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data demodulation and crystal selection for the AT86RF211S 1. introduction this document gives an overview of the receiver chain of the AT86RF211S and its associated embedded features: ? discriminator: demodulation of the rf signal (principle, measurement/tuning of output voltage) data slicer:from analog to digital world (different modes of functioning, how to set up the data slicer threshold) it also emphasizes the new po ssibilities of the AT86RF211S selection of lower cost crystal data rate up to 128 kbps frequency deviation wider than 100 khz 2. from analog to digital 2.1 demodulation 2.1.1 principle the fsk modulation used by the AT86RF211S consists in coding each bit as follows: ?0?: transmission of an rf signal at a frequency f0 ?1?: transmission of an rf signal at a frequency f1 the channel frequency (or carrier) is the middle frequency fc = (f0 + f1)/2 f1 - fc = fc - f0 is called the frequency deviation the receiver therefore has the overall task to: down-convert the signal at lower frequencies (for filtering purposes): 10.7 mhz and 455 khz convert the frequencies into voltages (= discriminator) make a decision to separate ?0? from ?1? levels (= data slicer) AT86RF211S fsk transceiver for ism radio applications application note rev. 5418a?wire?04/05
2 5418a?wire?04/05 AT86RF211S application note figure 2-1. principle of demodulation the AT86RF211S discriminator is analog: the output voltage is proportional to the input fre- quency. it was particularly designed to accept a very long sequence of ?zeros? or ?ones? (i.e. a constant input frequency). this is not the case for all receivers (in other words, with some transceivers it is necessary to use dc-free data encoding). figure 2-2. classic discriminator without dc ability l01 fv converter discriminator synchronous reshaped cmos levels clock recovery vcc gnd data slicer threshold l02 vcc gnd signal down-converted at 10.7 mhz 2 down-conversion at 455 khz 1st filtering stage 2nd filtering stage embedded function dataclk datamsg 10.7 mhz bandwidth = hundreds of khz 455 khz bandwidth = tens of khz f rf nd 01011010 sequence
3 5418a?wire?04/05 AT86RF211S application note figure 2-3. at86rf211: discrimin ator with dc ability 2.1.2 standard/narrowband modes of the discriminator since the output swing of the discriminator is proportional to the input frequency deviation, small frequency deviations (used in narrowband applications) lead to smaller peak-to-peak values of discriminator output. for this reason, the discriminator?s gain can be selected. the AT86RF211S features four different gains ? ndb, sdb, mdb and wdb ? where ndb and sdb are fully compatible with the at86rf211. the slope of the demodulator is: standard discriminator bandwidth mode: 14 mv/khz at 2.4v (+5 mv per volt of power supply) narrow discriminator bandwidth mode: 28 mv/khz at 2.4v (+10 mv per volt of power supply) medium discriminator bandwidth mode: 9 mv/khz at 2.4v (+4 mv per volt of power supply) wide discriminator bandwidth mode: 5.5 mv/khz at 2.4v (+2 mv per volt of power supply) 2.1.3 system requirements in order for the system to operate properly, the basic requirements are the following: the frequency deviation must be in accordance with the data rate (the higher the data rate, the larger the frequency deviation). the down-converted frequencies must remain within the if filters over the entire operating conditions (temperature range, ageing), part icularly when a narrow if2 filter is used. the typical values are: => 10.7 mhz filter: 50 to 150 khz (ceramic filter) => optional 455 khz second if filter: 2 to 17.5 khz (ceramic filter). the output of the discriminator must not exceed the maximum allowed voltage range. the level on discout depends on several parameters: the received signal frequency, the receiver local oscillator, the amplifier offsets, etc. important notes: the temperature drifts of the crystal are ofte n given in ppm (parts per million) over a given temperature range. 1 ppm is 0.9 khz at 900 mhz and 0.4 khz at 400 mhz, with a 10.245 mhz crystal. sequence 0101101
4 5418a?wire?04/05 AT86RF211S application note the crystal specifications in this document (ageing and temperature drifts) are given for the 868 to 915 mhz bands. for an application in the 400 mhz to 480 mhz band, these specifications can be relaxed and multiplied by 2. these specifications (temperature and ageing) include the tx and rx sides: the overall drift must meet these requirements. thanks to the high resolution of the AT86RF211S synthesizer (typically 200 hz) a given accuracy can be achieved by software: a small shift of the frequency (made by software) is able to compensate a temperature or ageing drift with no additional hardware cost. 2.2 data slicing once the frequency has been converted into a voltage , a decision must be made to identify the ?0? and ?1? levels and convert them into cmos levels. this is achieved thanks to a com- parator. the AT86RF211S offers two data-slici ng possibilities. figure 2-4. data slicing with the AT86RF211S, a hold is possible on the skfilt capacitor. this helps to maintain the average value of the signal captured without any discharge during reception of the message. 2.2.1 external mode comparing the signal to its average value a first possibility consists in comparing the signal to its own average value: a capacitor on skfilt (pin 25) is charged to the average value of the signal. demodulated signal on discout pin data slicer threshold set on skfilt pin data slicer threshold level set by internal dac (dsref) data slicing options
