Distributing a frequency reference over optical fiber
These days most professional telecommunications run over fiber optic cables. No wonder – these inconspicuous, hair-thin strands can carry information over enormous distances and are immune to electromagnetic interference, flooding and similar misfortunes. Fiber technology gets cheaper by the day and the basic building blocks of fiber optics can be found on auction sites at very affordable prices. What used to keep fiber out of amateur projects – the cost and availability of splicing – is gone thanks to ready-made, factory-terminated fiber cables. They come in many different versions and are available practically off the shelf.
Having access to quite a few broken Ethernet-to-fiber media converters, I decided to take a closer look at the optical transceivers inside and try to put them to use in our amateur setups. Since this coincided with the launch of the OSCAR-100 satellite and the need for a stable frequency reference for transverters sitting somewhere far out in the garden or a few floors up on the roof, the first thing I tried was sending a reference signal.
Optical cable has undeniable advantages: its attenuation is negligible (approx. 0.2 dB/km) and it is completely insensitive to strong RF fields – which makes it an ideal medium for signals as sensitive as a frequency reference.
How it works
After digging through datasheets and a few experiments I managed to build a system that delivers a frequency reference to every device in my shack and beyond. I decided to transmit the signal of a standard 10 MHz reference – it can be a plain TCXO, a rubidium standard or a GPSDO, it makes no difference here – 10 MHz goes in, 10 MHz comes out at the receivers.
The system uses passive splitting of the optical power, based on PLC splitters. A laser beam fed into the input of such a splitter is divided (usually) equally between its outputs. No specialised fiber or test equipment is needed to build the system. All components are readily available both from distributors and on popular auction sites.
The system consists of 4 parts:
- a reference frequency source
- an optical transmitter
- a distribution network with optical power splitting
- optical receivers
The reference frequency source can be any high-stability oscillator, e.g. an OCXO, a GPSDO or a rubidium standard. Ideally it should run 24/7 in stable environmental conditions: in a closed cabinet or another quiet place. This gives constant access to the reference without waiting for the oscillator to "warm up" and its frequency to settle. Since the reference is not electrically connected to any equipment (apart from its power supply, of course), it is not sensitive to lightning strikes, static or RF interference.
The optical transmitter converts the electrical reference signal into a laser beam and launches it into the fiber. The input has a Schmitt trigger with symmetry adjustment, the output is a fiber port.
The distribution network is built from prefabricated fiber patch cords, which can be bought in practically any length, terminated with SC/PC connectors (that is what we will be using). The signal is split with PLC splitters – also with factory-fitted connectors. To join it all together we only need some fiber adapters (couplers).
The optical receivers convert the received light back into an electrical signal. In addition, each receiver has a three-output CDCE913 PLL from TI, which can be programmed to deliver 3 different clock signals locked to our reference source.
I have been using this system in my lab for over a year and it has never let me down. This is how I deliver the reference to the 13 cm transverters on the roof, to the QO-100 satellite station in the garden, and to the ham shack and the lab, where radios, test equipment and SDRs are hooked up to it. Grzegorz SP8NTH is the beta tester.
Setup and PLL configuration
Once powered up, the only thing the system needs is setting the symmetry of the output signal. The easiest way is with an oscilloscope or a frequency counter that measures duty cycle. Connect the instrument to the output of an optical receiver and, with the reference connected, adjust the duty cycle of the output signal to 50%.
The PLL configuration is easiest to calculate with TI's ClockPro. Then copy the calculated values and send them to the chip over I2C. That can be an Arduino, a Raspberry Pi, a debugging interface (I use a Bus Pirate) or even two push buttons and a lot of determination ;-). The chip has a built-in EEPROM, so the configuration can be stored permanently and the outputs do not need to be reprogrammed after every power loss.
Below is an example configuration for output frequencies of 10, 25 and 100 MHz.
Bus Pirate (I2C mode):
[0xca 0x01 0x06 0x05 0x34 0x01 0x02 0x50] [0xca 0x14 0x0c 0x6D 0x02 0x08 0x02 0xff 0x00 0x02 0x83 0xff 0x00 0x02 0x80] # write EEPROM [0xca 0x86 0x01]
Raspberry Pi (i2c-tools):
i2ctransfer -y -v 1 w7@0x65 0x01 0x05 0x05 0x34 0x01 0x02 0x50
i2ctransfer -y -v 1 w14@0x65 0x14 0x0c 0x6D 0x02 0x08 0x02 \
0xff 0x00 0x02 0x83 0xff 0x00 0x02 0x80
# write EEPROM
i2ctransfer -y -v 1 w2@0x65 0x86 0x01
What to build it from
For the transmitter and receiver boards I used parts salvaged from TP-LINK MC112CS and MC111CS converters – these are WDM (so-called single-fiber) converters. Keep in mind that if the transmitter uses a module from an MC112CS, the receivers must use modules from MC111CS (or the other way round – depending on what you can get). If you use transceivers from classic dual-fiber media converters, remember to connect the splitter input to the TX port of the transmitter and the splitter outputs to the RX ports of the receivers.
With a 1:16 split the system distributes the reference over more than 1 km, which covers all amateur needs with plenty of margin.
Schematics and PCBs
Downloads
- article in PDF (13 MB, in Polish)
- schematics and PCBs [eagle] - transmitter (CLOCK_TX_buff) and receiver (CLOCK_RX)