Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Reexamination Certificate
2000-09-27
2003-01-07
Wamsley, Patrick (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
C341S118000, C331S017000
Reexamination Certificate
active
06504498
ABSTRACT:
FIELD OF INVENTION
The field of invention relates to wireless communication generally; and more specifically, to canceling an offset in a received signal.
BACKGROUND
Super Heterodyne and Frequency Shift Keyed (FSK) Modulation/Demodulation
FIG. 1
shows a portion
106
of a receiving device
166
referred to as a demodulator. A demodulator
106
provides a signal (commonly referred to as a baseband signal b(t) in various applications) that is representative of the information being sent from a transmitting device
165
to a receiving device
166
. The demodulator
106
extracts (i.e., demodulates) the baseband signal b(t) from a high frequency wireless signal that “carries” the baseband signal b(t) through the medium (e.g., airspace) separating the transmitting and receiving devices
165
,
166
.
The particular demodulator
106
example of
FIG. 1
is designed according to:
1
) a demodulation approach that is commonly referred to as super heterodyne detection (hereinafter referred to as a heterodyne detection for simplicity); and
3
) a modulation/demodulation scheme referred to as Frequency Shift Keying (FSK). The industry standard referred to as “BLUETOOTH” (the requirements of which may be found in “Specification of the Bluetooth System”, Core v.1.0B, Dec. 1, 1999, and published by the Bluetooth Special Interest Group (SIG)) can apply to both of these approaches and, accordingly, will be used below as a basis for reviewing the following background material.
Heterodyne detection is normally used when dedicated channels are allocated within a range of frequencies
111
(where a range of frequencies may also be referred to as a “band”
111
). For BLUETOOTH applications within the United States, 89 channels
110
1
,
110
2
,
110
3
, . . .
110
79
are carried within a 2.400 GHz to 2.482 GHz band
111
. Each of the 79 channels are approximately 1 Mhz wide and are centered at frequencies 1 Mhz apart.
The first channel
110
1
is centered at 2.402 Ghz, the second channel
110
2
is centered at 2.403 Ghz, the third channel
110
3
is centered at 2.404 Ghz, etc., and the seventy ninth channel
110
79
is centered at 2.480 Ghz. The heterodyne demodulator
106
accurately receives a single channel while providing good suppression of the other channels present within the band
111
. For example, if channel
110
2
is the channel to be received, the baseband signal b(t) within channel
110
2
will be presented while the baseband signals carried by channels
110
1
, and
110
3
through
110
79
will be suppressed.
An FSK modulation/demodulation approach is commonly used to transmit digital data over a wireless system. An example of an FSK modulation approach is shown in
FIG. 1. A
transmitting modulator
105
within a transmitting device
165
modulates a baseband signal at a carrier frequency f
carrier
into an antennae
102
. That is (referring to the frequency domain representation
150
of the signal launched into the antennae
102
) if the baseband signal corresponds to a first logic value (e.g., “1”), the signal
150
has a frequency of f
carrier
+fo. If the data to be transmitted corresponds to a second logic value (e.g., “0”), the signal has a frequency of f
carrier
−fo.
Thus, the signal launched into the antennae
102
alternates between frequencies of f
carrier
+fo and f
carrier
−fo depending on the value of the data being transmitted. Note that in actual practice the transmitted signal
150
may have a profile
151
that is distributed over a range of frequencies in order to prevent large, instantaneous changes in frequency. The carrier frequency f
carrier
corresponds to the particular wireless channel that the digital information is being transmitted within. For example, within the BLUETOOTH wireless system, f
carrier
corresponds to 2.402 Ghz for the first channel
110
1
. The difference between the carrier frequency and the frequency used to represent a logical value is referred to as the deviation frequency fo.
Referring now to the heterodyne demodulator
106
, note that the signal received by antennae
103
, may contain not only every channel within the frequency band of interest
111
, but also extraneous signals (e.g., AM and FM radio stations, TV stations, etc.) outside the frequency band
111
. The extraneous signals are filtered by filter
113
such that only the frequency band of interest
111
is passed. The filter
113
output signal is then amplified by an amplifier
114
.
The amplified signal is directed to a first mixer
116
and a second mixer
117
. A pair of downconversion signals d
1
(t), d
2
(t) that are 90° out of phase with respect to each other are generated. A first downconversion signal d
1
(t) is directed to the first mixer
116
and a second downconversion signal d
2
(t) is directed to the second mixer
117
. Each mixer multiples its pair of input signals to produce a mixer output signal. Note that the transmitting modulator
105
may also have dual out of phase signals that are not shown in
FIG. 1
for simplicity. Transmitting a pair of signals that are 90° out of phase with respect to one another conserves airborne frequency space by a technique referred to in the art as single sideband transmission.
The frequency f
down
of both downconversion signals d
1
(t), d
2
(t) is designed to be f
carrier
−f
IF
. The difference between the downconversion frequency f
down
and the carrier frequency f
carrier
is referred to as the intermediate frequency f
IF
. Because it is easier to design filters
118
a,b
and
127
a,b
that operate around the intermediate frequency, designing the downconversion that occurs at mixers
116
,
117
to have an output term at the intermediate frequency f
IF
enhances channel isolation.
The mixer
117
output signal may be approximately expressed as
kb
FSK
(
t
)cos(2
&pgr;f
carrier
t
)cos(2
&pgr;f
down
t
). Eqn. 1
Note that Equation 1 is equal to
kb
FSK
(
t
)[cos(2&pgr;(
f
carrier
−f
down
)
t
)+cos(2&pgr;(
f
carrier
+f
down
)
t
)] Eqn. 2
which is also equal to
kb
FSK
(
t
)cos(2
&pgr;f
IF
t
)+kb
FSK
(
t
)cos(2&pgr;(
f
carrier
+f
down
)
t
) Eqn. 3
using known mathematical relationships. The b
FSK
(t) term represents a frequency shift keyed form of the baseband signal (e.g., a signal that alternates in frequency between +fo for a logical “1” and −fo for a logical “0”). The constant k is related to the signal strength of the received signal and the amplification of amplifier
114
. For approximately equal transmission powers, signals received from a nearby transmitting device are apt to have a large k value while signals received from a distant transmitting device are apt to have a small k value.
Equation 3 may be viewed as having two terms: a lower frequency term expressed by kb
FSK
(t)cos(2&pgr;f
IF
t) and a higher frequency term expressed by kb
FSK
(t)cos(2&pgr;(f
carrier
+f
down
)t) Filter
118
b
filters away the high frequency term leaving the lower frequency term kb
FSK
(t)cos(2&pgr;f
IF
t) to be presented at input
119
of amplification stage
125
. Note that, in an analogous fashion, a signal kb
FSK
(t)sin(2&pgr;f
IF
t) is presented at the input
126
of amplification stage
170
.
Amplification stage
125
has sufficient amplification to clip the mixer
117
output signal. Filter
127
b
filters away higher frequency harmonics from the clipping performed by amplification stage
125
. Thus, amplification stage
125
and filter
127
b
act to produce a sinusoidal-like waveform having approximately uniform amplitude for any received signal regardless of the distance (e.g., k factor) between the transmitting device and the receiving device.
After filter
127
, a signal s(t) corresponding to Ab
FSK
(t)cos(2&pgr;f
IF
t) is presented to the frequency to voltage converter
128
input
129
(where A reflects the uniform amplitude discussed above). The spectral content S(f) of the signal s(t) at the frequency to voltage converter
128
input
129
is shown at FIG.
1
. The signal s(t) alternates between a fre
Parthus Ireland Limited
Wamsley Patrick
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