where always issues, like RF interference, and power supply shorts. I started to add things like galvanically decoupled audio interface, a step down power supply. It was working, but it looked like a mess.

This piece of hardware integrates all needed equipment in one nice hat. It is designed to put the RPI in a metallic box to further isolate from RF signals.

It was designed using KiCad.

Applications

  • HF/VHF digital modes (PSK, FT8, etc)
  • HF/VHF AX.25 based modes (Packet, APRS, etc)
  • CW keying

Features

  • 3A power supply with wide input voltage range: 8 to 35V
  • Ublox MAX-8 GPS receiver for position and time synchronization
  • Inductive audio couplers to minimize GND loops. Up to 4kHz.
  • PWM controlled cooling fan
  • GPIO controlled PTT from the PI
  • GPIO controlled CW key from the PI
  • RTC

GPS antenna is not integrated in the device, so an external antenna connector is mounted on the PCB.

Ufortunately, the sound card of the PI can not record audio, so an external sound card must be used. I use this particular device with good results. I usually break the plastic housing, and solder a shielded cable directly to the audio interface (J109).

The GPS is needed for APRS and time synchronization. There is an onbard RTC to have some redundancy.

Schematic

Grab the schematic for a quick look.

BoM can be found here.

Mechanics

It is designed to fit on the Raspberry PI 4. Be aware, that if the heatsink on the CPU is too high, it might conflict with the screws of the fan. If that happens, mount the heatsink 45 degrees.

Connection to the radio

The hat comes with a DB9 connector to connect up your rig. I wanted to have a screw lock connector, and this was the best choice. The pin out looks like this.

Signal name Pin of the DB9 Comment
TX audio 1 Audio input of your rig
KEY 2 CW key of the rig
PTT 3 PTT input of your rig
NC 4 Not connected
RX audio 5 Audio output of your rig
GND 6 Audio ground
GND 7 Audio ground
GND 8 Audio ground
GND 9 Audio ground

PTT is connected to GPIO17. It is buffered, and logic one means transmit.

KEY is connected to GPIO21 of the PI.

Connection to the soundcard

J109 provides connection to the soundcard.

Signal name Pin of J109 Comment
RX audio 1 Mic input of your soundcard
TX audio 4 Speaker output of your soundcard
GND 2,3 Audio ground

Setting up audio levels

There are two potentiometers (RV101 and RV102) to adjust sound level in RX and TX direction respectively. Adjust TX and RX audio levels to a certain point where you are not over driving either input. If you are using it for HF, monitor your ALC, and make sure it is not active.

Power connection

J101 provides power connection. The maximum input voltage is 35Volts.

GPS

Connect your passive or active GPS antenna to J104. Use 3.3V active antenna. PPS signal is provided on GPIO18.

Here is an excellent article how to set up GPIO-PPS for ntpd.

RTC

DS3231 based precision RTC is provided for backup if the GPS can't provide accurate timing. It has internal temperature compensated crystal oscillator.

A nice tutorial how to set up the RTC on an RPI.

Connect a 3V battery to BT101 for offline RTC operation.

FAN control

PWM shall be enabled on GPIO13.

If you don't want PWM control, short JP101, and the fan will spin at full power all times.

There's a non-PWM control of the fan that works out of the box using the Raspberry PI OS.

Open raspi-config, under Performance Options, you shall select the desired temperature, and the GPIO pin which is 13.

Accessories

These items needed for the hat for full functionality.

All sourced from Farnell, unless othervise noted. (Sorry, I'm Hungary based.)

Item Source designator Comment
RTC Battery 1892670 The idea is to use heatshrink tube and some wires.
RTC Battery holder 3583203 External battery holder for CR2032
Heatsink 1211701 or https://www.alza.hu/raspberry-pi-rb-hutoborda-d4147799.htm?o=21
Standoff 1466844 It must be 12mm in length
Mounting screws M2.5x6
GPS antenna This or this Use 3V active antenna
RPI4 3369503 (8GB) or 3051887 (4GB)
USB audio interface Alza Any other do the job. Check Linux driver availability.
Monitor Ebay
Keyboard https://www.pcland.hu/genius-luxemate-100-usb-fekete-hun-billentyuzet-31300725113 Any USB Keyboard. I use the one without keypad, for its compact size.
Mice Any USB, or BLE mice
SD card with Raspbian image.
uHDMI to HDMI cable Alza
DB9 Male connector To connect to your radio
Data connector for your radio
Shielded audio cable To connect the soundcard to the hat, and for the DB9 connector

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Raspberry PI hat for amateur radio operators

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