Crystal Vision
Appendix
8 Appendix
8.1 Fibre distance calculations guide
The maximum distance for a reliable fibre optic link will depend on several factors. The power
of the lasers, the sensitivity of the receiver, the type of fibre optic cable used, the quality and
cleanliness of the connectors, the data rate of the digital signal, the amount of jitter on the
digital signal applied to the system, even possibly the number and sharpness of bends in the
cable. Therefore, working an exact limit is very difficult and the number will vary from one
installation to another.
The calculations shown below provide a simple basis to know if you are going exceed the
maximum length which is defined by the power lasers and sensitivity of the receiver. This is
often referred to as the link budget.
Link budget = Laser Power output - cable loss - connector loss
The Link budget must be greater than the receiver sensitivity.
This will tell you if the length you wish to achieve is possible, assuming the other factors
mentioned are not significant. By this we mean normal or average conditions.
The Crystal Vision FTX-L 3G SDI to fibre converter is quoted as having an output power of
typically -2dbm but there’s a worst case figure of -5dbm and so you should use that.
The FRX 3G sensitivity is defined as the minimum input level being -20dBm.
Typical cable loss could be 0.4db per km, for 1310nm light. Connector loss could be 0.5dB
each.
So, if we wanted to send the signal down 25km of cable then we would then do the following
calculation…
(-5 for FTX) – (25Kmx0.4 for cable) – 1 (for the 2 connectors) = -16
which is more than -20 by a long way.
It is recommended that to be very safe you do not get within -3dBm of the limit of receiver
sensitivity. This would mean that you have a link budget maximum of -17. This then works
back to a maximum cable length of 32.5km.
However, these calculations are based purely on power loses. As the data rate rises from
270Mbs to 1.5Gbs or higher there are other effects which start to reduce the performance.
These optical effects introduce jitter on the signal and then reduce the maximum cable length
which can be achieved. Therefore, what we would recommend is working to these guidelines.
With the FTX-L 3G and using a SMPTE 297-2006 compatible receiver (e.g. FRX 3G) over
single-mode
fibre, the typical distance which can be achieved is 21km for HD-SDI, and 30km
for SD-SDI.
Note - If you are using multi-mode fibre then the cable lengths are always lower, probably by
at least a factor of 10 or more.
FTX-L 3G User Manual R1.2
22
19 May 2017