5 5418a?wire?04/05 AT86RF211S application note figure 2-5. ?external? comparison mode: signal compared to its average value. the value of the capacitor is a trade-off: it must be low enough to make the charging time as short as possible, but high enough to ?memorize? the level during the length of the maximum number of similar consecutive bits. the lower the data rate, the higher the capacitor. practical values are: data rate = 2400 bps => c = 22 nf data rate = 4800 bps => c = 10 nf data rate = 9600 bps => c = 4.7 nf data rate 19200 bps => c = 2.2 nf this procedure makes it impossible to receive a signal containing a dc component (= a long sequence of ?0? or ?1?): the signal and data slic er thresholds become very close to one another and the decision can no longer be made. ther efore, an adequate data encoding technique must be used to prevent any dc component. manchester encoding is a popular way of preventing the existence of any dc component. it consists in encoding the data as follows: ? logical ?0?: 01 ? logical ?1?: 10 this way, a long sequence of ?0s? will be transformed into a ?0101010101?? sequence. a maximum of two similar successive low or high levels can be seen. note: many other dc-free data encoding techniques are possible that increase the effectiveness of the encoding, but the principle of operation is the same. thanks to the new ?charge & hold? feature of the AT86RF211S, it is possible to suppress message encoding. a dedicated application note entitled ?benefits of charge & hold? refer- ence 5420, is available that details this. demodulator output (dsin) dac 100k + - + - skfilt 0 1 datamsg data slicer data slicer threshold a b
6 5418a?wire?04/05 AT86RF211S application note 2.2.2 internal mode comparing the signal to a fixed level a second possibility consists in comparing the signal to a fixed threshold (that does not depend on the signal itself). this is a very powerful mode: it enables you to take advantage of the receiver?s full bandwidth without need for a preamble (the first bit can be data sliced cor- rectly and processed by the microcontroller). figure 2-6. internal comparison mode: signal compared to a fixed level in this mode, the system designer must make sure that the data slicer threshold is tuned close to the middle of the demodulated signal. figure 2-7. ?internal? comparison mode notes: 1. if an active low-pass filter with a transist or (sallen-key for instance) is inserted between dis - cout and dsin, the internal comparison mode must be avoided as the vbe offsets the demodulated signal out of the data slicer windo w. the use of an op. amp. does not offset and the signal thereafter is allowed for active filtering. 2. it is recommended to use a simple r/c low-pass filter instead of an active one. 2.2.3 manchester vs. nrz 2.2.3.1 manchester the frequency spectrum is uniform (sam e number of ?0? and ?1? frequencies). the demodulator can be simple (no dc level). data slicing is very easy (the default threshol d is at the middle of the demodulated signal). a preamble to charge the capacitor to a correct value is required. two transitions are required to send one bit. since the number of transitions per second is limited by the hardware, the number of ?useful? bits is divided by two. for instance, with 128 000 transitions per second allowed by the AT86RF211S, 64 000 useful bits can be transmitted in this mode. signal - + discout - + dsin fixed threshold (internally set up by software, dtr[5:2]) - + datamsg discout: demodulated data ok not ok comparator threshold time
7 5418a?wire?04/05 AT86RF211S application note 2.2.3.2 nrz enhanced encoding effectiveness. if the internal mode is used: ? there is no need for a preamble once the threshold is properly set-up (the first bit can be demodulated). ? 128 000 transitions per sec ond = 128 000 useful bits. ? the threshold must be tuned (this can be done automatically through the software). if the external mode is used: ? the hold mode grants the data slicer long streams of ?0? and ?1?. ? there is a need for a preamble to ?charge? the c skfilt . 3. embedded tools for discriminator/data slicer tuning the AT86RF211S has all the relevant embedded tools to tune the output of the discriminator and the data slicer threshold to meet any requirements. all these operations are carried out by the software.
