Radio time station transmitter using the Raspberry Pi and Nvidia Jetson

I am living in a country where there is no DCF77 sender nearby for my European radio controlled wristwatch to get its time. This vintage Junghans Mega doesn't have any buttons to set the time, so to bring it back to life, I built my own 'transmitter', taking the NTP time of a Raspberry Pi and generating a modulated signal via GPIO pins to then magnetically couple it into the watch ferrite.

Since many other long-wave time stations around the world use similar concepts of sending amplitude modulated time, other time services have been added.

This program is useful if you have a clock that otherwise does not get any reception. This magnetical coupling is very low power and only works over a few centimeters, but before running this program, make sure you follow your local laws with regard to restrictions on radio transmissions.

Platform

txtempus supports Raspberry Pi series and Nvidia Jetson Series (experimental).

Raspberry Pi

So far, it has been tested on a Pi3 and a Pi Zero W. There has been a report of different frequencies generated with an older Pi (Bug #1), so until we have a definitive list of available clock sources inside these, check out that bug for a workaround.

Nvidia Jetson Series (experimental)

So far, it has been tested only on a Jetson Nano, but all Jetson devices except for TX1 and TX2 (there is no available pwm pin) are supported.

Supported Time Services

DCF77

The DCF77 (Germany) signal is a 77.5kHz carrier, that is amplitude modulated with attenuations every second of the minute except the 59th to synchronize. The length of the attenuation (100ms and 200ms) denotes bit values 0 and 1 respectively so in each minute, 59 bits can be transferred, containing date and time information.

The Raspberry Pi has ways to create frequencies by integer division and fractional jitter around that, which allows us to generate a frequency of 77500.003Hz, which is close enough. Can be chosen with -s DCF77 option.

WWVB

The WWVB (USA) is on a 60kHz carrier, and also transmits one bit per second with different attenuation times (200ms zero, 500ms one; 800ms sync) and multiple synchronization bits. Use -s WWVB option for this one.

MSF

The MSF (United Kingdom) has yet another encoding, transferring two bits per second. Carrier is 60kHz. Option is -s MSF.

JJY

The JJY (Japan) is similar to WWVB, with same timings of carrier switches, but reversed power levels. Some bits are different. Two senders exist in Japan with 40kHz and 60kHz carrier; their simulations can be chosen with command line options -s JJY40 and -s JJY60. If you're in/or want to display a different time-zone, issue #17 might be of interest to you.

Minimal External Hardware

Raspberry Pi

The external hardware is simple: we use the frequency output on one pin and another pin to pull the signal to a lower level for the regular attenuation.

To operate, you need three resistors: 2x4.7kΩ and one 560Ω (precision not critical), wired to GPIO4 and GPIO17 like so:

Schematic	Real world

GPIO4 and 17 are on the inner row of the Header pin, three pins inwards on the Raspberry Pi GPIO-Header.

You don't need GPIO17 and the 560Ω resistor for MSF, as that works with switching the signal (on-off keying) instead of attenuating. In that case, you can replace the sequence of two 4.7kΩ resistors with a single 10kΩ.

Now, wire a loop of wire between the open end of the one 4.7kΩ and ground - this loop acts as coupling coil to the watch ferrite antenna. The signal is very weak, so bring this wire-loop close to your radio watch/clock.

In the following image, which was the first experiment, it is wrapped around the antenna, but it is not strictly needed: anything within a few centimeters should work.

Being too close to the clock can confuse a sensitive receiver, so you might need to experiment with the distance. If your clock/watch is not receiving, add more turns to your transmission coil. In the picture at the bottom of the page you see that I am using about 10-20 turns on the coil (reddish oval lying on the Pi).

This set-up should work for most watches if you have them in close vicinity.

The antenna set-up is intentionally not optimal to just be good enough for a local watch but hopefully not causing interference. Further improvements of course can be done to the antenna for increased transmission distance, such as using a ferrite, making it an LC circuit or adding an amplifier. Only go in this direction after familiarizing yourself with allowances of radio transmissions in your area on your frequency of interest.

Nvidia Jetson Series (experimental)

(*Please read the external hardware for the Raspberry Pi above first.) On Jetson, the external hardware setup is slightly different from the Raspberry Pi.

We need one "PWM Pin" for a frequency output, and one "Attenuation Pin" for modulating the signal. These pins vary by the Jetson model. Please check the following table.

To operate, you need three resistors: 2x4.7kΩ and one 560Ω (precision not critical) and one NPN transistor (nearly any NPN transistor should work. I'm using KTC3198).

