Cannot get TTL modulation to work on flexmod

[polishedball]

If I connect the mod input to a 5v line wouldn't that disable modulation?

Yes, simulate a TTL high though, but what are you trying to prove? Are you confirming drlava's spec sheet or what? Set bias set gain hook up diode, modulate with DAC and watch it work.

[catalanjo]

Well the oscilloscope is measuring output voltage, which is only a few millivolts. Exactly what would be the purpose of adding a transistor? Can you clarify?

V= IR is basically what it's all about, but you have to look at it back to front when talking about impedance.

I = current in Amps, V= potential in Volts, R = resistance in Ohms (or if "I" & "V" are varying "R" can mean impedance also measured in Oh! umm's)

Impedance is similar to resistance except that that it applies to resistance to CHANGES in "V" and "I" rather than the constant versions. A coil can have almost no resistance since it's made out out copper wire but a very high impedance since it resists high frequency changes, (due to it's topology causing interacting electrical and magnetic fields).

Signal generator produces a 0-5V signal when connected to nothing (which is VERY high impedance or to put it another way "R" is big). To make the formula work, (and believe me, it does), if you connect one to a low impedance then "R" would be small. If "R" is small then "I" must be big (low impedance output) to compensate or "V" will be reduced, (= almost no signal), and get dragged down to ground.

If you connect a transistor (especially if it is a FET), the gate of the transistor needs very little "I" (whereas the driver may need a little bit more). The transistor therefore works like a water valve allowing a lot more "I" to flow straight from the PSU to the laser driver without requiring the signal generator to supply it. The signal generator in effect just tells the transistor what to do and all the effort (that is the "I") is done by the transistor.

The flexmod is a FAIRLY high impedance input, but some signal generators are often downright MEAN, and supply very very little current "I", hence the suggested buffering,(which just means more ummff. or "I")



HOWEVER ............having just read your post about how you are looking at the current (with a multimeter), I have realized that your laser driver probably IS modulating without the need for buffering.

Your meter would not be able to show you a 1k modulation since it has no method of showing you a current which changes size a thousand times a second.

To measure this changing current:
If you are just using diodes as the load you will NOT see a change in voltage since a diode is like a waterfall at a fixed height or "V". (How ever much water flows over it, the height doesn't change).
You must put a (very low value) resistance in series with the diodes and measure the voltage change across this resistor using your Oscope.
Again since V= IR and "R" (low value) is fixed, the "V" (low value) will vary with "I" . Your Oscope ..CAN ..show you these changes, occurring both at 1K and also at much higher frequencies. If you have a laser diode and you stick a mirror on the front of a fan and bounce the laser off it, then you will see the modulation clearly as soon as you get the frequency set to a harmonic of the fan rotation.

Hope this helps! It looks a bit long winded but shit we are talking about invisible mysterious forces here and even Harry Potter had to go to school to learn magic !

Unfortunately trying to explain electronics sometimes sounds patronizing which has NOT been my intention.

Nuff said , all that should make it as clear as mud !

Cheers

[DLProcessor]

Yes, simulate a TTL high though, but what are you trying to prove? Are you confirming drlava's spec sheet or what? Set bias set gain hook up diode, modulate with DAC and watch it work.

Well, I do not have a DAC. (Im not clear on how would it would work on a DAC). How should I set bias and gain currents? Do I set the bias current below the lasing threshold and the gain current at my desired operating current? Sorry, Im trying to understand to the best of my ability.

Thanks catalanjo for your explanation I am familiar with ohm's law but I do not have much experience in circuit electronics. Do you recommend any particular FET or will any one do?

[polishedball]

Well, I do not have a DAC. (Im not clear on how would it would work on a DAC). How should I set bias and gain currents? Do I set the bias current below the lasing threshold and the gain current at my desired operating current? Sorry, Im trying to understand to the best of my ability.

not sure if you have this, but it explains setup pretty well.
http://hacylon.case.edu/ebay/laser_d...2%20Manual.pdf

[DLProcessor]

not sure if you have this, but it explains setup pretty well.
http://hacylon.case.edu/ebay/laser_d...2%20Manual.pdf

I do have that and I followed the instructions to the best of my understanding but I could not get any modulation. I would be nice if someone can do some type of video tutorial. I am a good visual learner and it would help me tremendously.

