AXSEM AX5043 Programming Manual
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Contents
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Programming the Chip 313.5.1. RE C O M M E N D E D PR E A M B L EThe main purpose of the preamble is to allow for the receiver to acquire vital transmissionparameters before the actual packet data starts. The minimum duration of the preamble isdependent on how much time the receiver needs to acquire these parameters to sufficientprecision. More specifically, it depends on:• the time needed for the receiver adaptive gain control (AGC) to acquire the signalstrength• the time needed for the receiver to acquire the maximum possible frequency ofset(registers MAXRFOFFSET0, MAXRFOFFSET1 and MAXRFOFFSET2)• the time needed for the receiver to acquire the maximum possible data rate ofset(registers MAXDROFFSET0, MAXDROFFSET1 and MAXDROFFSET2)• the time needed for the receiver to acquire the exact bit sampling time (registersTIMEGAIN0, TIMEGAIN1, TIMEGAIN2 and TIMEGAIN3)• the time needed to acquire the actual frequency deviation in 4-FSK mode (registersFSKDMAX0, FSKDMAX1, FSKDMIN0 and FSKDMAX0)On the AX5043, these loops run in parallel. An AGC that is significantly of however causesthe received signal to fall outside the IF strip dynamic range, and thus prevents the otherloops from working. And a frequency ofset that is compensated insufficiently causes thereceived signal to fall (partially) outside the IF filter, thus also preventing the timing and 4-FSK loops from working.The minimum possible preamble duration can be achieved under the following conditions:• Use a transmitter with a sufficiently precise bit timing. If the maximum deviation ofthe transmitter data rate from the receiver data rate is less than approximately 0.1%,then the data rate acquisition loop should be switched of completely (settingregisters MAXDROFFSET0, MAXDROFFSET1 and MAXDROFFSET2 to zero). TheAX5043 is able to track the remaining small ofset without the data rate ofset loop.All Axsem transmitters derive the bit rate timing from the crystal reference and cantherefore easily meet this requirement.• Use an FSK frequency deviation that is larger than the maximum frequency ofsetbetween transmitter and receiver. In this case, receiver frequency ofset acquisition isnot needed. Do not use 4-FSK.Version 1.9 Programming Manual AX5043 |
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