Re: matching impedance

From: Fred Bloggs (nospam_at_nospam.com)
Date: 02/19/05 

Date: Sat, 19 Feb 2005 17:58:23 GMT

Winfield Hill wrote:
> James 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 data*** 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

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