optical sniffer for Gbit/s fibre systems

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Last year, I wanted to have a look on the eye diagram of my digital optical link system. As these so-called opto-electrical converters do exist as commercial equipment, I tried to get one and was shocked by the price (about CHF 10000).. so I decided to build one myself. Actually, this is not so difficult... so I called it the 'optical sniffer'. It is not as sophisticated as commercial equipment, but the results are quite surprising.

The key component to the sniffer is a fast PIN diode. These come today included with a pre-amplifier in a single package for direct fibre coupling. In my case, I used a HFD3381-102 from Honeywell, which has excellent bandwidth of 2.5 GHz. In principle, it has a direct differential output capable of driving 50 Ohms for each leg, so this could well drive directly a little bit of cable and a scope input. To isolate the PIN diode from any load variations (cable mating/demating) and have a little bit more power, we just have to add a GHz bandwidth amplifier behind. That's easier as it sounds, as the communications industry has developed MMICs, which are internally matched gain blocks with 50 Ohm impedance. These devices, which most popular representatives are the ERA series from Mini-Circuits and their successors GALI (same company), have gains between 12 and 20 dB in a 4-pin package: 2 ground, one in and one out. The supply voltage is inserted with a bias resistor at the output, that's all. Bandwidth of those devices: 8-10 GHz, price: around USD 2, depending where you get them. Their high bandwidth is in principle good, as our sniffer will only be bandwidth limited by the PIN diode (which is 2.5 GHz). However, 8 GHz bandwidth means careful layout as 3 mm of trace can be too much for a ground trace. You will see late, that the circuit is rather cramped and I would try to disentangle it.. This ERAs are very robust against all sorts of mis-handling, so they provide an excellent isolation of the PIN diode to the outside world.

In addition, you need some 8 V regulator (7805) for the ERA, which is regulated a second time down to 3.3 V (LP2985-3.3 or similar) for the PIN diode. Soe resistors and caps.. finished.

Part List:

PIN diode HFD3381-102 any distributor for Honeywell Inc.
CHF 30
ERA-1 (-2 or -3 is also ok)
any distributor for Mini-Circuits
 personal recommendation for
 Germany: www.giga-tech.de
 United Kingdom: www.ghengineering.co.uk
your favourite component supplier..
any distributor for www.national.com

passive components

SMA or BNC plug
tin box

PCB 0.8 mm FR4
make it yourself..
pin-detector.zip : EAGLE 4.09 file
pin-det_gerber.zip : gerber data

In total, the material cost is less than CHF 50. But how does it perform? Judge yourself..

This is a comparison between the optical sniffer and the OE425 from LeCroy (CHF 10000). Connected to a 6 GHz scope from LeCroy (SDA6000). While both devices are connected to a VCSEL driven by the GOL 1.0, I did not record the very same bit pattern, but just have a look on the over-/undershoots. Can you tell, which is the homebrew one and which the commercial?  (the commercial is the upper one..)

Any more questions? Contact me!