Demodulators – Frequency modulation demodulator
Reexamination Certificate
2001-12-21
2003-08-12
Mis, David C. (Department: 2817)
Demodulators
Frequency modulation demodulator
C329S327000
Reexamination Certificate
active
06605989
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a demodulation circuit, and more particularly relates to an apparatus using a single input signal and integrating all demodulation devices therein.
BACKGROUND OF THE INVENTION
Wireless communication uses electromagnetic waves as the medium to send a signal (data). This use of electromagnetic waves to carry a signal (data) is called a modulation process. Conversely, retrieving data from an electromagnetic wave is called demodulation. An electromagnetic wave that carries a signal is called a carrier wave and the signal thus carried is called a signal wave or a modulating wave. A modulated carrier wave is called a “modulated wave”. There are three methods of modulation, the “amplitude modulation method”, the “frequency modulation method”, and the “phase modulation method”.
This means that the frequency of the carrier wave changes according to the frequency of the modulating wave in the frequency modulation method. For example, assuming that the signal of the carrier wave is Vc and the signal of the modulating wave is Vs, the frequency modulation wave V
FM
is as follows:
v
c
=&egr;
c
sin &ohgr;
t
(&ohgr;=2
&pgr;f
)
v
s
=&egr;
s
cos
pt
(
p
=2
&pgr;f
)
v
FM
(
t
)=&egr;
c
sin(&ohgr;
t=&bgr;
F
sin
pt
)
where &bgr;
F
is the index of the frequency modulation.
The frequency modulation method shifts the original frequency value of carrier wave by a value proportional to the frequency of the modulating wave. The process of drawing the modulating wave Vs out from the frequency modulation wave V
FM
is called frequency demodulation. Reference is made to
FIG. 1
, a schematic drawing of the demodulator
101
being used to demodulate the modulating wave from the frequency modulation wave.
Different demodulating methods can be used in the demodulator
101
. For example, the frequency variation can be transferred to amplified variation, after which an amplified demodulation process is performed to draw out the modulating wave. Alternately, the frequency variation can be transferred to phase variation, after which a phase demodulation process is performed to draw out the modulating wave.
The conventional quadrature demodulator uses the second method to perform the demodulation process. Reference is now made to
FIG. 2
, a description of the demodulation process. Assuming that the frequency modulation wave is V
FM1
,
v
FM1
(
t
)=&egr;
1
sin(&ohgr;
t+&bgr;
F
sin
pt
)=&egr;
1
sin &agr;
&agr;=
wt+&bgr;
F
sin
pt
The quadrature phase-shift apparatus
201
receives the frequency modulation wave V
FM1
and generates a quadrature output signal V
FM2
:
v
FM2
(
t
)=&egr;
2
sin(&agr;+&pgr;/2)
The two waves V
FM1
and V
FM2
are then multiplied together by the multiplier stage
202
to generate an output signal:
v
FM1
(
t
)
×
v
FM2
(
t
)
=
ϵ
1
sin
α
×
ϵ
2
sin
(
α
+
π
2
)
=
ϵ
1
sin
α
×
ϵ
2
cos
α
=
1
2
ϵ
1
ϵ
2
sin
2
α
From the foregoing description, the phase of the output signal demodulated by the conventional quadrature demodulator gives rise to a second order frequency component (
2
&agr;). Therefore, it is necessary to uses the low pass filter
203
to eliminate the second order frequency component.
However, the foregoing method has a major drawback. An output wave having the second order frequency component (
2
&agr;) is generated in the phase when the two waves V
FM1
and V
FM2
are input into the multiplier stage
202
. Therefore, a high quality low pass filter
203
is needed in the conventional demodulator circuit to eliminate the second order frequency component (
2
&agr;). The low pass filter
203
must be highly selective because the second order frequency component (
2
&agr;) is very close to the required frequency &agr; of the low pass filter
203
is required. The high selectivity means that the slope of the frequency response curve is high. Therefore, the low pass filter
203
and the demodulator circuit may not form an integrated circuit because the low pass filter
203
is composed of discrete devices.
FIG. 3
illustrates another demodulation method. The Philips company discloses the method in the U.S. Pat. No. 5,341,107. The in-phase (I) and quadrature (Q) modulated signals are input mutually into the complex circuit
301
to generate two modulated signals having time delay, respectively, in the phase (I
d
) and quadrature (Q
d
) signals. Then, the in phase and delay time modulated signal (I
d
) and the quadrature modulated signal (Q) are multiplied together by the multiplier stage
302
to generate an output signal. At the same time, the quadrature and delay time modulated signal (Q
d
) and the in phase modulated signal (I) are multiplied together by the multiplier stage
303
to generate an output signal. Then, the two output signals are subtracted from each other in the differential stage
304
to generate the modulating signal Vs.
Although the method provided by the Philips Company may integrate all demodulation devices including the low pass filter into one circuit, two modulated signals, in phase and quadrature ones, are necessary in the input terminals. Therefore, two circuits are needed to process the in phase and quadrature modulated signals at the same time before these two signals are received by the complex circuit
301
. This will not only requires additional circuits but also consumes more power.
SUMMARY OF THE INVENTION
To resolve the forgoing problems, the main purpose of the present invention is to provide a new design demodulator that eliminates the second order frequency of demodulated signal. Therefore, use of a low pass filter having high selectivity to filter the frequency is no longer necessary. In other words, all demodulation devices and the low pass filter may be combined into an integrated circuit according to the present invention. Moreover, the demodulator of the present invention only requires one modulated input signal to perform the demodulating process. Therefore, only one circuit is required to handle the modulated signal before the demodulator begins to process the signal. This is not similar to Philips' invention that requires two circuits and so may save power.
The demodulator of the present invention comprises two, the first and the second, I-Q splitters with constant group delay apparatuses. When operated, the first I-Q splitter with constant group delay apparatus first receives the modulated input signal I and then generates two modulated output signals. One is modulated signal I
&tgr;1
which is in phase and has delay &tgr;
1
time compared with I and the other is modulated signal Q
&tgr;1
which is in quadrature and has delay &tgr;
1
time compared with I. Then, the second I-Q splitter with constant group delay apparatus receives the modulated signal I
&tgr;1
and generates two modulated output signals. One is modulated signal I
&tgr;1+&tgr;2
which is in phase and has a delay &tgr;
2
time compared with I
−1
and the other is modulated signal Q
&tgr;1+&tgr;2
which is in quadrature and has a delay &tgr;
2
time compared with I
&tgr;1
. Next, in phase and having delay &tgr;
1
time modulated signal I
&tgr;1
and in quadrature and having delay &tgr;
1
+&tgr;
2
time Q
&tgr;1+&tgr;2
are multiplied together by the first multiplier stage to generate an output signal. At the same time, in phase and having delay &tgr;
1
+&tgr;
2
time modulated signal I
&tgr;1+&tgr;2
and in quadrature and having delay &tgr;
1
time modulated signal Q
&tgr;1
are also multiplied together by the second multiplier stage to generate another output signal. Finally, the two output signals are subtracted from each other in the differential stage to demodulate the modulated signal I. It is noted that &tgr;
1
is the delay time of the first I-Q split
Industrial Technology Research Institute
Mis David C.
Nath & Associates PLLC
Novick Harold L.
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