Digital I/Q imbalance compensation

Details Digital I/Q imbalance compensation Digital I/Q imbalance compensation

1998-09-25

2001-12-11

Le, Amanda T. (Department: 2634)

Pulse or digital communications

Receivers

Angle modulation

C375S349000, C455S314000, C329S306000

Reexamination Certificate

active

06330290

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a demodulator and more specifically to an arrangement for compensating I/Q imbalances caused by imbalances in the receive chain of a communication terminal.
BACKGROUND OF THE INVENTION
The need for lower cost transceivers is continuously increasing as the use for wireless communication terminals is expanding at a remarkable rate. Among one of the various designs employed in such terminals is an arrangement that includes a super heterodyne receiver, which includes an image-reject filter at the antenna input. Although this arrangement provides for a good quality reception, it tends to be costly and complicated.
Recently, the super heterodyne receiver has been replaced by a less costly design referred to as a low IF receiver which applies. RF image-reject mixing. RF image-reject mixers avoid the need for image-reject filters at the input and enable conversion of radio-frequencies at a greatly reduced cost.
A disadvantage of RF image-reject mixing designs is signal imbalances that are generated by the signal splitter unit that is coupled to the local oscillator employed for demodulation.
FIG. 1
illustrates a typical low-IF receiver
10
that employs an image-reject mixing design. Antenna
12
receives radio-frequency signals that are filtered via low-noise amplifier
14
, and fed to a mixing demodulator
18
via low-noise amplifier
16
. Mixing demodulator
18
includes an RF mixing stage
30
, which functions as an intermediate frequency converter of receiver
10
. RF mixing stage
30
is configured as a quadrature demodulator comprising an in-phase and quadrature-phase branches respectively. A local oscillator
60
provides a sinusoidal signal to a signal splitter
20
. The output ports of signal splitter
20
provide an in-phase frequency signal and a quadrature frequency signal to each of these branches via mixers
22
and
24
respectively, so as to demodulate and shift the frequency range of the received signal from radio-frequency, such as 900 Mhz to an intermediate range such as 100 Khz. Each branch also includes an automatic gain control and filtering unit
26
and an analog to digital converter
27
, so as to provide IF digital signals to a second IF mixing stage
28
. The IF mixing stage of demodulator
18
functions as a base band demodulator, which is designed to shift the frequency range of signals provided by first mixing stage to a baseband region.
Intermediate frequency (IF) mixing stage
28
includes an in-phase branch that subdivides into two branches
32
and
34
. IF mixing stage
28
also includes a quadrature phase branch that subdivides into two branches
36
and
38
. Each branch
32
and
34
includes a mixer
40
and
44
respectively, which are configured to mix the in-phase component received from RF mixing stage
30
with an in-phase and quadrature phase local-oscillator signal received from local oscillator
60
so as to provide baseband in-phase signal I
1
and baseband quadrature signal IQ. Similarly, each branch
36
and
38
includes a mixer
42
and
48
respectively, which are configured to mix the quadrature phase component received from RF mixing stage
30
with an in-phase and quadrature phase local-oscillator signal received from local oscillator
60
so as to provide baseband in-phase signal Q
1
and baseband quadrature signal QQ.
Adders
52
and
54
are configured to add and subtract various baseband components obtained from second mixing stage as will be discussed later in more detail, so as to provide a signal with substantially small image components. It is noted that the image band component signals are caused by interference from adjacent channels which are mixed into the desired signal band intended for receiver
10
due to imbalances in the I/Q paths. The output signal of adders
52
and
54
are then provided to a digital signal processing
56
via digital filters
58
and
60
respectively.
As mentioned above, a significant disadvantage with receiver
10
is the need for extremely accurate splitter unit for the local oscillator to achieve the desired image rejection. Thus, it is important for such receivers that the in-phase and the quadrature phase components of the RF local oscillator
20
are exactly in quadrature and have equal amplitudes. Any phase or amplitude imbalances may directly decrease the image-reject capabilities of the receiver.
A common way to acquire the quadrature signal is by using a RC-CR circuit. When these circuits are employed in an integrated circuit (IC) arrangement, a desired tolerance may not be achieved resulting in a worse than acceptable image rejection. Some designs include poly-phase filters to generate accurate quadrature signals. However, such filters consume relatively high power.
Thus, there is a need for a receiver that provides accurate demodulation with substantially low image band components.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the invention a communications receiver provides a compensation arrangement that overcomes the phase and amplitude imbalances caused by the local oscillator employed in the receiver. The receiver includes a first RF mixing stage and a second IF mixing stage. The RF mixing stage includes an in-phase and a quadrature phase branch respectively that provide IF demodulated signals to the IF mixing stage via a phase splitter compensator unit. In accordance with one embodiment, the phase splitter compensator unit multiplies the in-phase and quadrature phase signals provided by the RF mixing stage by a predetermined compensation factor that among other things depend on the amplitude and phase imbalances caused by the local oscillator in the RF mixing stage.
During operation, a test tone signal is generated and provided to the receiver. Both desired and image band signals are measured and based on those measurements the value of the compensation factors are derived. The test tone signal may be generated externally at the fabrication stage of the receiver. Alternatively, the receiver may also generate a test tone to periodically adjust the compensation factor as it becomes necessary.


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José Páez-Borrallo and Francisco J. Casajús Quirós. Self adjusting digital image rejection receiver for mobile communications. In proceedings of the VTC'97, pp. 686-690, 1997.
Behzad Razavi. Design considerations for direct-conversion receivers. IEEE Transactions on Circuits and Systems II, 44(6):428-435, Jun. 1997.

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