Here's the full schematic of the external hardware for the Jetson Series:

Schematic	Real world (Jetson Nano)

Like the Raspberry Pi, you don't need the Attenuation Pin and the 560Ω resistor for MSF, and a wire-loop between the 4.7kΩ register and the ground acts as coupling coil. Bring this wire-loop close to your radio watch/clock.

Build

 sudo apt-get install git build-essential cmake -y
 git clone https://github.com/hzeller/txtempus.git
 cd txtempus
 mkdir build && cd build

Rapberry Pi

 cmake ../ # or cmake ../ -DPLATFORM=rpi
 make

Nvidia Jetson Series (experimental)

Before you build txtempus on your Jetson:

• You should install JetsonGPIO which is a library that enables the use of Jetson's GPIOs.
• The system pinmux must be configured to connect the hardware PWM controlller(s) to the relevant pins. Read the L4T documentation for details on how to configure the pinmux.

 cmake ../ -DPLATFORM=jetson
 make

Transmit!

 sudo ./txtempus -v -s DCF77

With -s, you set the type of time signal you want to transmit.

There are a few options you can set. The -r option is useful to have the program run only for the few minutes it might take for a clock to synchronize.

By default, the current system time is transmitted. The -t option allows different times for testing.

usage: ./txtempus [options]
Options:
        -s <service>          : Service; one of 'DCF77', 'WWVB', 'JJY40', 'JJY60', 'MSF'
        -r <minutes>          : Run for limited number of minutes. (default: no limit)
        -t 'YYYY-MM-DD HH:MM' : Transmit the given local time (default: now)
        -z <minutes>          : Transmit the time offset from local (default: 0 minutes)
        -v                    : Verbose.
        -n                    : Dryrun, only showing modulation envelope.
        -h                    : This help.

Don't connect monitor (Raspberry Pi)

Don't connect a monitor to the Pi, just operate it headless.

The internal oscillator used is also used for HDMI in the Rasbperry Pi; it will be changing its frequency if a monitor is connected and the transmission will fail. There should probably be a flag added to generate the frequency from an alternative oscillator instead; but until that is implemented, just don't connect a monitor and it will work. See the very wrong frequency bug for details.

Action video - watch a watch synchronize

In the video below, you can see how a watch is set with this set-up. After it is manually reset, it waits until it sees the end-of-minute mark (which does not have any amplitude modulation) and then starts to count on from second 59, then gathering the data that is following.

An interesting observation: you see that the watch already gets into fully set mode after about 50 seconds, even though there is the year data after that. This particular watch never shows the year, so it just ignores that.

Showing the modulation envelope

Mostly for understanding the protocol, the -n option allows to observe how the amplitude modulation of each second looks like. Unlike the regular transmission, don't need to be root or run on the Raspberry Pi to use this option. Underscores (_) show low power carrier, hashes (#) high power:

$ ./txtempus -n -s wwvb
2018-08-17 13:22:00 -> tx-modulation
:00 [________##]
:01 [__########]
:02 [_____#####]
:03 [__########]
:04 [__########]
:05 [__########]
:06 [__########]
:07 [_____#####]
:08 [__########]
:09 [________##]
:10 [__########]
:11 [__########]
  ... and so on for the whole minute ...

Limitations

In some of these protocols, there are additional bits that contain information about upcoming daylight saving times, leap seconds or difference to astronomic time. These are currently not set, but usually clocks are fine with it.

Some time stations also phase-modulate their carrier, txtempus does not.

The frequency generation does not seem to work on a Raspberry Pi4. Please use older Pis for now until that is figured out (also pull requests accepted if you know details).

Installation

Software

After building, you can install the binary in some standard location

 sudo make install

Watch holder

Each set-up will be different. In my case, I need my DCF77 radio watch getting set over night. So I built this watch holder that presents the watch upright while the antenna (in the wristband) is close to the 'transmission coil' that is lying flat on the Pi. The bottom of the 3D printed case is filled with lead shot in epoxy to provide a stable base.

The Raspberry Pi Zero W runs ntpd, PLL locking the system time to various stratum 1 NTP servers keeping it at atomic time within ±50ms. This particular watch only checks the radio twice a day at 2am and 3am, so there is a cron-job that runs txtempus around these times for a few minutes.

Crontab

If you put the following line in your /etc/crontab txtempus will be started at 1:57 and 2:57 at night and runs for 10 minutes.

57 1,2    * * *   root    /usr/bin/txtempus -s DCF77 -r 10

(this requires that you have installed txtempus so that it can be found in /usr/bin : sudo make install).

watch holder	... with watch

tx common telecommunication abbreviation for 'transmit'
tempus, n Latin. Time; period; age