DESCRIPTION
This demonstration board implements the IrDA standards for infrared data communications. It is a complete transmitter/receiver for use in SIR, FIR, and 4PPM infrared data links. The LT1328 is a photodiode receiver that supports IrDA data rates up to 4Mbps, as well as other modulation methods such as Sharp-ASK and TV remote control. This board can be configured for IrDA-SIR (Infrared Data Association Serial Infrared), IrDA-FIR (Infrared Data Association Fast Infrared), and 4PPM (4MHz pulse-position modulation).
The LT1328, available in SO-8 and MSOP packages, contains all the necessary circuitry to convert current pulses from an external photodiode to a digital TTL output, while rejecting unwanted lower frequency interference. The LT1328 requires only six external components to make an IrDA compatible receiver. Figure 1 is a block diagram of the LT1328. The preamp converts the photodiode current into a voltage that trips the comparator. The internal servo action suppresses only frequencies below the Rgm/CFILT pole. This highpass filtering attenuates interfering signals, such as sunlight or incandescent and fluorescent lamps, and is selectable at Pin 7 for low or high data rates. The upper frequency limit for the servo is set by an external capacitor C4. The pole is found by the formula f(Hz) = 25[1/(2π•60k•C4)]; a 330pF capacitor for C4 will pass signals above 200kHz (FIR and 4PPM). To lower the passband for SIR, move the jumper on the demo board from FIR, 4PPM to SIR. This takes the MODE pin (Pin 7) from ground to VCC, which switches C1 (on Pin 3) in parallel with C4. The required filtering for VBIAS (Pin 8) is provided by C6, a 1000pF cap to ground. The DATA pin (Pin 5) is the output of the comparator. The quiescent, no signal state is a TTL HIGH; a TTL LOW occurs when the photodiode receives light. Power requirements for the LT1328 are minimal: a single 5V supply and 2mA of quiescent current. The LT1328's ease of use and flexibility make it an ideal solution for numerous other photodiode receiver applications. The LT1328 is available in the 8-pin SO and the tiny MSOP package for size-critical applications.
An IrDA transmitter is also provided on the demo board. Figure 2 shows the LED driver circuit and the LT1328 receiver. The LED transmits light when the transmit input pin on the demo board is high (5V), and is off when the Transmit Input is low (0V). The receiver output is the DATA pin on the demo board (which is Pin 5 on the LT1328). The Data pin is high for no received light and a TTL LOW for a light pulse. To switch from SIR to the faster data rates, a jumper is provided. Remote switching can be accomplished by removing the jumper and accessing the MODE pin: TTL HIGH for SIR and TTL LOW for faster data rates.
Actual transmit and receive waveforms can be seen in Figures 3, 4, and 5 for SIR, FIR, and 4PPM, respectively. The lower waveform in each photo is the transmit input to the demo board. The upper waveform is the demo board DATA pin (Pin 5 of the LT1328).
LTC and LT are registered trademarks of Linear Technology Corporation. IrDA is a registered trademark of the Infrared Data Association.
BOARD PHOTO
A photograph of the DC112A demo board shows the physical layout of components. Key components visible include the LT1328 IC (U1), photodiode (D1), LED (D2), various capacitors (C1-C6), resistors (R1-R3), transistors (Q1, Q2), and jumpers (JP1, XJP1). The silkscreen markings indicate pin functions like IN, FILTER, MODE, DATA, VCC, and GND.
PACKAGE AND SCHEMATIC DIAGRAMS
Figure 1. LT1328 Block Diagram
The LT1328 block diagram illustrates its internal architecture. It features a Preamp, Filter, DC Servo, and Comparator. The photodiode connects to the IN pin (Pin 1). The FILTER pin (Pin 2) and FILTER SWITCH pin (Pin 3) control filtering characteristics. The MODE pin (Pin 7) selects between SIR and FIR/4PPM modes. The DATA pin (Pin 5) provides the digital output. VBIAS (Pin 8) is supplied by C6, and VCC (Pin 6) is the power supply. GND is Pin 4.
LT1328CMS8 (MSOP Package)
This is the 8-lead plastic MSOP package for the LT1328, with pins labeled IN, FILTER, FILTER SWITCH, GND, DATA, VCC, MODE, and VBIAS.
