Finkenzeller K.RFID handbook.2003
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5.3 NATIONAL LICENSING REGULATIONS IN EUROPE |
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Table 5.13 Permitted transmission power in accordance |
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with EN 300 440 |
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Frequency (GHz) |
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Class |
1.0–5.0 |
5.0–20 |
>20 |
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I |
10 mW |
25 mW |
100 mW |
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II |
500 mW |
500 mW |
500 mW |
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III |
500 mW |
2 W |
2 W |
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Reflective transponder systems.
The following frequency ranges are reserved for ISM applications:
•2.400–2.4835 GHz
•5.725–5.875 GHz
•24.00–24.25 GHz
This standard also defines testing methods and limit values for spurious emissions, which we will not, however, consider in more detail here.
5.3 National Licensing Regulations in Europe
In Europe, the recommendations of the ERC serve as the basis for national legislative and licensing regulations for radio systems. For RFID systems REC 70-03 (short range devices, SRD) applies. The website of the ERO (European Radio Office) provides current notes on the national regulation of SRDs in the member states of CEPT (see Section 5.2.1).
In all member states of the EU and the member states of CEPT that apply the EU Directive 1999/5/EC (‘Radio and Telecommunications Terminal Equipment Directive’, R&TTE Directive), SRDs can be offered for sale without further licensing (ERC, 2000). This is the case, under the prerequisite that the applicable licensing regulations for the frequency ranges and applications in question are adhered to. The manufacturer needs only to confirm that the relevant regulations have been adhered to for each product (EC Declaration of Conformity), which it does by displaying a CE mark upon the product.
Notes on the procedure regarding the CE marking and sale of radio and telecommunications systems can be found on the R&TTE homepage of the EU at http://europa.eu. int/comm/enterprise/rtte.
Basic notes on the new legislation regarding the CE marking of products can be found at http://europa.eu.int/comm/enterprise/newapproach/legislation/guide/legislation.htm.
5.3.1 Germany
In Germany, the licensing of RFID systems is regulated by two decrees (REGTP, 2000a,b) which were published in the Amtsblatt der Regulierungsbeh¨orde f¨ur Telekommunikation und Post (RegTP) (Gazette of the Regulatory Authorities for Telecommunications and Post) in summer 2000. These decrees converted the recommendations
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5 FREQUENCY RANGES AND RADIO LICENSING REGULATIONS |
of the REC 70-03 into national legislation. Here, too, national restrictions (e.g. lower power level in some frequency ranges below 135.00 kHz) should be taken into consideration.
All radio systems that have a German licence or were put into operation in accordance with the provisions of Directive 1999/5/EC (R&TTE Directive), and are marked accordingly (CE marking), may be operated. Of course, national restrictions must still be adhered to.
The licensing of inductively coupled RFID systems is regulated in decree 61/2000, entitled ‘Allgemeinzuteilung von Frequenzen fur¨ die Benutzung durch die Allgemeinheit fur¨ induktive Funkanlagen des nichtoffentlichen¨ mobilen Landfunks (nomL)’¨ (‘General allocation of frequencies for use by the general public for inductive radio systems relating to private mobile national radio’). General allocation covers numerous applications of inductive radio systems, such as lorry barriers (RFID), EAS, traffic control systems, metal detectors, recognition systems for people, animals and goods (RFID), but also data and voice transmission over short distances (e.g. for alarm systems). See Figure 5.6.
Only the frequency ranges listed in Table 5.14 may be used for the above-mentioned radio applications.
The licensing of RFID systems in the frequency ranges up to 24 GHz is regulated in decree 73/200 entitled ‘Allgemeinzuteilung von Frequenzen fur¨ die Benutzung durch die Allgemeinheit fur¨ Funkanlagen geringer Leistung des nichtoffentlichen¨ mobilen Landfunks (nomL)¨ in ISM Frequenzbereichen; SRD (Short Range Devices)’ (‘General allocation of frequencies for use by the general public for low power radio systems relating to private mobile national radio in ISM frequency ranges; SRD (Short Range
80
H field strength limit dBµA/m at 10 m
60
40
20
0 |
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−20 |
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1 × 103 |
1 × 104 |
1 × 105 |
10 |
100 |
Frequency range
Figure 5.6 The permitted frequency range up to 30 MHz and the maximum field strength at a distance of 10 m in Germany
5.4 NATIONAL LICENSING REGULATIONS |
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Table 5.14 Permitted frequency ranges and field strengths |
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at a distance of 10 m |
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Frequency range (MHz) |
Field strength H @ 10 m |
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(dBµA/m) |
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0.009 |
–0.057 |
42 |
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0.057 |
–0.05975 |
69 |
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0.05975–0.06025 |
42 |
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0.06025–0.067 |
69 |
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0.067 |
–0.119 |
42 |
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0.119 |
–0.127 |
69 |
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0.127 |
–0.135 |
42 |
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0.135 |
–30.000 |
5 |
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3.155 |
–3.400 |
13.5 (EAS) |
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6.765 |
–6.795 |
42 |
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7.400 |
–8.800 |
9 (EAS) |
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13.553–13.567 |
42 |
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26.957–27.283 |
42 |
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Table 5.15 Permissible frequency ranges and power levels for |
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short range devices |
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Frequency range (MHz) |
Power level |
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6.765–6.975 |
42 dBµA/m @ 10 m |
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13.553–13.567 |
42 dBµA/m @ 10 m |
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26.957–27.283 |
42 dBµA/m @ 10 m, 10 mW ERP |
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40.660–40.700 |
10 mW ERP |
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433.05–434.79 |
10 mW ERP |
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2400.0–2483.5 |
10 mW ERP |
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5725.0–5875.0 |
25 mW ERP |
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24.000–24.250 |
100 mW ERP |
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Devices)’). The general allocation covers numerous SRD applications and is not limited to RFID systems. Only the frequency ranges listed in Table 5.15 may be used for these applications with the corresponding maximum magnetic field strength or the maximum radiated power.