[polishedball]

I do have that and I followed the instructions to the best of my understanding but I could not get any modulation. I would be nice if someone can do some type of video tutorial. I am a good visual learner and it would help me tremendously.

I wont have time till the weekend but I can shot a basic setup of it then if you still need it. So do you get bias set to just a dim glow and 5v input on the mod line to full rated current of the diode getting full power? You are now just trying to see modulation on / off/ ramping now?

Or are you now there yet?

[DLProcessor]

I wont have time till the weekend but I can shot a basic setup of it then if you still need it. So do you get bias set to just a dim glow and 5v input on the mod line to full rated current of the diode getting full power? You are now just trying to see modulation on / off/ ramping now?

Or are you now there yet?

I will be in my school's electronics lab tomorrow. I will try a couple of things and report back.

[polishedball]

I will be in my school's electronics lab tomorrow. I will try a couple of things and report back.

Also fill out your profile, if we know where you are there might be someone close that can help ya out.

[drlava]

If I connect the mod input to a 5v line wouldn't that disable modulation? The oscilloscope that I use only measures voltage. I measured the output using a multimeter and it reads constant current.

Also what is the difference b/w the 0bias and gain pot? Do they have to be set equal to each other? Im kinda new to this. I have to modulate lasers as part of a school project and was recommended to use this driver.

When you connect a 5V power supply to the driver power AND modulation inputs at the same time, it will output a constant current of a level set by the sum of 0-bias (standby current) and gain (full-modulation current) settings. The standby current setting you must set with 0V on the modulation input. This should be set below the lasing threshold of the diode.

[catalanjo]

Well, I do not have a DAC. (Im not clear on how would it would work on a DAC). How should I set bias and gain currents? Do I set the bias current below the lasing threshold and the gain current at my desired operating current? Sorry, Im trying to understand to the best of my ability.
Thanks catalanjo for your explanation. I am familiar with ohm's law but I do not have much experience in circuit electronics. Do you recommend any particular FET or will any one do?

...........................................Oops sorry !

Originally Posted by DLProcessor
Using my function generator I set a 0-5v square wave signal

Now that you mentioned that, it could be possible. My output is very small (several mV's). My frequency is at 1khz.
Well the oscilloscope is measuring output voltage, which is only a few millivolts.
"(i used a 5 regular diodes in series as a test load). The output is just a regular DC signal."
I measured the output using a multimeter and it reads constant current.


But I still don't understand why you are trying to see a 1K modulation on a current meter and why are you measuring a diode voltage drop with a Oscope? Are you interested in the temperature of the diodes or something ?

Assuming the "only a few millivolts" reading is your signal generator AFTER connecting to the driver then your signal generator is VERY STINGY INDEED (very high impedance).

As far as the FET is concerned it doesn't really y matter much which one you use. I doubt the docs input impedance is low enough or insensitive enough to require much current and a 5v PSU is not exactly high voltage, also IK is a walk in the park for most transistors let alone a FET. (A normal LF darlington type 33BXD or similar with 5K in series with input and a pull up resitor of about 5k aswell will also probably work OK.) Cuation with an LD connected it would relax to ON.

The DAC which was mentioned is just to be able to have a better, buffered function generator controlled via PC. This would enable you to DRAW the wave form you use to modulate, and give you any shape at any frequency between DC and about 5Khz.

If you are experimenting with a FET it might be nice to be able to attach/detach small capacitors and resistors across the input side after a 10k resistor, that would enable you to see the effects this has on the wave forms you see on your Oscope once it is connected correctly.
This is also a great way to SEE the effects of audio preamp tone controls at different frequencies and different wave forms and explore harmonic structures and Fourier transforms in a totally visual way.

Once you get a laser diode you can use the motorized fan mirror INSTEAD of the Oscope so a whole bunch of people can see it at the same time. Then if you use scanners instead of the fan motor things really start to take off.
I never did understand why schools haven't jumped on lasers as the perfect multicolour Oscope system for audio frequencies to show kids how "modulation", "resonance", "interference" and "Fourier analysis" works.
They are all tricky concepts which become dead simple once you can actually see them.

" (i used a 5 regular diodes in series as a test load)"

I doubt you will see any "glow" from normal diodes and if you do get your hands on a laser diode please do NOT look down it to see the glow.... always watch its projection on a surface other than your eyeball.

Cheers