Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
2002-05-29
2004-08-31
Nguyen, Tony T. (Department: 2685)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S323000, C455S296000, C455S302000
Reexamination Certificate
active
06785529
ABSTRACT:
BACKGROUND
1. Technical Field
The disclosed system and method generally relates to the field of wireless communications. More particularly, the disclosed system and method relates to a system and method for compensating for I-Q mismatch in a low intermediate frequency (IF) or zero IF receiver.
2. Description of Related Art
Radio receivers, such as a heterodyne receiver, have long been used for radio communication. With a heterodyne architecture, a radio frequency signal is detected by a tuner, or other radio frequency device coupled to an antenna. An internal oscillator, called a local oscillator, is supplied to a mixer along with the radio frequency signal. The mixer produces output signals at both the sum and difference between the radio frequency and the local oscillator. The output of this stage is usually designated as an intermediate frequency (IF). Because the IF is still relatively high frequency, conventional filtering techniques may be used to eliminate one set of output signals from the mixer (i.e., either the sum or the difference signals).
Heterodyne techniques have been used in many kinds of receivers. For example, wireless communication devices, such as cellular telephones, often use heterodyne architecture. However, this architecture requires additional circuitry, power consumption and additional expense to build the device. Thus, new system architectures are arising in which the IF circuitry is eliminated. Receivers employing this architecture are sometimes referred to as zero IF receivers. In this application, the local oscillator mixes the radio frequency signal directly to baseband frequencies. In a similar architecture, designated as a low IF architecture, the local oscillator mixes the RF signal down to an IF. However, the IF is a very low frequency and thus does not permit the conventional filtering to remove the undesirable image band interference as is common in heterodyne architectures, as described above.
The zero IF and low IF receivers have virtually identical front end circuitry. An example of this system architecture is illustrated in
FIG. 1
in which a quadrature receiver employs zero IF or low IF architecture. As illustrated in
FIG. 1
, a conventional system
10
includes an antenna
12
coupled to a radio frequency (RF) stage
14
. The RF stage
14
may include amplifiers, filters, tuning elements, and the like. Details of the RF stage
14
are known to those skilled in the art and need not be described herein. The RF stage
14
operates in conjunction with the antenna
12
to detect a modulated RF signal and generates an electrical signal corresponding thereto.
The conventional system
10
also includes an RF splitter
16
, which generates two identical copies of the signal from the output of the RF stage
14
. The RF splitter
16
may be an electrical circuit or, in its simplest implementation, it may simply be a wire connection. In some implementations, the RF splitter
16
may be implemented as part of the RF stage
14
.
The two identical signals are provided to RF inputs of a mixer
20
and a mixer
22
. The mixers
20
and
22
each include a local oscillator input. The mixer
20
is provided with the local oscillator signal, designated as an “I” local oscillator. The mixer
22
is provided with a local oscillator signal, designated as a “Q” local oscillator. The local oscillator signals I and Q are identical in frequency, but have a 90° phase shift with respect to each other. Techniques for producing these quadrature signals are known in the art and need not be described in detail herein. The output of the mixers
20
and
22
are provided to low-pass filters
24
and
26
, respectively. In an exemplary embodiment, the filters
24
and
26
are low-pass filters. The resultant signal generated by the filter
24
is a baseband (or near baseband) signal I(t). Similarly, the resultant signal generated by the filter
26
is a baseband (or near baseband) signal Q(t).
In ideal circumstances, the quadrature signals provided by the I local oscillator and the Q local oscillator are separated by precisely 90°. The resulting I and Q outputs would, ideally, have equal amplitudes. Further, an ideal system would have precisely matched mixers
20
and
22
and matched filters
24
and
26
. Under these ideal circumstances, the output I(t) and Q(t) are truly orthogonal. That is, there is no projection of the I(t) signal into the Q(t) signal and vice-versa.
Unfortunately, such ideal circuits do not exist. Even with close matching of the mixers
20
and
22
and the filters
24
and
26
, some phase and/or gain errors will result. This undesirable circuit mismatch in the I and Q circuits results in output signals I(t) and Q(t) that are not truly orthogonal. That is, the I(t) signal may project onto the Q(t) signal and vice-versa. The results of this circuit mismatch are illustrated in
FIGS. 2A and 2B
. The results of circuit mismatch affect both I(t) and Q(t); thus, we will consider the complex spectrum of the quadrature signals in the discussion with respect to
FIGS. 2A and 2B
.
FIG. 2A
is an RF spectrum. Those skilled in the art will recognize that, for the sake of convenience, the RF spectrum is not drawn to scale. The RF spectrum includes a line
30
representing the I local oscillator signal. The desirable signal is indicated by a portion
32
of the spectrum.
FIG. 2A
also illustrates what are designated as “jammer” signals that are present due to adjacent channels or alternate channels. The adjacent channel, separated from the carrier frequency of the desired signal by 30 kilohertz (kHz), is indicated by a portion
34
of the spectrum labeled as the J_
30
signal.
Telecommunications standard IS-98B, entitled “RF Performance for Dual-Mode Mobile Telephones,” specifies the measurement of certain interference signals using a jammer signal that is separated from the desired carrier frequency by 60 kHz. A portion
36
of the spectrum indicates the presence of the J-
60
jammer signal. In addition,
FIG. 2A
illustrates a portion
38
of the spectrum resulting from a jammer signal J_
120
, which is separated from the carrier frequency of the desired signal
32
by 120 kHz.
Those skilled in the art will appreciate that the spectrum is symmetrical about the DC axis (0 Hz). Thus, the spectrum
32
of the desired signal has a mirror image spectrum
32
′, which is centered at the minus carrier frequency. Similarly, the spectrum
34
,
36
, and
38
each have mirror image spectra
34
′,
36
′, and
38
′, respectively.
FIG. 2A
also illustrates a line
40
indicating a portion of the spectrum resulting from a local oscillator signal due to mismatch between the I and Q portions of the circuit illustrated in the example circuit of FIG.
1
. The mixers
20
and
22
multiply the signals in the RF spectrum by the value of the local oscillator
30
. The result of processing the portions
32
-
38
and
32
′-
38
′ by the local oscillator
30
is effectively a shift in frequency of all components in the spectrum of FIG.
2
A. Following processing by the mixers (e.g., the mixer
20
) and the filters (e.g., the filter
24
), the I circuit of
FIG. 1
produces the baseband spectrum illustrated in FIG.
2
B. The spectral portions
32
-
38
and
32
′-
38
′ have effectively been shifted to the right by the frequency of the local oscillator. As a result, the portion
32
′ of the spectrum, which represents the desired signal, is now centered at 15 kHz. Similarly, the portions
34
′,
36
′, and
38
′ of the spectrum have been frequency shifted and are now centered at −15 kHz, −45 kHz, and −105 kHz, respectively. At the same time, the portions
32
-
38
of the spectrum (see
FIG. 2A
) have been shifted to a much higher frequency level and are not illustrated in FIG.
2
B. Those portions of the spectrum are undesirable and are readily removed using conventional techniques.
The mismatch local oscillator
40
also interacts with the portions
32
-
38
and
32
′-
38
′ of the
Ciccarelli Steven C.
Raghupathy Arun
Reeves Robert
Brown Charles D.
Nguyen Tony T.
Qualcomm Incorporated
Sao Howard H.
Wadsworth Philip
LandOfFree
System and method for I-Q mismatch compensation in a low IF... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System and method for I-Q mismatch compensation in a low IF..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System and method for I-Q mismatch compensation in a low IF... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3360622