LT1328CS8 (SO Package)
This is the 8-lead plastic SO package for the LT1328, with pins labeled IN, FILTER, FILTER SWITCH, GND, DATA, VCC, MODE, and VBIAS.
Figure 2a. IrDA Receiver
This schematic shows the IrDA receiver circuit. It uses the LT1328 IC (U1), a photodiode (D1 BPV22NF), and supporting components C1, C4, C6, and jumper JP1. The input is from the photodiode, and the output is the DATA pin. The MODE pin is controlled by jumper JP1.
Figure 2b. IrDA Transmitter
This schematic shows the IrDA transmitter circuit. It uses an LED (D2 HSDL-4220) driven by two 2N7002 transistors (Q1, Q2) controlled by resistors R1, R2, R3 and capacitors C2, C3. The transmit input signal controls the LED.
PARTS LIST
| REFERENCE DESIGNATOR | QUANTITY | PART NUMBER | DESCRIPTION | VENDOR | TELEPHONE |
|---|---|---|---|---|---|
| C1 | 1 | 12065C103KATMA | 0.01μF 50V 20% X7R Capacitor | AVX | (803) 946-0362 |
| C2 | 1 | 12065C104KATMA | 0.1μF 50V 10% X7R Capacitor | AVX | (803) 946-0362 |
| C3 | 1 | TAJC106K016R | 10µF 16V 10% Tantalum Capacitor | AVX | (207) 282-5111 |
| C4 | 1 | 1206A331KATMA | 330pF 50V 10% NPO Capacitor | AVX | (803) 946-0362 |
| C6 | 1 | 12065C102KATMA | 1000pF 50V 10% X7R Capacitor | AVX | (803) 946-0362 |
| D1 | 1 | BPV22NF | Diode | Temic | (408) 970-5700 |
| D2 | 1 | HSDL-4220 | LED | Hewlett Packard | (800) 235-0312 |
| E1 to E6 | 6 | 1502-2 | Turret | Keystone | (718) 956-8900 |
| JP1 | 1 | 3801S-3-G1 | 3-Pin Header | COMM CON | (818) 301-4200 |
| Q1, Q2 | 2 | 2N7002LTA | Transistor | Zetex | (516) 543-7100 |
| R1 | 1 | CR32-101J-T | 100Ω 5% Resistor | AVX | (803) 946-0524 |
| R2 | 1 | CR32-103J-T | 10k 5% Resistor | AVX | (803) 946-0524 |
| R3 | 1 | CRCW20106R8J | 6.8Ω 1/2W 5% | Dale | (408) 985-5733 |
| U1 | 1 | LT1328CS8 | IC | LTC | (408) 432-1900 |
| XJP1 | 1 | CCIJ230-G | Shunt | COMM CON | (818) 301-4200 |
OPERATION
The most straightforward way of evaluating IR links with the LT1328 demo board is to use a separate LED transmitter, similar to the one on the DC112A, placed at a measured distance from the receiver. If a second board is not available, the transmitter design shown in Figure 2 can be duplicated. Pulses to drive this transmitter can be obtained from a suitable pulse generator with a TTL output or from the system that will use the IR link. Use coax cable with proper termination on the transmitter board input for good pulse fidelity.
The onboard jumper must be set for the desired modulation. For example, for IrDA-SIR modulation, set the mode jumper to the SIR side. Connect an oscilloscope to the appropriate output of the demonstration board and apply a 5V power source capable of supplying greater than 25mA to the VCC and GND pins.
Set the transmitter close to the receiver (approximately 1cm), input an appropriate modulation signal to the transmit input pin, and verify basic receiver operation using the modulation photographs (Figures 3, 4, and 5) as a guide. Test the receiver over the desired range, transmitter power, angle of incidence, and other variables. It is helpful to set up an optical range clear of obstacles, reflections, and interference. For more sophisticated testing, a Bit Error Rate Test (BERT) set and circuits to modulate/demodulate digital signals are usually required.