5.4 National Licensing Regulations
5.4.1 USA
In the USA, RFID systems must be licensed in accordance with licensing regulation ‘FCC Part 15 ’. This regulation covers the frequency range from 9 kHz to above 64 GHz and deals with the intentional generation of electromagnetic fields by low and minimum power transmitters (intentional radiators) plus the unintentional generation of electromagnetic fields (spurious radiation) by electronic devices such as radio and television receivers or computer systems. The category of low power transmitters covers a
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5 FREQUENCY RANGES AND RADIO LICENSING REGULATIONS |
wide variety of applications, for example cordless telephones, biometry and telemetry transmitters, on-campus radio stations, toy remote controls and door openers for cars. Inductively coupled or backscatter RFID systems are not explicitly mentioned in the FCC regulation, but they automatically fall under its scope due to their transmission frequencies, which are typically in the ISM bands, and their low transmission power.
Table 5.16 lists the frequency ranges that are important for RFID systems. In all other frequency ranges the permissible limit values for spurious radiation given in Table 5.17 apply to RFID systems. It should be noted here that, unlike the European licensing regulation ETS 300 330, the maximum permissible field strength of a reader is principally defined by the electrical field strength E. The measuring distance is selected such that a measurement is made in the far field of the generated field. This also applies for inductively coupled RFID systems in the frequency range below 30 MHz, which primarily generate a magnetic high frequency field.
5.4.2Future development: USA–Japan–Europe
The USA and Japan are currently planning to adapt their national regulations for RFID systems to 13.56 MHz, and to the values permitted in Europe in accordance with REC 70-03. At the time of preparation of this book a final decision had not been taken on
Table 5.16 Permissible field strengths for RFID systems in accordance with FCC Part 15
Frequency range (MHz) |
Max. E field |
Measuring distance |
Section |
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(m) |
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1.705–10.000 |
100 µV/m |
30 |
15.223 |
13.553–13.567 |
10 mV/m |
30 |
15.225 |
26.960–27.280 |
10 mV/m |
30 |
15.227 |
40.660–40.700 |
1 mV/m |
3 |
15.229 |
49.820–49.900 |
10 mV/m |
3 |
15.235 |
902.0–928.0 |
50 mV/m |
3 |
15.249 |
2435–2465 |
50 mV/m |
3 |
15.249 |
5785–5815 |
50 mV/m |
3 |
15.249 |
24075–24175 |
250 mV/m |
3 |
15.249 |
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Table 5.17 Permissible interference field strength in all other frequency ranges in accordance with FCC Part 15, Section 15.209
Frequency range (MHz) |
Maximum E field |
Measuring distance |
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(m) |
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0.009 |
–0.490 |
2400/f µV/m |
300 |
0.490 |
–1.705 |
24/f mV/m |
30 |
1.705 |
–30.00 |
30 µV/m |
30 |
30.00 |
–88.00 |
100 µV/m |
3 |
88.00 |
–216 |
150 µV/m |
3 |
216–960 |
200 µV/m |
3 |
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>960 |
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500 µV/m |
3 |
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5.4 NATIONAL LICENSING REGULATIONS |
181 |
Electric field strength (magnetic)
47 544 µV/m (42 dBµA/m)
1061 µV/m (9 dBµA/m)
316 µV/m (−1.5 dBµA/m)
150 µV/m (−8 dBµA/m)
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E |
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U. U |
J |
S. |
P |
A. |
N |
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±7 kHz
Frequency
±150 kHz
±450 kHz
Figure 5.7 Comparison of the permitted magnetic field strengths of the planned regulations for 13.56 MHz RFID systems in the USA, Japan and Europe (reproduced by permission of Takeshi Iga2, SOFEL, Tokyo)
any of the planned regulations, so at this point it is not possible to deal with these regulations in more detail. See Figure 5.7.
