Modulators – Phase shift keying modulator or quadrature amplitude modulator
Reexamination Certificate
2002-08-29
2003-12-30
Wells, Kenneth B. (Department: 2817)
Modulators
Phase shift keying modulator or quadrature amplitude modulator
C332S104000, C330S129000, C375S308000, C455S127200
Reexamination Certificate
active
06670861
ABSTRACT:
FIELD OF THE PRESENT INVENTION
The present invention is directed to wideband modulation in a communication device using a phase-locked loop and voltage-controlled oscillator. More particularly, the present invention is directed to a wideband modulation summing network that scales a voltage signal being fed to a voltage-controlled oscillator and a calibration gain circuit that controls the scaling of the modulated signal.
BACKGROUND OF THE PRESENT INVENTION
Phase-locked loops are used in a variety of applications such as clock recovery, frequency and phase modulation, and frequency synthesizers. A voltage-controlled oscillator is a central design element of the phase-locked loop, whereby the voltage-controlled oscillator produces an output frequency proportional to its input voltage. A typical drawback of a voltage-controlled oscillator is its uncertainty in output frequency to the applied input voltage due to integrated circuit process variations. This leads to the need for a voltage-controlled oscillator having a large gain if a wide output frequency range is required. The large voltage-controlled oscillator gain also has the effect of producing a large variation in the output frequency in response to any noise in the applied input voltage, also known as phase noise. This phase noise at the voltage-controlled oscillator output is undesirable as this limits the accuracy of the output signal.
As noted above, a common application of voltage-controlled oscillators are within wireless communication systems. Wireless communication systems typically require frequency synthesis in both the receive path circuitry and the transmit path circuitry. For example, cellular phone standards in the United States and Europe define a cellular telephone system with communication centered in two frequency bands at about 900 MHz and 1800 MHz.
A dual band cellular phone is capable of operating in both the 900 MHz frequency band and the 1800 MHz frequency band. Within the frequency bands, the cellular standards define systems in which base station units and mobile units communicate through multiple channels, such as 30 kHz (IS-54) or 200 kHz (GSM) wide channels. For example, with the IS-54 standard, approximately 800 channels are used for transmitting information from the base station to the mobile unit, and another approximately 800 channels are used for transmitting information from the mobile unit to the base station. A frequency band of 869 MHz to 894 MHz and a frequency band of 824 MHz to 849 MHz are reserved for these channels, respectively.
Because the mobile unit must be capable of transmitting and receiving on any of the channels for the standard within which it is operating, a frequency synthesizer must be provided to create accurate frequency signals in increments of the particular channel widths, such as for example 30 kHz increments in the 900 MHz region.
Phase-locked loop circuits including voltage-controlled oscillators are often used in mobile unit applications to produce the desired output frequency. An example of a phase-locked loop circuit in mobile applications is illustrated in
FIGS. 1 and 2
.
FIG. 1
is a block diagram example of a receive path circuitry
150
for a prior art wireless communication device, such as a mobile unit in a cellular phone system. An incoming signal is received by the antenna
108
, filtered by a band-pass filter
110
, and amplified by a low noise amplifier
112
. This received signal is typically a radio-frequency signal, for example a 900 MHz or 1800 MHz signal. This radio-frequency signal is usually mixed down to a desired intermediate frequency before being mixed down to baseband. Using a reference frequency (f
REF
)
106
from a crystal oscillator
105
, frequency synthesizer
100
provides an RF mixing signal RF
OUT
102
to mixer
114
. Mixer
114
combines this RF
OUT
signal
102
with the filtered and amplified input signal
113
to produce a signal
115
that has two frequency components. The signal is filtered by band-pass filter
116
to provide an IF signal
117
. This IF signal
117
is then amplified by variable gain amplifier
118
before being mixed down to baseband by mixers
122
and
124
.
Signal processing in mobile phones is typically conducted at baseband using in-phase (I) and quadrature (Q) signals. The Q signal is offset from the I signal by a phase shift of 90 degrees. To provide these two signals, an IF mixing signal
104
and a dual divide-by-two and quadrature shift block
120
may be utilized. Frequency synthesizer
100
generates an IF
OUT
signal
104
; for example, at about 500 MHz; that is divided by 2 in block
120
to provide mixing signals
119
and
121
. Block
120
delays the signal
121
to mixer
122
by 90 degrees with respect to the signal
119
to mixer
124
.
Block
120
may be implemented with two flip-flop circuits operating off of opposite edges of the signal
104
, such that the output of the flip-flops are half the frequency of the signal
104
and are 90 degrees offset from each other. The resulting output signals
123
and
125
have two frequency components.
Assuming the baseband frequency is centered at DC, the signal is filtered using low-pass filters
126
and
128
. The resulting baseband signal
123
is the Q signal, and the resulting baseband signal
125
is the I signal. These signals
123
and
125
may be further processed at baseband by processing block
130
and provided to the rest of the mobile phone circuitry as I and Q signals
131
and
132
.
FIG. 2
is a block diagram of a prior art phase-locked loop circuitry
200
for synthesizing one of the frequencies required by frequency synthesizer
100
. A second phase-locked loop circuit may be implemented to provide the second frequency.
The reference frequency
106
is received by a divide-by-R counter
204
, and the output frequency
102
is received by a divide-by-N counter
214
. The resulting divided signals
216
and
218
are received by a phase detector
206
. The phase detector
206
determines the phase difference between the phase of the divided signal
216
and the phase of the divided signal
218
. The phase detector
206
uses this phase difference to drive a charge pump
208
. The charge pump
208
provides a voltage output that is filtered by a loop filter
210
to provide a voltage control signal
220
. The voltage control signal
220
controls the output frequency
102
of a voltage-controlled oscillator
212
.
For a typical mobile phone application, the frequency
104
will remain constant, while the frequency
102
will change depending upon the channel of the incoming signal. Thus, a first phase-locked loop may be used to provide the frequency
104
, and its N and R values may be programmed once and then left alone. A second phase-locked loop may be used to provide the frequency
102
, and its N and R values may be selectively programmed to provide the desired signal
102
. If desired, the R value for this second phase-locked loop may be programmed once and left alone, while the N value may be used to select the desired signal
102
.
The typical transmit path circuitry (not shown) for a wireless communication device, such as a mobile unit in a cellular phone system, may include circuitry to move the outgoing signal from baseband to an RF transmission frequency. A transmit frequency band for cellular phone systems typically includes the identical number of channels as included within the receive frequency band. The transmit channels, however, are shifted from the receive channels by a fixed frequency amount.
As noted above, the phase-locked loop circuitry typically utilizes a phase detector to monitor phase differences between the divided reference frequency and the divided output frequency to drive a charge pump. The charge pump delivers packets of charge proportional to the phase difference to a loop filter.
The loop filter outputs a voltage that is connected to the voltage-controlled oscillator to control its output frequency. The action of this feedback loop attempts to drive the phase difference to zero to provi
Analog Devices Inc.
Samuels , Gauthier & Stevens, LLP
Wells Kenneth B.
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