IrDA-SIR
SIR systems transmit a pulse for a zero and nothing for a one. Figure 3a shows the NRZ data sequence for SIR. Figures 3b and 3c demonstrate SIR modulation with the receiver at 1 meter and 1cm, respectively. The lower trace in each photo is the transmitter input, and the upper trace is the receiver output. The receiver output is a TTL LOW for a transmitted light pulse. There is minimal difference in received data between 1cm and 1m.
Figure 3a: NRZ Data for IrDA-SIR. Data Rates: 2400BAUD TO 115KBAUD. Bit Interval: 417μs TO 8.7μs. Pulse Width: 3/16 OF INTERVAL OR 1.63μs.
Figure 3b: IrDA-SIR Modulation at 1 meter. Shows Transmitter Input and Receiver Output waveforms.
Figure 3c: IrDA-SIR Modulation at 1cm. Shows Transmitter Input and Receiver Output waveforms.
IrDA-FIR
FIR modulation is similar to SIR, differing mainly in data rate and pulse width. Figure 4a shows the NRZ data sequence for FIR. Figures 4b and 4c show the receiver at 1 meter and 1cm, respectively. A detected light pulse causes the receiver to switch to a TTL LOW state. Precautions regarding interference apply to both FIR and SIR.
Figure 4a: NRZ Data for IrDA-FIR. Data Rates: FIXED AT 1.15MBAUD. Bit Interval: 870ns. Pulse Width: 1/4 OF INTERVAL OR 217ns.
Figure 4b: IrDA-FIR Modulation at 1 meter. Shows Transmitter Input and Receiver Output waveforms.
Figure 4c: IrDA-FIR Modulation at 1cm. Shows Transmitter Input and Receiver Output waveforms.
IrDA-4PPM
4Mbps pulse position modulation (4PPM) positions one pulse, 125ns wide, within one of four data windows for every data bit pair. The bit interval is 500ns, as shown in Figure 5a. Figure 5a also maps data bit pairs to their 4PPM symbols. Figures 5b and 5c show the receiver output at 1 meter and 1cm, respectively, as the distance from the transmitter is varied.
Figure 5a: Data Bit Pair to 4PPM Symbol mapping and NRZ Data for IrDA-4PPM. Data Rates: FIXED AT 4MBAUD. Bit Interval: 500ns. Pulse Width: 1/4 OF INTERVAL OR 125ns.
Figure 5b: IrDA-4PPM Modulation at 1 meter. Shows Transmitter Input and Receiver Output waveforms.
Figure 5c: IrDA-4PPM Modulation at 1cm. Shows Transmitter Input and Receiver Output waveforms.
PCB LAYOUT AND FILM
This section details the physical layout of the DC112A demo board. It includes views of the component side and solder side, along with their respective silkscreen and solder mask layers. The LT1328CS8 part number and contact information are visible on the silkscreen.
PC FAB DRAWING
The PC FAB DRAWING provides specifications for the printed circuit board. It is a 2-layer FR-4 glass epoxy board, 0.062" thick, with 2oz copper clad. The drawing includes dimensions (1.25 units) and a table detailing hole sizes and counts for symbols A, B, C, and D. Notes specify plating requirements, silkscreen color, finish, and solder mask type.
Hole Specifications:
| SYMBOL | DIAMETER | NUMBER OF HOLES |
|---|---|---|
| A | 0.094 | 6 |
| B | 0.042 | 2 |
| C | 0.037 | 7 |
| D | 0.015 | 2 |
| TOTAL HOLES | 17 |
Notes:
- MATERIAL: 2 LAYERS, 0.062" THICK, FR-4 GLASS EPOXY 2OZ COPPER CLAD
- ALL HOLES SHALL BE PLATED THROUGH
- PLATE THROUGH HOLES WITH COPPER 0.0014 MIN THICKNESS. ALL HOLE SIZES IN HOLE TABLE ARE AFTER PLATING
- SILKSCREEN: WHITE EPOXY NONCONDUCTIVE INK
- FINISH: SMOBC
- SOLDER MASK: LPI, GREEN
- SCORING:
DISTRIBUTOR INFORMATION
Mouser Electronics is an authorized distributor. Information regarding pricing, inventory, delivery, and lifecycle can be found for Analog Devices Inc. part number DC112A-B.
