Material originally prepared by Prof. Prescott

9 Oct 2014

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Optical Infrared Communications

Project #3 will require you to build and test an infrared communication system for analog communications. The system concept is illustrated below;

Information is placed on a carrier by modulation. IR emitters (usually LED or Laser diodes) use light as the carrier. Remember light has a frequency, just as any other RF signal, so it can be represented as follows:

If we were implementing an RF system then it would be easy to vary the frequency and phase. However with optics, this is very difficult to do:

• The frequency of the IR device is set and we cannot change it.

• The phase of any optical carrier is extremely difficult, if not impossible to control.

All we can vary is the amplitude, but we can do this in interesting ways that will enable us to build a relatively sophisticated communication system.

cosx t a t t t

Material originally prepared by Prof. Prescott

9 Oct 2014

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Let first look at how we can use an optical device to implement a communications capability. First of all we consider digital communications:

1. Amplitude Shift (on/off keying -- OOK) Subcarrier Keying

High frequency “on-off” flashing of the IR diode

2. Amplitude-Frequency Subcarrier Shift Keying

Why can’t we just turn the LED on for “1” and off for “0”?

Material originally prepared by Prof. Prescott

9 Oct 2014

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3. Amplitude-Phase Subcarrier Shift Keying

Data Rate and Subcarrier Frequency

The “flashing-rate” of the LED is called the “subcarrier” frequency of the system. This is usually around between 20 kHz and 150 kHz and typically, for remote control applications, 40 kHz is standard.

In digital communications, the flashing rate is also sometimes called the “chip rate” as shown below.

Material originally prepared by Prof. Prescott

9 Oct 2014

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Bit Rate is the number of information bits transmitted per second.

An acceptable ratio is 10:1 – that is, 10 chips/bit

Material originally prepared by Prof. Prescott

9 Oct 2014

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Transmission and reception of the optical signal using OOK digital modulation techniques is simplified by using a modular receiver, a shown below.

The TSOP1138

Material originally prepared by Prof. Prescott

9 Oct 2014

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TSOP 1138Data from the spec sheet

BUT......we are using analog frequency modulation and devices of this type are not useful to us !!

So how do we use a fundamentally analog modulation technique with optics?

Material originally prepared by Prof. Prescott

9 Oct 2014

7

Analog Signaling using Optical Devices

Digital modulation using optical devices is rather straightforward –just a matter of flashing the LED at some subcarrier frequency corresponding to a bit-interval.

Returning to the mathematical model of the signal being transmitted from the IR LED:

We still can only change the amplitude of this signal. We might first think of “intensity modulation” as described below:

1. Intensity modulation: Varies the intensity of the light emitted from the diode in accordance with the amplitude of the analog information signal. This is really a bad idea! Why??

2. Subcarrier Frequency Modulation: Varies the frequency of the subcarrier in accordance with the amplitude of the analog information signal. This is the one we will use.

Here is the concept:

cosx t a t t t

analog information signal

analogfrequencymodulation

Subcarrierfrequencymodulation(VCO output)

VCO bias voltage

Material originally prepared by Prof. Prescott

9 Oct 2014

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Note that the frequency varies above and below some “rest” or quiescent frequency – this is the frequency transmitted when there is no input signal applied to the modulator.

For example, if your quiescent subcarrier frequency is 80 kHz, the input analog signal might drive the subcarrier frequency up to 100 kHz at the peak amplitude and down to 60 kHz in the min. amplitude of the sinusoidal waveform.

Now lets put it all together

A1Audio fromSound Source, adjusted to correct voltage level.

VoltageControlledOscillator IR LED

A2VCO

Active LPF LED Driver

VCO bias for rest frequency

inv

inv

outf

outf

of

3000 Hz

A3

OpticalDetector

A4

PreAmp

Audio AmpOpticalInput

Audio Output

Audio to PCSound Card

VCO

PC

SF

loop filter(LPF)

4046 PLL

demodulatedsignaloutput

invoutf

outf

of

VCO bias for rest frequency

inv

Material originally prepared by Prof. Prescott

9 Oct 2014

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Some interesting graphs regarding light. The graph below shows the distribution of spectral energy from the sun.

By comparison, here is the spectrum of a tungsten lamp

Characteristics of Optical Transmitting and Receiving Devices

Material originally prepared by Prof. Prescott

9 Oct 2014

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Here is the spectrum of the human eye

Here is the spectral response of a typical optical receiving device - but these devices are NOT receiver modules.

Material originally prepared by Prof. Prescott

9 Oct 2014

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Comparison of LED with Laser Diode

• Greater range possible with laser diode, by orders of magnitude. This is due to the narrow beamwidth and chromatic light characteristics of the laser.

• Narrow beamwidth makes a long range laser system more vulnerable to weather and atmospheric effects.

• Laser diodes requires far more power for operation than do LEDs.

Advantages of LED-Based Systems

• LEDs are available in packages containing integrated lenses that focus the optical power into beams with various degrees of divergence.

• LEDs have relatively simple power supply requirements, since they operate at low voltages and currents.

• LEDs are safer to work with. The danger in lasers is the highly intense collimated light.

• Modulation is more easily accomplished with LEDs, and can be accomplished directly. Lasers require external techniques.

You might be asking at this point...Why don’t we us Laser Diodes in our project?

Material originally prepared by Prof. Prescott

9 Oct 2014

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Infrared LEDs for Communications

A coupld of the LEDs available in the shop for this project are:

1. SFH 484 (880 nm)

2. SFH 485 (880 nm)

Angle of half radiation intensity: 8

Angle of half radiation intensity: 20

max subcarrier bandwidth = 900 kHz

Radiant intensity: 50 mW/sr @ 100 mA

Radiant intensity: 25 mW/sr @ 100 mA

Material originally prepared by Prof. Prescott

9 Oct 2014

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3. TSAL6100 (940 nm)

Angle of half radiation intensity: 10

Radiant intensity: 80 mW/sr @ 100 mA

4. QED 233 (940 nm)

5. QED 234 (940 nm)

Angle of half radiation intensity: 20

Radiant intensity: 10 mW/sr @ 100 mA

max subcarrier bandwidth = 500 kHz

27 mW/sr @ 100 mA

(QED 233)

(QED 234)

max subcarrier bandwidth = 625 kHz

Material originally prepared by Prof. Prescott

9 Oct 2014

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Silicon PIN Photodiodes

The PIN diodes available in the shop for this project are as follows:

1. BP104

Angle of half sensitivity: 65

1050 nm870 nm

(max subcarrier bandwidth = 5.0 MHz)

2. SFH206K

Angle of half sensitivity: ??

Range of spectral sensitivity: ??

(max subcarrier bandwidth = 25.0 MHz)

Material originally prepared by Prof. Prescott

9 Oct 2014

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4. QSD 122/123/124

• No difference indicated in these part numbers on the spec sheet.• This is a phototransistor, not a PIN diode

(max subcarrier bandwidth = 71 kHz)

Peak spectral sensitivity = 880 nm(no plot given in spec sheet)

Material originally prepared by Prof. Prescott

9 Oct 2014

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The final topic is the drive circuitry for the LED. There are two ways to increase the transmission distance in a LED communication link:

1. Increase the power of the transmitted signal

2. Focus the signal more precisely on the receiver

You can increase the power of the transmitted signal by driving the LED harder (be careful of the limitations of the diode) or driving more than one LED. Consider the circuits below:

We have the ZTX602 (an NPN Darlington transistor) that you can use to drive the LEDs.