MOTOROLA
Technical
Developments
CARRIER
SIGNAL PATTERN DEMODULATOR
by
Michael Burri
BACKGROUND
A
method of detecting data in an electronic reader interface, received from a
contact less smart card or tag transponder, in which the reader supplies power
to the contact less card or transponder via a radio frequency transmit loop
antenna. The same transmit antenna is also used to receive back scattered
signal from the card or tag device which is perfonned with an admittance load
switching device inside the card device.
The
transmitted data from the card can be modulated directly on the carrier or on
sub-carrier. The rate of the modulated data is synchronized with the internal
frequency generating the carrier frequency.
Different
type of modulation schemes may be used which have to be decoded to obtain the
NRZ data infonnation.
THE
SOLUTION
The
design proposed is a ROM pattern demodulator capable of detecting different
schemes of digital signals shown in Figure 1. For instance, BPSK and OOK
signals can be detected and demodulated taking into account their
characteristics such as the mark/space ratio and of the fact that they can be
coded in Manchester for instance. Also, a certain amount of noise disturbing
the pattern of the signal can be predetermined allowing the demodulator to
extract the data information.
Synchronized
Sampling Clock
Incoming
Encoded Signal
Shift
Register
ROM
Signal
Status Outputs
Fig. 1
ROM Pattern Demodulator
Figure
2 shows the electrical diagram of the Pattern demodulator configured to decode
BPSK/OOK signals. BPSK means Bi-Phase Shift Keying. Bi-Phase means they are two
conditions which will specify the data bit information lor 0 and Shift Keying
is defined by shifting of the carrier signal by 180 degrees each time the data
information bit changes its status from O to lor 1 to 0.
The
Figure 3 shows the wave fonn of modulated data at 107 kBaud rate. The
demodulated data has a constant output delay of one period of the carrier
signal related to the modulated input signal.
Motorola,
Inc. 2000
Different
Baud rate can be demodulated theoretically up to the half of the frequency of
the incoming encoded signal.
OOK
means On Off Keying. The carrier is ON (active) or Off (not active).
In the
example, shown in Figure 3, the BPSK/OOK signals have been encoded on a
847.5kHz carrier frequency and are input to the 8bit Shift Register which is
clocked at 3.39MHz.
Thus,
the sampling resolution is 4 samplings for one period of the carrier signal. The
parallel out-
148
May
2000
MOTOROLA
Technical
Developments
847.5
kHz encoded signa[s
D 8 bit
Shift Reg C
3.39
MHz
A7
A6A5A4A3A2A 1 AO DO 01 02 03 04 05 06 07
ROM
DEMODULATED
BPSK
RS FF
BPSR
SIGNAL
PATTERN OK
NOISE
(ev. trend to O or to 1} IDLE at -1IDLE at -0SUB-CARRIER ACTIVE .. .. MCU
.,......""""....""""",
"...."".."""""",
, ,
, ,
, ,
, ,
,
~ Q R C
3.39 MHz ~ ~ n n-bit counter ~
, ,
, I I I
, ftw I
~""
, .., 'S..~f..'S.." " , ~'} ,~, !~..e,c ,~~~y ,~, ,,~!~;
RS FF
OOK DEMOOULATEOOOK
Fig. 2
...
