[Laser] TX Circuit for high speed data.

[Chuck Hast]

On 7/2/07, James Whitfield <n5gui at cox.net> wrote:
> From: "Paul B. Webster VK2BZC"
> >
> >   Now, note John's circuit for modulating multiple diodes using 1N4007s!
> > This demonstrates that in this shunt regulation circuit, the voltage
> > drop of such a diode is no problem.................
>
> It is true that the voltage drop is not a problem.  The shunt circuit, which
> looks like a regulator but is not used as such for this application, need
> only "steal" enough current from the laser diode to quench the laser action.
> It may still have sufficient current flowing through the laser diode for it
> to continue to emit light, just not with the intensity of the laser.  If you
> are testing the circuit with an LED, its linearity will continue to emit
> significant light untill the LED trigger voltage is lost.  As such an LED
> test may show more degradation ( rounding ) of the signal than the laser
> diode will.
>
> >................and since that diode *also* prevents
> > the adverse effect of a gate-to-drain breakdown of the FET, it renders
> > the series resistors quite unnecessary (except for a very brief current
> > "spike" due to the discharge time of the slow 1N4007 at the moment of
> > the breakdown - perhaps a single 10 ohm resistor might still be in
> > order).
> >
> I do not see how the diode would prevent gate to drain breakdown.
>
> >
> > 74HC07 gates incidentally do not need balancing in parallel - they are
> > matched - on the same die - and self-balance due to their own source
> > resistance.
> >
> I looked at the data sheet which shows the "equivalent circuit".  The output
> circuit shown has the output pin connected to the drain and the source
> grounded.  Neither line has shows resistance to limit current in the "ON"
> state, but of course the "real" devices will have some small resistance in
> in the trace conductors and measureable voltage drop across each output FET
> which will vary with load current.  I made the suggestion to add balancing
> resistors based on my own, limited and ancient, experience with paralleling
> logic devices for power to drive atypical devices.   I presume that Paul has
> more experience than I with modern logic devices.  That seems to allow him
> to assert that all six buffers, being on the same die, are matched and will
> track each other sufficiently to be used in this circuit effectively "wire
> OR'ed".  To do so will simplify the modification to the circuit and I see no
> risk to any of the other components.
>
I looked around and found several circuits using buffers or inverters to drive
laser diodes, turns out all of them paralleled the buffer inputs in order to get
the current sink capability to where it was needed. I have got enough 'brain
pickin's" here to go and do some damage or come up with some good num-
bers.

I know that this list has a lot more NLOS people on it than speed/power
people (NOTE THIS IS NOT A COMPLAINT, the discussion of counting
photons no matter how many is always very INTERESTING and the info
has uses on both sides of the house) but is it good to see some work and
info on high speed links. I for one want to get these links running soon so
I can gather some real data on them. The more I look at what is on the
net the more I realize that most of what has been published has been short
haul or space craft to space craft. And a lot of what maybe interesting
demands that you dig up a credit card and pay someone big bux to down-
load a PDF of how they did it, it is more fun to try to do it your self instead
of paying the big bux to some collective who is selling white papers which
were probably paid for in part of whole by our taxes in some university some
where....

So all of you thank you so far for your golden nuggets, optical links of any
sort are fascinating at least to me. After I get these high speed ones going
I do want to test some of the NLOS stuff. We just do not have high places
except building tops and towers here in FL for testing from, but there is
always a cloud in the sky somewhere, except of course when we get a
cold front through and that only last a few days....



-- 
Chuck Hast  -- KP4DJT --
To paraphrase my flight instructor;
"the only dumb question is the one you DID NOT ask resulting in my going
out and having to identify your bits and pieces in the midst of torn
and twisted metal."