Cannot get TTL modulation to work on flexmod

[polishedball]

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

Cheers

opps missed that he was just using a test load.

[DLProcessor]

Ok, so I shorted the mod inputs and set the bias current to 60mA. Then I applied 5v to the mod+ input and set the gain current to 115mA. I hooked up a phr-803t diode in series with a 1-ohm resistor and measure the voltage across the resistor to be 80mV (so that means 80mA). The laser was on but not blinking so that means no modulation, right?

So if I want to use my funtion generator for modulation, i would have to use the FET so I could get the proper signal on my oscilloscope. Is that correct? Any schematics of such a setup would be helpful.

[catalanjo]

The laser was on but not blinking so that means no modulation, right?

NO.... 1khz is NOT blinking !

10HZ is VERY fast blinking .... 1kHZ well thats almost twenty times faster than the "blinking" of your average fluorescent light. A tiny sensation of flicker at the very best.

You would never be able to see blinking at this frequency....much too fast. (unless you bounce the beam off a rotating mirror.) Then you would see the projected circumference as discontinuous once you balance the frequency a bit.

I am not familiar with the phr-803t diode but it sounds vaguely like it might be only infrared (@803nm perhaps) if this is the case you wouldn't SEE anything and without goggles I wouldn't strenuously suggest you don't try to look at it.

It does however seem that you now have the setup right.

The 80mv you were measuring with the O scope I hope! ............and maybe the blinking you are referring to is the lack of activity in the Oscope trace ?

If this is the case please confirm, because this means you are getting somewhere.

Also make sure that you can see the signal generator output with the scope when it is disconnected from the laser drive. This will confirm that the Oscope is set up correctly and the generator is working, although the sensitivity will obviously need adjusting when you go back to measuring the series resistor.

If you can do this successfully then YES it would be time to connect the FET.

Do you know how to distinguish the base from the emitter and the collector and the difference between a PNP and an NPN transistor ?

I would suggest an NPN connected between the 5v+ and the mod+ input of the driver.

The function generator would then connect to the "gate" of the transistor.

If the transistor is wired back to front .... no sweat it just won't work so you turn it around and try again (check with the scope to make sure you can still see the signal on the gate with the FET connected.)

Then check (again with the scope) that the signal is getting to the mod+ input.
You should see the same size signal on both of these tests.

If this is OK check the series resistor of the LD and turn up the sensitivity of the scope so 80mv is almost full range. If you cant see the signal now the the LD or the diode driver is KAPUT but hopefully this will not happen since as far as I can make out you haven't done anything wrong yet.

As far as a circuit diagram is concerned, once you have it working you can draw it yourself.

It is much better to understand what is happening first and express it later!

Otherwise you will end up like a lot of music students who play perfectly well from a written score but find themselves incapable of making their own music!

If you have problems just come back here and tell us and we'll sort it.

Good luck.... you deserve it, for trying so hard.

If all this is now working OK ........ get a visible diode out of an old DVD player and have some fun with a rotating mirror!

But tread carefully, this stuff can get very addictive once you start getting into it.

Cheers

[DLProcessor]

The phr diode is a blu-ray diode. I extracted it from a drive sled. So yeah i can see a blue-ish light when it is turned on.

I measured the 80mV from a standard DMM. I did not use the oscilloscope. The blinking I am talking about is a visual on/off blink. But since you say 1khz is so fast, I wouldn't actually see it. BTW, is the driver running at 1khz by default? I have no type of signal generator connected to the mod input. Only the 5v. Am I on the right track so far?
I know the NPN/PNP connections of a transistor so I think I can wire it up no problem when I am up to that part.

My school lab is closed now so I will have to continue tomorrow. Your help is appreciated catalanjo, thanks

[catalanjo]

The phr diode is a blu-ray diode. I extracted it from a drive sled. So yeah i can see a blue-ish light when it is turned on.

That's great news!

I measured the 80mV from a standard DMM. I did not use the oscilloscope.

Right so you have just confirmed diode current and that driver is working.

The blinking I am talking about is a visual on/off blink. But since you say 1khz is so fast, I wouldn't actually see it.

Correct, unless you connect the O scope to the series resistor. ( Which you can do Right? )

BTW, is the driver running at 1khz by default? I have no type of signal generator connected to the mod input. Only the 5v. Am I on the right track so far?

No, (that was just a communications screw up), the driver is running CW with the 5vdc. but you are definitely now on the right track.

I know the NPN/PNP connections of a transistor so I think I can wire it up no problem when I am up to that part.

Great, but maybe you won't need the transistor since we are missing a couple of tests first, to see whether it's necessary or not.

My school lab is closed now so I will have to continue tomorrow.

No problem, all we have to do is to sort out what you have to do next and keep the communications simple so I don't go leaping to wrong conclusions about what you are telling me.

Have you tried connecting the function generator to the mod input of the LD driver since you got all the wiring set up correctly ?

If answer is NO, then this is exactly what you must try next after disconnecting the 5v+ from the mod input. (No transistor yet.)

Only if this does NOT work do you bother with the transistor.

1) First connect the O-scope to the function generator and sync it up so you can see the signal clearly. Disconnect it and increase the sensitivity so 80mv is almost full scale, then connect it to the series resistor to see if it all works since the frequency will be too high to see with the naked eye.
Connect the function generator to the mod input of the driver and switch everything on.

