Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
2000-11-22
2004-08-24
Urban, Edward F. (Department: 2685)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S076000, C455S317000, C455S324000, C331S012000
Reexamination Certificate
active
06782249
ABSTRACT:
FIELD
Disclosed embodiments of the present invention relate to circuits, and more particularly, to radio frequency communication circuits.
BACKGROUND
In a direct conversion radio receiver or transceiver, an RF (Radio Frequency) signal is directly down-converted to a baseband signal without first down-converting to an IF (Intermediate Frequency) signal. For many types of receivers, both the inphase and quadrature components of the baseband signal are employed for further demodulation or detection. A simplified functional diagram of a transceiver employing direct conversion is illustrated in FIG.
1
.
In
FIG. 1
, antenna
102
receives an RF signal. When the transceiver is in its receive mode, the received RF signal is filtered by bandpass filter
104
and mixed with quadrature RF local oscillator signals cos(&ohgr;
c
t) and sin(&ohgr;
c
t) by mixers
106
and
108
, respectively, where &ohgr;
c
is the carrier frequency in radians/sec. The output signals of mixers
106
and
108
are filtered by low pass filters
110
and
112
, respectively, to provide the inphase (I) and quadrature (Q) components. Detector
114
further processes the I and Q components to provide the final digital data to the end user. Detector
114
may employ matched filtering, error detection and correction, spread-spectrum filtering (de-spreading), and other forms of detection appropriate for the particular modulation and coding scheme that is employed. When the transceiver is in its transmit mode, digital data provided by the end user is encoded by encoder
116
, modulated to a baseband signal by modulator
118
, up-converted via mixer
120
by mixing with cos(&ohgr;
c
t) and band pass filtered by filter
122
. The quadrature RF local oscillator signals cos(&ohgr;
c
t) and sin(&ohgr;
c
t) may be obtained from VCO (Voltage Controlled Oscillator)
126
within PLL (Phase Lock Loop)
124
.
If the oscillation frequency of PLL
124
is substantially the same as the carrier frequency, &ohgr;
c
, then VCO
126
may be susceptible to “frequency pulling” during transmission of an RF signal. This may be caused by VCO
126
locking on to the frequency of the transmitted RF signal during transmission because of unintended feedback. If FSK (Frequency Shift Key) modulation is employed, then the transmitted RF signal has a varying instantaneous frequency, possibly centered around &ohgr;
c
. VCO
126
cannot instantly return back to the carrier frequency &ohgr;
c
when the transceiver enters its receive mode, in which case a received RF signal may not be accurately mixed down to its baseband I and Q components.
VCO
126
is less susceptible to frequency pulling if its oscillation frequency is substantially different from &ohgr;
c
. Thus, one approach to mitigating frequency pulling of VCO
126
is to run VCO
126
at twice the carrier frequency, and then synthesize the quadrature mixing signals by a divide-by-two division circuit. This is illustrated in
FIG. 2
, where VCO
202
oscillates at 2&ohgr;
c
and JK flip flop
204
is configured as a divide-by-two division circuit. One of the quadrature mixing components is taken from output port
206
of JK flip flop
204
. The other quadrature mixing component is taken at output port
208
of XOR (exclusive-OR) gate
210
.
The quadrature mixing signals in
FIG. 2
are square waves, whereas in
FIG. 1
the quadrature mixing signals are represented by analog sinusoids, e.g., cos(&ohgr;
c
t) and sin(&ohgr;
c
t). It is to be understood that
FIG. 1
is a representation of a communication circuit at a functional level, and that for particular implementations, the quadrature mixing signals may be square waves of frequency &ohgr;
c
rather than analog sinusoids of frequency &ohgr;
c
. It is to be understood throughout these letters patent that sinusoids may be interpreted as square waves, or other discrete-valued periodic waveforms.
As carrier frequencies are increased for communication circuits, running a VCO at twice the carrier frequency may add unwanted cost. For example, in a communication circuit for the consumer market, such as a transceiver circuit in a cordless phone, the carrier frequency may be on the order of 2.5 GHz, so that running a VCO at 5 GHz may add to its manufacturing cost.
REFERENCES:
patent: 4246539 (1981-01-01), Haruki et al.
patent: 5822366 (1998-10-01), Rapeli
patent: 5896562 (1999-04-01), Heinonen
patent: 5953643 (1999-09-01), Speake et al.
patent: 6016422 (2000-01-01), Bartusiak
Intel Corporation
Kalson Seth Z.
Le Lana
Urban Edward F.
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