Adeunis RF ARF26 User Manual

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Adeunis RF ARF26 User Manual Manual pdf 7 page image

05-11-V3-psn 7On account of the ARF26 system transmission principle and of the modulation in phase lockloop, the modulating signals must have a frequency comprised between 1 kHz and 25kHz.Lower and higher frequencies cannot be transmitted, as they are filtered. This means, amongother things, that the link cannot transmit constant states with a duration of more than 500 μs.• Level - Bit EncodingMost conventional hardwired serial links transmit bytes in asynchronous manner using anNRZ type encoding (1 logic bit = 1 electrical state). This results in the spectrum occupation ofthe signals thus generated being about one decade. Furthermore, sampling of the bytes thusreceived supposes perfect synchronisation of decoding on the start bit.In the case of radio transmission, it is preferable to limit the spectrum of the signals to betransmitted as far as possible, in particular towards low frequencies, even if this meansincreasing the main frequency. Moreover, it is advantageous to reset the synchro on eachtransmitted bit; receivers are in fact the cause of large duty cycle errors on the electrical states.Bit encodings should therefore be preferred, for example those proposed below:Manchester or two-phase:Differential PWM:• Level - Frame EncodingOnce the bits have been encoded, it is then imperative that they be transmitted according to aframe structure; the radio link is very sensitive to binary flow breaks which can only beminimised by “serialising” the bytes.A conventional frame structure is shown below:PROTOCOL