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matching impedance

P

Pazzeo

Jan 1, 1970
0
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?

Thanks
Pazzeo
 
J

James Meyer

Jan 1, 1970
0
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?

Thanks
Pazzeo

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.

Jim
 
W

Winfield Hill

Jan 1, 1970
0
James Meyer wrote...
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. :>) But as to whether he can make good transmission
lines and evaluate them, that may be another matter.
 
F

Fred Bloggs

Jan 1, 1970
0
Winfield said:
James Meyer wrote...



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. :>) But as to whether he can make good transmission
lines and evaluate them, that may be another matter.

http://www.minicircuits.com/dg03-222.pdf
 
P

Pazzeo

Jan 1, 1970
0
Hi all, I thank all for your help. But I don't resolve my problem. I would
send my circuit but it 's very big file.
My email is true, if you want you can contact me so I can send you by email
my circuit.

Thank for your help
Pazzeo
 
P

Pazzeo

Jan 1, 1970
0
for Winfield Hill
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.

I search discussion but I don't find anything, can you help me? What is it
a.b.s.e.?
If you can, you contact me so I send you my circuit, my email is true.
I thank you very very much
Bye
Pazzeo
 
R

Rich Grise

Jan 1, 1970
0
for Winfield Hill

I search discussion but I don't find anything, can you help me? What is it
a.b.s.e.?
If you can, you contact me so I send you my circuit, my email is true.
I thank you very very much
Bye
Pazzeo

a.b.s.e is an abbreviation for .
It's another newsgroup where binaries (for example, schematics) can be
posted. You can attach your schematic to your post, and it's OK, and
everyone (everyone who gets that group on their server) can see it.

Also, you can search newsgroups at http://groups.google.co.uk or
http://groups.google.com.au - don't use the American beta version,
because it's an awful interface.

Good Luck!
Rich
 
J

JeffM

Jan 1, 1970
0
...schematics...can be posted
you can search newsgroups at http://groups.google.co.uk or
http://groups.google.com.au
--don't use the American beta version, because it's an awful interface.
Rich Grise

Using the beta has always been a matter of learning the tricks.
Clicking *show options*, *Show original* gives a fixed-font display.
The *Reply* link under *show options* allows a clean display
if you want to post from Google and blockquote ASCII graphics.

They have patched the page display as well.
For ~3 weeks now there has been a Fixed font - Proportional font
selection
at the top of the thread page.
 
P

Pazzeo

Jan 1, 1970
0
If you want i have posted in a.b.s.e. my schematic circuit.
Please help me I'm very disperated :(.

Pazzeo
 
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