In <7899e2ca-3755-46d8-b0de-dc7725c68108@s36g2000prg.googlegroups.com>,
My polygonal mirror rotates scan the beam at about 2000 passes/second.
Say i am trying to write 100,000 pixels per scan, this comes out to
100Mhz for the worst case of alternating black and white pixels. The
laser scanner will be moved over the plot at a rate of about 5 seconds/
inch. So not quite 200 prints per second, but much better resolution.
Just about all emulsion is sensitive to 405nm. For one thing, gelatin/
chromium emulsion certainly are.
Believe me, many, probably most photographic emulsions do very well at
405 nm. I don't consider that the higgest obstacle.
It is difficult to estimate the effect of reprocity, but as a rough
calculation, the film will be exposed to about 5 minutes of 5mw radiation
/ sq meter. Which is more than enough for any film i know of.
Also, square wave is not so important. From what i gather the laser
should be modulated between threshold and some high value. The scan
speed/filtering need to be adjusted to make this work for the film.
You could only need to pass a sine wave of 100 MHz with frequency
response at least somewhat flat from DC to somewhat over 100 MHz, and not
too much ringing at any frequencies at or near where gain starts varying
heavily inversely with frequency.
There are blu-ray burners, and 1x blu-ray is ~30MB a second, so say
240Mhz, probably higher to deal with reed-soloman, 8/10b encoding and
such.
I think the problem is not so much the RF as getting it to work from
RF all the way down to DC.
<<SNIP from here to edit for space>>
I see the big challenge making a broadband RF circuit that works from DC
to a little past 100 MHz. This touches heavily upon such things as stray
inductance, stray capacitance, and within that distributed inductance,
distributed capacitance, stripline PCB principles, knowledge of a
stripline transmission line, and principles of transmission lines such as
characteristic impedance and need for matching load impedance to
characteristic impedance to flatten frequency response, and need to match
source impedance to characteristic impedance to minimize resending any
reflections from the load if the load mismatches the transmission line.
This gets a bit easier if you understand characteristic impedance,
impedance matching to characteristic impedance, use of the Smith chart to
determine theoretically effects (and degree thereof) of for mismatches,
what the characteristic impedance of a coax cable and a twinlead cable are
as a function of dimensions and dielectric constant of insulating material
involved, "velocity factor" and relationship to dielectric constant of
insulating material exposed to electric field, and principles of
characteristic impedance and velocity factor of a stripline transmission
line.
Heck - video amplifiers, which only need to work from DC to about 4 or 5
MHz or so, get to be a slight bit of a challenge! If you want practice,
try making a video amplifier set and a pad-down to gain and pad back down
a signal set for an SVGA monitor. (No, I have not done that!) If you can
do this without horizontal blurring or ghosting, then you are probably
most of the way there to amplifying or buffering a signal of DC to 100
MHz. I do suggest this as practice!
One more note - if a transistor has Ft of 300 MHz, then it's "beta" or
Hfe at 100 MHz is close to 3!
If you use a transistor good for microwave frequencies, then your
circuit needs to be well-behaved at frequencies up to a goodly fraction of
the transistor's Ft. Watch for parasitic oscillations! Know
common-collector and common-base as well as common-emitter transistor
amplifier circuit theory! One classic oscillator is the Colpitts, which
uses a transistor in common-base mode! Know the treatments for parasitic
oscillations, and I suspect they are best-published for different
frequencies and different applicable impedances than those you will run
into! Know how to translate component values to different frequencies and
different "working impedances", and hope that you are treating the same
oscillation mode that the treatment is for, hope for lack of 2nd and 3rd
or whatever order complications in your application, etc!
Also, you might be exploring into territory that has not all of its
charts well-published, which means you could be a prime victim for
Murphy's law finding a way to effectively supersede Ohm's law!
Oh, one more thing - if you have a bipolar transistor in a Class A
amplifier (better know what that is), if the transistor is a fast one then
design for lowish supply voltages preferably 30 volts or less, with the
transistor having average collector-emitter voltage while conducting
around or under 20 volts. For that matter, in Classes B and AB (better
know what those are), if the transistor is conducting close to half the
time its average C-E voltage could easily need to be under 25 volts or so
if the transistor is a fast one.
One key here: "SOA" or "Safe Operating Area", and that is a graph of
allowable combinations of C-E voltage and collector current. It is common
for bipolar transistors to have allowable power dissipation decreased when
C-E voltage is more than somewhere around 30 volts, and this gets worse
for faster ones. The principle here is "forward bias second breakdown",
where a hotspot develops in a localized region within the transistor
"chip" / die.
I have made my living doing stuff other than this for the past 23 years,
and I have this little bit of knowledge of this that could be "dangerous"
here... I suspect a possible hazard is need to kick in a few dozen hours
of extra work time on weekly salary or per-diem time or unbillable hourly
as "continuing education", and that gets to be hell when deadlines make
you spend an extra 15 or 50 hours fixing something in a matter of days to
a week!
Best Regards,
- Don Klipstein (
[email protected])