2 Takeshi Iga: Publisher and translator of the Japanese edition of the RFID handbook. See also http://RFIDhandbook.com/japanese
RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification, Second Edition
Klaus Finkenzeller Copyright 2003 John Wiley & Sons, Ltd.
ISBN: 0-470-84402-7
6
Coding and Modulation
The block diagram in Figure 6.1 describes a digital communication system. Similarly, data transfer between reader and transponder in an RFID system requires three main functional blocks. From the reader to the transponder — the direction of data transfer — these are: signal coding (signal processing) and the modulator (carrier circuit) in the reader (transmitter), the transmission medium (channel), and the demodulator (carrier circuit) and signal decoding (signal processing) in the transponder (receiver).
A signal coding system takes the message to be transmitted and its signal representation and matches it optimally to the characteristics of the transmission channel. This process involves providing the message with some degree of protection against interference or collision and against intentional modification of certain signal characteristics (Herter and Lorcher,¨ 1987). Signal coding should not be confused with modulation, and therefore it is referred to as coding in the baseband .
Modulation is the process of altering the signal parameters of a high frequency carrier, i.e. its amplitude, frequency or phase, in relation to a modulated signal, the baseband signal.
The transmission medium transmits the message over a predetermined distance. The only transmission media used in RFID systems are magnetic fields (inductive coupling) and electromagnetic waves (microwaves).
Demodulation is an additional modulation procedure to reclaim the signal in the baseband. As there is often an information source (input) in both the transponder and the reader, and information is thus transmitted alternately in both directions, these components contain both a modulator and a demodulator. This is therefore known as a modem (Modulator — Demodulator), a term that describes the normal configuration (Herter and Lorcher,¨ 1987).
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Noise |
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Transmitter |
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n(t) |
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Receiver |
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Information |
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To information |
source |
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s(t) |
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r (t) |
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sink (user) |
m(t) |
Signal |
Carrier |
Channel |
Carrier |
Signal |
m(t) |
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processing |
circuit |
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circuit |
processing |
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Figure 6.1 Signal and data flow in a digital communications system (Couch, 1997)
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6 CODING AND MODULATION |
The task of signal decoding is to reconstruct the original message from the baseband coded received signal and to recognise any transmission errors and flag them as such.
6.1Coding in the Baseband
Binary ones and zeros can be represented in various line codes. RFID systems normally use one of the following coding procedures: NRZ, Manchester, Unipolar RZ, DBP (differential bi-phase), Miller, differential coding on PP coding (Figure 6.2).
NRZ code A binary 1 is represented by a ‘high’ signal and a binary 0 is represented by a ‘low’ signal. The NRZ code is used almost exclusively with FSK or PSK modulation.
Manchester code A binary 1 is represented by a negative transition in the half bit period and a binary 0 is represented by a positive transition. The Manchester code is therefore also known as split-phase coding (Couch, 1997).
The Manchester code is often used for data transmission from the transponder to the reader based upon load modulation using a subcarrier.
NRZ coding:
Manchester coding: (bi-phase)
Unipolar RZ coding:
DBP
Miller coding:
Modified Miller coding:
Differential coding:
1
1 |
0 |
1 |
1 |
0 |
0 |
1 |
0 |
1 |
0 |
1 |
1 |
0 |
0 |
1 |
0 |
1 |
0 |
1 |
1 |
0 |
0 |
1 |
0 |
1 |
0 |
1 |
1 |
0 |
0 |
1 |
0 |
1 |
0 |
1 |
1 |
0 |
0 |
1 |
0 |
1 |
0 |
1 |
1 |
0 |
0 |
1 |
0 |
1 |
0 |
1 |
1 |
0 |
0 |
1 |
0 |
Figure 6.2 Signal coding by frequently changing line codes in RFID systems
6.1 CODING IN THE BASEBAND |
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Unipolar RZ code A binary 1 is represented by a ‘high’ signal during the first half bit period, a binary 0 is represented by a ‘low’ signal lasting for the entire duration of the bit.
DBP code A binary 0 is coded by a transition of either type in the half bit period, a binary 1 is coded by the lack of a transition. Furthermore, the level is inverted at the start of every bit period, so that the bit pulse can be more easily reconstructed in the receiver (if necessary).
Miller code A binary 1 is represented by a transition of either type in the half bit period, a binary 0 is represented by the continuance of the 1 level over the next bit period. A sequence of zeros creates a transition at the start of a bit period, so that the bit pulse can be more easily reconstructed in the receiver (if necessary).