8 5418a?wire?04/05 AT86RF211S application note 3.1 measuring the output of the discriminator the voltage on the discout pin can be read by setting ctrl1[24] = ?1? and ctrl1[1] =?1?: the result is stored into status[23:18]. an unmodulated rf carrier must be at the input of the receiver (at least 10 db above sensitivity). 3.2 tuning the output level of the discriminator the output voltage of the discriminator can be shifted (up or down) by setting: dischigh (dtr[1]=?1?) adds a positive offset to the output = +220 mv + 100 mv (v cc - 2.4v) for the sdb. disclow (dtr[0]=?1?) adds a negative offset to the output = -220 mv - 100 mv (v cc - 2.4v) for the sdb. one obtains 220 mv at v cc = 2.4v and 340 mv at v cc = 3.6v. this is for the sdb selection. for ndb, the shift is (sdb) 2. for mdb, the shift is (sdb) 2/3. for wdb, the shift is (sdb) 2/5. 3.3 tuning the data slicer threshold dtr[5:2] make it possible to tune the data slicer threshold around v cc /2. 16 levels are possible (lsb = 15 mv per volt of supply voltage). v cc /2 corresponds to dtr[5:2] = ?0111?, and the reset value is 1000. lsb?s value for usual values of power supply voltage: 2.4v: 36 mv 2.7v: 41 mv 3v: 45 mv 3.3v: 50 mv 3.6v: 54 mv
9 5418a?wire?04/05 AT86RF211S application note 4. deviations and crystal specifications 4.1 overall procedure rule 1: the frequency deviation is in accordance with the data rate so that the demodulation is operated properly. rule 2: the down-converted signals (10.7 mhz, 455 khz) remain within the if filters over the whole operating conditions (t range, ageing). rule 3: the output of the discriminator stays within the guaranteed range over the whole operating conditions (t range, ageing). rule 4: the data slicer threshold must be tuned close to the middle of the demodulated signal if the ?internal? mode is used. if not, the drift compensation is naturally done thanks to the skfilt capacitor charge. note: the ?internal? mode for the data slicer is no t mandatory as the skfilt capacitor can be used as the reference level (the discriminator has the abi lity to demodulate cw). the user must choose the relevant value for this capacitor to ensur e that the capacitor remains charged during the ?0000...? and ?1111...? sequences. in this case, it is not possible to send an infinite sequence of ?0? or ?1? as with the internal mode. for inst ance, using c = 4.7 nf with a 9600 bps data rate sets the limit for a correct sequence to be received to 10 similar bits. if no encoding is used, then the ?charge & hold? mode can be operated. it allows you to set the skfilt capacitor to ?hold?. the average level captured during the preamble is then kept during the entire message. an application note dedicated to this subject is available, entitled ?benefits of charge & hold? reference 5420. 4.1.1 meeting rule no. 1 the chosen deviation must be as wide as possibl e to maximize sensitivity. our recommended minimum deviations are the following: <4800 bps: 4 khz 9600 bps: 6 khz 19,200 bps: 10 khz 38,400 bps: 19.2 khz 50,000 bps: 25 khz 100,000 bps: 50 khz 128,000 bps: 64 khz 4.1.2 meeting rule no. 2 the limitation very often comes from the narrow bandwidth of the 455 khz filter. a margin must be considered between the total deviation and the bandwidth otherwise a distortion may appear. for example, with a 3 db bandwidth of 15 khz, it is not recommended to have a devi- ation wider than 10 khz. in addition, any ppm drift of the crystal implies a khz drift of the signal into the filter. of course, thanks to the fine software tuning of the synthesizer, this can be compensated for very easily.
