LED modulation at high frequency and VCO usage

Hi, I got 2 questions:

We are interested in optical imaging of human tissue (specifically brain) by finding out the amounts of some molecules like hemoglobin. For this, we need to modulate infrared LEDs with a relatively high frequency like 120 MHz. How can we achieve that? What kind of LED driving circuits would be necessary? Would a single-transistor driver circuit do the job for us? Also, can we obtain such modulation on a prototype board initially for testing or do we have to make a PCB layout with RF considerations?

We plan to use a voltage controlled oscillator (VCO) to obtain the desired frequency. First of all, how can we obtain such a precise frequency with a VCO? We will also need a second modulated signal, e.g. at 120.0004 MHz for heterodyning, so that we will obtain a signal with a 400 Hz cross-correlation frequency (actually it does not have to be that precise, 4 KHz or 40 KHz might also be sufficient). How can we arrange voltage so precisely to get such frequencies from the VCO? Are there any VCO modules that you can recommend? Also, do we simply apply DC voltage to the VCO's to get a specific frequency or are there any other driving circuits for applying the voltage?

Thanks a lot.

Edit: added a block diagram of what we want to do.. We are actually planning on using Avalanche Photodiodes (APDs) since we don't have any PMT's and they are, as far as I know, harder to use than APD's. Signal levels on our detector will be pretty low, in the order of nA's; since only a small portion will be reflected from the head.

You might need a laser rather than an LED to be able to modulated that fast. DDS may be an option instead of VCO.

Keith

Thanks Keith, but what if I modulate at a lower frequency like 50-60 MHz? Can I not do that with LEDs?..

I am unhappy with the idea of running a 120MHZ VCO and a 120000400 HZ oscillators on the same board. The isolation between the two must be >100 Db or if any of the 120+400, gets into the VCO, it will jump in frequency to 120+400 and lock there irrespective of the control voltage. It can be done but each oscillator must be built in a screened box with feed through Vcc decouplers and VERY good isolation amps. I would go for two VXCOs, with their frequencies off set about 10MHz or above add their outputs together, then upconvert in a mixer with the VCO. The output will need a tunable filter to remove the VCO - 10MHZ component.
Frank

My concern about modulating LEDs at 60MHz is whether they are really that fast - storage time etc. The data sheets probably don't give such details unless you manage to find one intended to high speed modulation. You may be able to "slope modulate" by sinking extra current into the LED i.e not turning it on & off.

Keith

Thanks for the answer Frank. Our purpose is to extract phase and amplitude information to determine the optical characteristics of the tissue. We will send the low frequency signal to an ADC after demodulation, so its frequency should be as low as possible, shouldn't it?
Also, I didn't understand why the 120+400 signal lock the other VCO to its own frequency.. Is that some kind of cross-talk between oscillators?

You could look at something like the AD8302 (I think that is the right part number from memory) to get phase & amplitude information directly.

Keith

serhannn said:

Also, I didn't understand why the 120+400 signal lock the other VCO to its own frequency.. Is that some kind of cross-talk between oscillators?

Click to expand...

It's called injection locking. When you have two oscillators running at very close frequency, like in your case 120000000 and 120000400, they will eventually run at the same frequency. It's the electrical version of the Huygen's principle.

For high speed digital switching of LED or Laser diodes you could look at a laser switch, like the iC-HG (200MHz) or the iC-HKB(155MHz) (see https://www.ichaus.biz/images/productgroups/laser_drivers.pdf ). At these frequencies all high speed lines should be as short as possible to reduce inductance and provide a fast rise and fall time of the light pulse. Another topic is the proper measurement of the optical output. An optoscope would be needed and other tools (see iC-Haus Homepage - product: iC212 ).