Modified Miller code In this variant of the Miller code each transition is replaced by a ‘negative’ pulse. The modified Miller code is highly suitable for use in inductively coupled RFID systems for data transfer from the reader to the transponder.
Due to the very short pulse durations (tpulse Tbit) it is possible to ensure a continuous power supply to the transponder from the HF field of the reader even during data transfer.
Differential coding In ‘differential coding’ every binary 1 to be transmitted causes a change (toggle) in the signal level, whereas the signal level remains unchanged for a binary zero. Differential coding can be generated very simply from an NRZ signal by using an XOR gate and a D flip-flop. Figure 6.3 shows a circuit to achieve this.
Pulse-pause coding In pulse-pause coding (PPC) a binary 1 is represented by a pause of duration t before the next pulse; a binary 0 is represented by a pause of duration 2t before the next pulse (Figure 6.4). This coding procedure is popular in inductively coupled RFID systems for data transfer from the reader to the transponder. Due to the very short pulse durations (tpulse Tbit) it is possible to ensure a continuous power supply to the transponder from the HF field of the reader even during data transfer.
Data in |
Data out |
(NRZ) |
(differential) |
XOR |
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D |
Q |
Clock |
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Figure 6.3 Generating differential coding from NRZ coding
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6 CODING AND MODULATION |
Pulse/Pauselength coding:
START |
SYNC 1 |
0 |
1 1 |
0 |
0 |
1 |
0 |
Figure 6.4 Possible signal path in pulse-pause coding
Various boundary conditions should be taken into consideration when selecting a suitable signal coding system for an RFID system. The most important consideration is the signal spectrum after modulation (Couch, 1997; Mausl,¨ 1985) and susceptibility to transmission errors. Furthermore, in the case of passive transponders (the transponder’s power supply is drawn from the HF field of the reader) the power supply must not be interrupted by an inappropriate combination of signal coding and modulation procedures.
6.2Digital Modulation Procedures
Energy is radiated from an antenna into the surrounding area in the form of electromagnetic waves. By carefully influencing one of three signal parameters — power, frequency, phase position — of an electromagnetic wave, messages can be coded and transmitted to any point within the area. The procedure of influencing an electromagnetic wave by messages (data) is called modulation, and an unmodulated electromagnetic wave is called a carrier.
By analysing the characteristics of an electromagnetic wave at any point in the area, we can reconstruct the message by measuring the change in reception power, frequency or phase position of the wave. This procedure is known as demodulation.
Classical radio technology is largely concerned with analogue modulation procedures. We can differentiate between amplitude modulation, frequency modulation and phase modulation, these being the three main variables of an electromagnetic wave. All other modulation procedures are derived from one of these three types. The procedures used in RFID systems are the digital modulation procedures ASK (amplitude shift keying), FSK (frequency shift keying) and PSK (phase shift keying) (Figure 6.5).
In every modulation procedure symmetric modulation products — so-called sidebands — are generated around the carrier. The spectrum and amplitude of the sidebands are influenced by the spectrum of the code signal in the baseband and by the modulation procedure. We differentiate between the upper and lower sideband.
6.2.1 Amplitude shift keying (ASK)
In amplitude shift keying the amplitude of a carrier oscillation is switched between two states u0 and u1 (keying) by a binary code signal. U1 can take on values between u0 and 0. The ratio of u0 to u1 is known as the duty factor m.
6.2 DIGITAL MODULATION PROCEDURES |
187 |
P 
Carrier
Sideband
f
Figure 6.5 Each modulation of a sinusoidal signal — the carrier — generates so-called (modulation) sidebands
To find the duty factor m we calculate the arithmetic mean of the keyed and unkeyed amplitude of the carrier signal:
uˆ m = uˆ 0 + uˆ 1
2
The duty factor is now calculated from the ratio of amplitude change
the mean value uˆ m: |
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uˆ m |
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uˆ 0 − uˆ m |
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uˆ 0 − uˆ 1 |
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m |
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u |
u |
= u |
u |
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ˆ m |
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ˆ m |
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ˆ 0 |
+ ˆ 1 |
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(6.1)
uˆ 0 − uˆ m to
(6.2)
In 100% ASK the amplitude of the carrier oscillation is switched between the carrier amplitude values 2uˆ m and 0 (On-Off keying; Figure 6.6). In amplitude modulation using an analogue signal (sinusoidal oscillation) this would also correspond with a modulation factor of m = 1 (or 100%) (Mausl,¨ 1985).
The procedure described for calculating the duty factor is thus the same as that for the calculation of the modulation factor for amplitude modulation using analogue
∆ûm |
û0 |
ûm |
û1 |
t |
m = 0.5; (ASK 50%) |
Figure 6.6 In ASK modulation the amplitude of the carrier is switched between two states by a binary code signal