10 5418a?wire?04/05 AT86RF211S application note 4.1.3 meeting rule no. 3 the minimum and maximum levels on discout that must not be exceeded are: from 0.4v to 2.2v for v cc = 2.4v (total range = 1.8v) from 0.7v to 3.5v for v cc = 3.6v (total range = 2.8v) the dc level on discout is the sum of the frequency deviation on the t x, the crystal drifts and to the natural dc level of the discriminator the natural dc level of the discriminator, understood with a pure 455 khz on if2in, depends on the ndb, sdb, mdb and wdb selection but not on the temperature. the temperature drift can be neglected ? only the crystal drifts need be considered. these parameters lead to the equivalent deviation range ( = crystal drifts + deviation). without any dischigh/disclow co mpensation, the edr limits are: notes: 1. as previously noted, the crystal drifts must be understood as (rx +tx) drifts. the deviations can be set as expected by the user. the drifts are thereby modified in accordance with the edr. 2. in the 433 mhz band, the crystal drifts can be relaxed by 2. 3. the initial tolerance of the crystal is not considered when mentioning drifts. use of the dischigh/disclow compensation is possible for all selections. however, it is not recommended for the sdb, mdb and wdb as the natural dc level on discout is low enough not to require any compensation. the above-described deviations and tolerances are wide enough to enable any application requirements. this compensation may be of interest for the nd b selection to relax constraints over crystals. moreover, once the dischigh/disclow co mpensation is done on the production bench, there are no expected drifts except for those of the crystals. the edr then becomes approxi- mately 19 khz providing a relaxed crystal constraint of 4 ppm compared to the non-compensated one. the dischigh/disclow compensation means t hat the best setting leading to the smallest offset from v cc /2 has been programmed. equivalent deviation range example of deviation/drifts (868/915 mhz bands) maximum deviation crystal drifts ndb 15 khz 8 khz 7 ppm sdb 50 khz 30 khz 20 ppm mdb 75 khz 55 khz 20 ppm wdb 125 khz 100 khz 25 ppm
11 5418a?wire?04/05 AT86RF211S application note 4.1.4 meeting rule no. 4 one must ensure that the demodulated data can be correctly data-sliced and reshaped. if the ''external'' mode is used, the average level of the demodulated signal is the reference of the data slicer. there is no problem for meeting rule 4. if the ''external'' mode is used in combinati on with the ''charge & hold? mode, the average level of the demodulated signal is captured and kept as the reference of the data slicer. there is no problem for meeting rule n4. if the ''internal'' mode is used, the reference le vel of the data slicer is the dac output. there- fore, you must ensure that the demodulated signal stays in the window of the data slicer comparator. the ?internal? level can be set on the production bench, then stored in the memory of the appli- cation module. afterwards, the drifts to be considered and compensated for are only the drifts of the crystals, as the discriminator offers a constant dc level over the operating temperature range. the lsb of the data slicer dac is from 36 mv (at v cc = 2.4v) to 54 mv (at v cc = 3.6v). the conversion gains of the discriminator can be expressed in mv/ppm (instead of mv/khz) as 1 ppm is equivalent to 1 khz in the 868/915 mhz bands. therefore, whatever v cc , the rule is: nbd: 1 ppm corresponds to 0.80 lsb sbd: 1 ppm corresponds to 0.40 lsb mbd: 1 ppm corresponds to 0.25 lsb wbd: 1 ppm corresponds to 0.15 lsb we recommend that the ?internal? dac level be checked and modified in accordance with the following rule, where v pp is the dsin peak-to-peak demodulated signal and lsb is the dac lsb corresponding to the current application: v pp < 