RF to Fiber and back to RF??? - RF Cafe Forums

 Post subject: RF to Fiber and back to RF???

Posted: Wed Oct 10, 2007 4:23 pm 

I have a unit that converts RF and places it on a fiber line. How does this happen? What takes place with the conversion? Why does this method of data transfer down a fiber line use so much bandwidth? How in the world does an increase or decrease in RF amplitude on the Fiber Transmit end show up on the other end where the RF is received by the Fiber receiver and then re-transmitted. I have played with the system and I know it like the back of my hand. I do not know any of the inner workings. I would greatly appreciate any and all help with my questions.

 Post subject:

Posted: Tue Oct 16, 2007 4:16 am 

Hi kylenm
It sounds like you have a wide-band modem, an electrical interface to a Light Emitting Diode. Some have bandwidths up to 2.5GHz using solid-state LASER diodes, where the digital signal is a complex set of subcarriers with different phase relationships. The simpler types are purely analog, and by using low-capacitance LEDs they ensure a wide bandwidth up to say 100MHz. The light intensity received at a photo-diode is more or less proportional to the analog signal fed to the LED at the transmitter.
It is not a question of using so much bandwidth, as having the bandwidth available, the limiting factor being not the fibre but thebandwidth of the electrical to light converter, the LED. Intrinsic parasitic capacitance and inductance limit thebandwidth at that point. The bandwidth of a monomode fiber is at least 10GHz, but the bandwidth of a purely analog LED is far less as noted above. I have seen another interesting application of these devices where it was required to isolate a variable frequency HF oscillator from frequency drift caused by load impedance "pulling". The oscillator fed an analog opto-isolator (integrated LED and photo-diode). Due to the coupling being optical, the isolation was > 100dB.