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TXM-315-LC Datasheet(PDF) 5 Page - List of Unclassifed Manufacturers |
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TXM-315-LC Datasheet(HTML) 5 Page - List of Unclassifed Manufacturers |
5 / 11 page Page 9 Page 8 TYPICAL APPLICATIONS The LC Series transmitter is ideal for the transmission of remote control / command data. One of the easiest way to transmit on / off data or switch closures is to use an encoder and decoder. These ICs provide a number of data lines that can be connected to switches or buttons or even a microcontroller. When a line is taken high on the encoder, a corresponding line will go high on the decoder as long as the address matches. The figure below shows an example using the Linx MS Series encoder. This circuit uses the LC Series transmitter and the MS Series encoder to transmit button presses. The MS Series has eight data lines, which are connected to buttons that will pull the line high when pressed. When not used, the lines are pulled low by 100k Ω resistors. The encoder will begin a transmission only when the SEND line is taken high. Diodes are used to pull this line high when any data line is pulled high while isolating the data lines from each other. The MS Series Encoder Data Guide explains this circuit and the many features of the encoder in detail, so please refer to that document for more information. A 750 Ω resistor is used on the LADJ line of the transmitter to reduce the output power of the transmitter. This is appropriate for some antennas, but may need to be adjusted depending on the design. Typically, a resistor pad will be placed on the board and a potentiometer used by the FCC test lab to adjust the output power to the maximum legal limit. The potentiometer value would then be measured and the closest standard value resistor placed for final testing. If the level adjust resistor does not provide enough attenuation, a T-pad attenuator can be placed between the transmitter and antenna. This is a network of three resistors that will provide a set amount of attenuation while maintaining a 50 Ω match between the antenna and the transmitter. Application Note AN-00150 gives the formulas for calculating the resistor values. If not needed, the series resistors can be zero ohms or shorted and the parallel one not placed. 100k 100k 220 100k 100k 100k 100k 100k 100k 100k LICAL-ENC-MS001 D6 D7 SEL_BAUD0 SEL_BAUD1 GND GND GND TX_CNTL DATA_OUT MODE_IND D5 D4 D3 D2 VCC VCC D1 D0 SEND CREATE_ADDR 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 220 TXM-xxx-LC 1 2 3 45 6 7 8 GND VCC GND ANT GND DATA GND LADJ/GND 750 00 OPEN Figure 12: Typical Remote Control Example PROTOCOL GUIDELINES While many RF solutions impose data formatting and balancing requirements, Linx RF modules do not encode or packetize the signal content in any manner. The received signal will be affected by such factors as noise, edge jitter, and interference, but it is not purposefully manipulated or altered by the modules. This gives the designer tremendous flexibility for protocol design and interface. Despite this transparency and ease of use, it must be recognized that there are distinct differences between a wired and a wireless environment. Issues such as interference and contention must be understood and allowed for in the design process. To learn more about protocol considerations, we suggest you read Linx Application Note AN-00160. Errors from interference or changing signal conditions can cause corruption of the data packet, so it is generally wise to structure the data being sent into small packets. This allows errors to be managed without affecting large amounts of data. A simple checksum or CRC could be used for basic error detection. Once an error is detected, the protocol designer may wish to simply discard the corrupt data or implement a more sophisticated scheme to correct it. INTERFERENCE CONSIDERATIONS The RF spectrum is crowded and the potential for conflict with other unwanted sources of RF is very real. While all RF products are at risk from interference, its effects can be minimized by better understanding its characteristics. Interference may come from internal or external sources. The first step is to eliminate interference from noise sources on the board. This means paying careful attention to layout, grounding, filtering, and bypassing in order to eliminate all radiated and conducted interference paths. For many products, this is straightforward; however, products containing components such as switching power supplies, motors, crystals, and other potential sources of noise must be approached with care. Comparing your own design with a Linx evaluation board can help to determine if and at what level design-specific interference is present. External interference can manifest itself in a variety of ways. Low-level interference will produce noise and hashing on the output and reduce the link’s overall range. High-level interference is caused by nearby products sharing the same frequency or from near-band high-power devices. It can even come from your own products if more than one transmitter is active in the same area. It is important to remember that only one transmitter at a time can occupy a frequency, regardless of the coding of the transmitted signal. This type of interference is less common than those mentioned previously, but in severe cases it can prevent all useful function of the affected device. Although technically it is not interference, multipath is also a factor to be understood. Multipath is a term used to refer to the signal cancellation effects that occur when RF waves arrive at the receiver in different phase relationships. This effect is a particularly significant factor in interior environments where objects provide many different signal reflection paths. Multipath cancellation results in lowered signal levels at the receiver and, thus, shorter useful distances for the link. |
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