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Old 06-02-2017, 05:28 PM #1
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Default What is an RF Oscillator in a driver IC for? How fast can a diode switch on/off

This data sheet mentions an RF Oscillator https://media.digikey.com/pdf/Data%2...85_Summary.pdf

It says "An on-chip, low-EMI RF oscillator is available to reduce laser mode hopping noise". What does this mean?

I have a phr-803t optical head from an xbox DVD player and want to switch one of the lasers on and off at a chosen frequency. The assembly has a built in driver (datasheet above) and two switches for turning on each of the three lasers. The IR laser is on when switch 1 is low and switch 2 is high. I was thinking I could leave switch 1 low and use a pwm signal from an arduino on switch 2 to make it jump between high and low which would turn the infrared laser on and off. This seems to work up to about 2kHz, and I would like to go Can the oscillator in the driver IC be used to do this also? I'm using a photodiode to see how if the beam attenuates at higher frequencies. at about 3kHz no light hits the photodiode, and I'm not sure if this is because the photodiode can't keep up or if the diode can't switch on/off that fast. Thanks for any insight you guys may have, I'm new at this!


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Old 06-02-2017, 05:53 PM #2
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Default Re: What is an RF Oscillator in a driver IC for? How fast can a diode switch on/off

Quote:
Originally Posted by mkoll4 View Post
This data sheet mentions an RF Oscillator https://media.digikey.com/pdf/Data%2...85_Summary.pdf

It says "An on-chip, low-EMI RF oscillator is available to reduce laser mode hopping noise". What does this mean?

I have a phr-803t optical head from an xbox DVD player and want to switch one of the lasers on and off at a chosen frequency. The assembly has a built in driver (datasheet above) and two switches for turning on each of the three lasers. The IR laser is on when switch 1 is low and switch 2 is high. I was thinking I could leave switch 1 low and use a pwm signal from an arduino on switch 2 to make it jump between high and low which would turn the infrared laser on and off. This seems to work up to about 2kHz, and I would like to go Can the oscillator in the driver IC be used to do this also? I'm using a photodiode to see how if the beam attenuates at higher frequencies. at about 3kHz no light hits the photodiode, and I'm not sure if this is because the photodiode can't keep up or if the diode can't switch on/off that fast. Thanks for any insight you guys may have, I'm new at this!

It means it uses RF modulation of the pump current to reduce mode noise. Basically it removes noise in the output beam, resulting in a lower noise signal.

What are you trying to achieve? What modulation frequency are you looking to achieve?

The diode should have no problem modulating at that frequency, they should be capable of significantly higher frequencies than that.

Are you providing that modulation signal from one of the Arduino's output pins? IIRC the switching speed on those tops out at around 2.5Khz.

Can you show us your test setup? Schematics? Pictures?
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Last edited by diachi; 06-02-2017 at 05:54 PM.
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Old 06-02-2017, 07:39 PM #3
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Default Re: What is an RF Oscillator in a driver IC for? How fast can a diode switch on/off

Quote:
What are you trying to achieve? What modulation frequency are you looking to achieve?
I am trying to see if the diode can modulate up to 100kHz. Using the ATmega PWM pins directly (https://www.arduino.cc/en/Tutorial/SecretsOfArduinoPWM) I know the arduino can output a 50% duty cycle square wave at 100kHz, I have checked the output on a o-scope as well.

Here is an image of the set up, and a link to the board schematic I'm using to control the PHR-803t

Arduino set up https://imgur.com/a/3shxo

schematic (http://www.diyouware.com/sites/defau...iverV3.3_0.JPG)

It might be that the photodiode I am using can't pick it up quick enough
in these images the blue is the square wave switching the laser on and off, the orange is what the photodiode picks up. as the frequency increses the photodiode doesn't output a signal of comparable amplitude

200 Hz - https://imgur.com/a/PTPhM
2.7 kHz - https://imgur.com/a/Nw84U

thanks again for the help!
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Old 06-02-2017, 07:57 PM #4
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Default Re: What is an RF Oscillator in a driver IC for? How fast can a diode switch on/off

Quote:
Originally Posted by mkoll4 View Post
I am trying to see if the diode can modulate up to 100kHz. Using the ATmega PWM pins directly (https://www.arduino.cc/en/Tutorial/SecretsOfArduinoPWM) I know the arduino can output a 50% duty cycle square wave at 100kHz, I have checked the output on a o-scope as well.

Here is an image of the set up, and a link to the board schematic I'm using to control the PHR-803t

Arduino set up https://imgur.com/a/3shxo

schematic (http://www.diyouware.com/sites/defau...iverV3.3_0.JPG)

It might be that the photodiode I am using can't pick it up quick enough
in these images the blue is the square wave switching the laser on and off, the orange is what the photodiode picks up. as the frequency increses the photodiode doesn't output a signal of comparable amplitude

200 Hz - https://imgur.com/a/PTPhM
2.7 kHz - https://imgur.com/a/Nw84U

thanks again for the help!
Why do you want to modulate it at 100Khz though, just curiosity or do you have some project that requires this?

Input signal certainly looks fine.

What does the modulation look like at the output of the diode driver? Your issue may be there... Good to rule that out.

Which photodiode are you using? Datasheet should tell you if it's fast enough or not.
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Last edited by diachi; 06-02-2017 at 07:59 PM.
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Old 06-02-2017, 09:40 PM #5
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Default Re: What is an RF Oscillator in a driver IC for? How fast can a diode switch on/off

Quote:
Why do you want to modulate it at 100Khz though, just curiosity or do you have some project that requires this?
It is for small scale periodic heating for a research project.

Quote:
What does the modulation look like at the output of the diode driver? Your issue may be there... Good to rule that out.
Good idea on checking the output after the driver, I will do that.

Quote:
Which photodiode are you using? Datasheet should tell you if it's fast enough or not.
I checked the photodiode rise time and it is 1ns, so it should be able to keep up.

I will also look at using the pwm signal on the laser power instead of the select pin that chooses which laser is on.
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Old 06-02-2017, 11:05 PM #6
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Default Re: What is an RF Oscillator in a driver IC for? How fast can a diode switch on/off

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It is for small scale periodic heating for a research project.
How have you determined a laser is the best heating device for the project?

What is your reasoning for wanting 100kHz over your proven 200Hz?
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Old 06-03-2017, 12:43 AM #7
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Default Re: What is an RF Oscillator in a driver IC for? How fast can a diode switch on/off

I reckon this chip is mostly intended for -reading- data from a disc.

In any case, the oscillator frequency is defined as 200 to 500 MHz, probably fairly typical of dvd or blu-ray data rate drives operated at normal speed.

If you want to modulate a high power diode at 100 kHz or so this would be little problem with available driver circuits. It's a bit faster than most scanning applications really require, but not much. I wouldn't be surprised if the laser drivers used for say a 30.000 pps scanning system could handle 100 kHz right out of the box - the problem is getting the mirrors moving that fast and accurately, not modulating the diode.
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