5 lsb: check each time the drift corresponds to 1 lsb v pp < 10 lsb: check each time the drift corresponds to 2 lsb v pp = 10 lsb: check each time the drift corresponds to 3 lsb the following table derives from the two afore-described rules, providing the maximum drifts allowed between ?internal? dac corrections: if the current application does not exceed thes e limits, only the production bench setting is mandatory. if subsequent applications do exceed these limit s, a compensation will be neces- sary (based on the temperature conditions). v pp < (5 lsb) v pp < (10 lsb) v pp > (10 lsb) ndb 1.5 ppm 3 ppm 4.5 ppm sdb 2.5 ppm 5 ppm 7.5 ppm mdb 4 ppm 8 ppm 12 ppm wdb 6.5 ppm 13 ppm 20 ppm
12 5418a?wire?04/05 AT86RF211S application note examples: sdb selection with a modulation of 20 khz, low data rate, operated at v cc = 3v and in the 868 mhz band: the peak-to-peak signal after demodulation has a range of 340 mv. the dac?s lsb is 45 mv. therefore, the voltage swing equals 15 times the lsb. a compensation of the dac is required if 7.5 ppm drifts are expected, whether from the rx or tx side, or a combination of both. if operated in the 433 mhz band, this constraint is relaxed to a drift of 15 ppm. wdb selection with a modulation of 80 khz, 100 kbps, operated at v cc = 3v and in the 915 mhz band: the peak-to-peak signal after demodulation has a range of 450 mv. notice that at low data rates, it would be 550 mv but because of the f cut-off of the discriminator, at 100 kbps, the signal has less swing. the dac?s lsb is 45 mv. therefore, the volt age swing equals 20 times the lsb. a compen- sation of the dac is required if approximately 20 ppm drifts are expected, whether from the rx or tx side. if operated in the 433 mhz band, this drift is relaxed to 40 ppm. this means that for such applications, it may be that no compensation is necessary.
13 5418a?wire?04/05 AT86RF211S application note 5. appendix 1 5.1 setting up the dtr[5:2] this appendix is relevant only if the internal data slicer mode is used. a given sequence feeds the receiver with 010101... (50% duty cycle). the dtr[5:2] is tuned from ?0000? up to ?1111?. for the lowest values, the comparator level is too low, thereafter the datamsg pin is showing only ?1? as the discriminator output is always below the comparator threshold. for the highest values, the comparat or level is too high thereafter the datamsg pin is showing only ?0? as the discriminator out put is always above the comparator threshold. the value to be chosen is the one for which the datamsg pin shows a correct message on datamsg pin. this value is in the middle bet ween the ?last? dtr[5:2] showing only ?1? at datamsg pin and the ?first? dtr[5:2] showing only ?0? at datamsg pin. the value of the dtr [5:2] register is stored in the application memory. 5.1.1 duty cycle method (internal data slicer mode only) a given sequence feeds the receiver = 010101... (50% duty cycle). dtr[1:0] and dtr[5:2] are tuned to have a duty cycle at the output of th e data slicer as close as possible to 50%.
14 5418a?wire?04/05 AT86RF211S application note 6. appendix 2 6.1 AT86RF211S relevant registers ctrl1[24] : mvcc: selection of the adc mode (rssi or voltage) ctrl1[6] : fskbw: selection of the bandwidth of the discriminator ctrl1[4] : dsref: selection of the dat a slicing mode (internal or external) ctrl1[1] : moffset: measurement of level on discout pin (no modulation) dtr [5:2] : dsref: dac word for reference level adjustment of data slicer threshold dtr [1] : dischigh: positive shif t of the dc level on discout pin dtr [0] : disclow: negative shift of the dc level on discout pin status : mvcc: stores the word [23:18] extended : ctrl1[0]=1 mode: dtr[26:25]: : discrange ? selection of the bandwidth range of the discriminator.
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