If you can possibly get hold of a PC fan and stick a mirror in the middle of it (slightly angled) and point the laser at it then you could use this during the first test since it will give us valuable feedback about what's happening independently to the scope results.

If you can't get it together in time don't worry, the scope should work OK anyway, and you can always muck about with the fan and mirror later on just for kicks.

2) If this doesn't work you need to do the tests I already mentioned below.

First reduce the sensitivity of the scope again and make sure that you can see the signal generator output with the scope when it is disconnected from the laser drive. This will confirm that the O-scope is set up correctly again and that the generator is still working OK, although the sensitivity will obviously need re-adjusting when you go back to measuring the series resistor.

Then connect the function generator back to the LD driver and check the signal on the mod input with the scope again (this time with everything connected and switched on).
You will only need the transistor if your signal disappears at this stage.

If this is what happens we can discuss it further tomorrow.

Your help is appreciated catalanjo, thanks

My pleasure, I am sure you will end up helping others, once it's you who has the experience, and someone a lot younger needs a hand !

I got quite a bit of help yonks ago when I was in your situation.

Good luck tomorrow !

Cheers

[DLProcessor]

Sorry for delay but I do I have good news!

So I checked the 0-5v square wave signal on my scope, all is well. The bias is set to 65mA and gain is set to 100mA on the driver. I hooked up 4 regular diodes in series along with an 1-ohm resistor. Applied 5 volts to mod+ and got 90mV across the resistor with my DMM (which is really 90mA of current). Fine, all is well.

Now I hooked up the square wave signal to the mod inputs and put a probe at the series resistor. I turned it on and...success! I got a square wave at the output end with a voltage of 96mV (peak to peak)

Now I tried with my phr laser diode. This time the oscilloscope read a voltage of 320mV across the resistor. The peaks of the square wave were shorter than the dips. My DMM however read a voltage of 80mV across the resistor. Any clue to this discrepancy?

I hope I got it right this time. I have absolutely no clue as to why I got it working this time even though I pretty much did the same thing except I used a test resistor.

[-bart-]

Your DMM is clearly not capable of measuring these waveforms.

Most DMM's only measure DC and AC (sine 50/60Hz) well.

Beyond that use a scope

[catalanjo]

Sorry for delay but I do I have good news!
So I checked the 0-5v square wave signal on my scope, all is well.

My DMM however read a voltage of 80mV across the resistor. Any clue to this discrepancy?

Hi, DLP good to know that it is now all working!
Remember we are talking about a 1khz = 1000hz frequency. (see earlier notes)

As Bart correctly points out your DMM won't be able to see this frequency either.
[/B]
I hope I got it right this time. I have absolutely no clue as to why I got it working this time even though I pretty much did the same thing except I used a test resistor.

So you DO know why ......................"except I used a test resistor." ................
it's a name problem again!

This is a "series resistor" NOT a "test resistor". It does NOT affect the workings of the circuit itself.......... It's function is precisely to be able to SEE the wave forms using an O-scope, (which ONLY sees voltages), instead of a DMM (which CAN see current but only at 50-60hz NOT at 1khz frequency. The same way you CAN see visible light but NOT infrared,UV,radio xrays etc.).

As I suspected earlier your setup worked from the beginning but you couldn't SEE that it was working!
"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. " (see posts below).

I suggest you re-read the earlier posts and all will become a bit clearer to you.

I also re-suggest If you can possibly get hold of a PC fan and stick a mirror in the middle of it (slightly angled) and point the laser at it then you could ......... (for)...valuable feedback about what's happening independently to the scope results.

Also: As far as a circuit diagram is concerned, once you have it working you can draw it yourself.
Congratulations on your success.

Cheers

[DLProcessor]

Yes, I meant series resistor!

I will try to find a fan and see if I can do that.

So is my scope correct for reporting 320mA of current running to the laser diode? If so, that is way too much and it would kill the diode instantly (according to laser diode specs). However my diode was running fine. Something is not right...

[catalanjo]

Yes, I meant series resistor!

I will try to find a fan and see if I can do that.

So is my scope correct for reporting 320mA of current running to the laser diode? If so, that is way too much and it would kill the diode instantly (according to laser diode specs). However my diode was running fine. Something is not right...

Hi DLP,

I am afraid I don't know the specs for your diode but if you have a one ohm series resistor and the scope is reading 320mv peak across it then you have 320mw peak current (always assuming you have the scope set up right.)

First forget the DMM since it can't understand this type of high frequency. with a bit of luck it "MIGHT" measure the average current, but would never be reliable since you would get aliasing with it's sampling frequency.

Now a few questions just to get things clear!

Is the scope (or drive output) isolated from the "ground" ?

Are you measuring just across the resistor (not LD and resistor)?

Do you see a 320mv DIFFERENCE between peaks and troughs when modulating?

Have you set it up to measure DC (not AC, which might well have a capacitor involved)?

Does the scope still register only 80mv when you put 5vdc on the modulation input instead of connecting the wave form from the signal generator?

Does the scope report correctly the zero to 5v when connected just to the signal generator?

If answers to the above are all YES, then you have a genuine mystery here!
I which case you are totally correct................. "Something is not right...[/QUOTE]"


BTW what VOLTAGE does this LD require?

Good luck with the mirror and fan !

Cheers