First you must measure the laser's impedance for the AC signal over the
I search discussion but I don't find anything, can you help me? What is it
.While your answer is correct, Jim, it may over-complicate the problem.
But as to whether he can make good transmissionhttp://www.minicircuits.com/dg03-222.pdfJames Meyer wrote...
Pazzeo <matteo.silvestrin@virgilio.it> wroth:
Hi all, i have this problem: in my circuit i must insert a laser.
The problem is that i do matching impedance at 75 ohm. This matching
is on a wide band: 80- 900 Mhz. How do i realize this?
First you must measure the laser's impedance for the AC signal over the
range of frequencies of interest. Make sure that the DC operating point,
if any, is set to the final value during the measurements.
Hopefully, the AC signal will be a small percentage of the DC bias.
If it isn't, then the load will be non-linear and matching will be
"problematical" at best.
Once you have the measurements, convert them into an equivalent lumped
circuit configuration of L, R, and C elements. At that point, if the
matching circuit isn't obvious to you, post the circuit here and
someone will do the math for you.
While your answer is correct, Jim, it may over-complicate the problem.
Most lasers operate at currents of at least 40mA, and as a result have
very low intrinsic dynamic impedances. Taking this as an assumption,
I matched my wideband 50-ohm RF signal to a Hitachi laser diode with
a small 47-ohm SMD series resistor, and added a Picosecond Pulse Labs
bias-T to inject the dc current. My network analyzer and TDR tests
showed a reasonable 50-ohm termination was obtained. Overall light
throughput response was flat, dropping by 1dB at 1200MHz. It should
be noted I took special care to maintain effective double-terminated
50-ohm transmission lines for both the laser and PIN-diode receiver.
It's possible the latter was responsible for the 1.2GHz rolloff. I
noted with amusement that Hitachi's datasheet curves showed a peak
at 600MHz, and dropped off rapidly thereafter. I surmised this was
the response of their optical detector, or due to bad laser wiring.
Hint 1: All my open-wiring distances were kept to under 1mm.
Hint 2: Learn about making high-performance bias-T networks. Pazzeo
can read the recent discussion here, and photo postings on a.b.s.e.
Ahem. Pazzeo can "do the math" by replacing my 50-ohm resistor with
a 75-ohm part. :>
lines and evaluate them, that may be another matter.