Slope drift and offset compensation in zero-if receivers

Details Slope drift and offset compensation in zero-if receivers Slope drift and offset compensation in zero-if receivers

1996-08-13

2002-10-29

Vo, Don N. (Department: 2631)

Pulse or digital communications

Receivers

Particular pulse demodulator or detector

C375S346000

Reexamination Certificate

active

06473471

ABSTRACT:

FIELD OF THE DISCLOSURE
The present invention relates to radio receivers and more particularly to compensating radio receivers that are a special case of superheterodyne receivers having an intermediate frequency of zero.
BACKGROUND OF THE DISCLOSURE
In the field of radio receivers, there has been a concentrated effort to reduce the amount of tuned circuitry used in the receivers. By reducing the number of tuned circuits, large portions of the receiver can be integrated resulting in smaller receivers. These compact receivers can then be used in many areas such as cellular telephones. A major advance in the design of such receivers is a technique known as the “zero-IF” technique.
According to theory, an IQ radio receiver can be constructed according to
FIG. 1
, in which the radio signal S from the antenna
1
is applied directly to two balanced, quadrature mixers
2
a
,
2
b
(mathematically-multiplying devices) where the signal is multiplied respectively by a sine and cosine wave at the carrier frequency of signal S generated by a local oscillator
3
. In this manner, the I-channel or in-phase signal and the Q-channel or quadrature signal are generated. The multiplication devices yield outputs containing both sum frequency components around
2
f
and difference frequency components around zero frequency. DC or low pass filters
4
a
,
4
b
eliminate the former and accept the latter. The zero frequency components can then be amplified to any convenient level by low-frequency amplifying stages
5
a
,
5
b
instead of high frequency amplifiers. Essentially, the zero-IF receiver eliminates the interim conversion to an intermediate frequency by converting the incoming signal directly to baseband in a single operation.
In practice, this so-called zero-IF approach is beset with a variety of practical problems, one of which concerns the imperfection of the balanced mixers as compared to perfect mathematical multipliers. The most troublesome aspect of this imperfection is the generation of a DC offset or standing voltage that can be many orders of magnitude greater than the desired signal. The low frequency amplifiers, which receive the mixer outputs, can be forced into saturation by the large DC offset long before the desired signal has been amplified sufficiently.
To avoid premature saturation, RF amplifers can be added ahead of the mixers to raise the desired signal voltage level. Unfortunately, a common source of the offset is leakage from the local sinusoidal oscillator back to the antenna, producing coherent interference. As a result, RF amplification is not a satisfactory solution because the desired signal and coherent interference are amplified equally.
Another proposed solution used in conventional superheterodyne radio receivers is partial amplification of the input signal at the original antenna frequency. The partially amplified signal is then converted to a convenient intermediate frequency IF for further amplification before being applied to the balanced quadrature mixers. In this situation, the locally generated sine and cosine waves are at the IF rather than the antenna frequency, so leakage back to the antenna is of no consequence. However, by adding IF tuning circuitry, the simplicity and reduced size of the zero-IF receiver are lost. An alternative method of overcoming DC offset from the IQ mixers may employ the technique variously called AC coupling, DC blocking, high-pass filtering or differentiation to eliminate the standing or DC offset voltage. The trade-off with this method is the result that the DC and low-frequency components are lost or gravely distorted. This trade-off is unacceptable in digital transmission systems which use QPSK (Quadrature Phase Shift Keying) or MSK (Minimum Shift Keying) modulation techniques. These modulation techniques generate low frequency components that must be preserved.
U.S. Pat. No. 5,241,702 discloses a method of compensating for low frequency offset without losing or distorting the DC and low-frequency components of the desired signal. Initially, the received signal is differentiated to filter out the DC offset The signal is amplified to a suitable level and then integrated to recapture the original DC and low frequency signal components. The integration essentially restores the filtered components to their original values in the amplified signal using an arbitrary constant of integration of bounded magnitude to generate a restored signal. Using various techniques that exploit predetermined signal patterns or inherent signal properties of the desired signal, the DC offset estimate is then subtracted out of the restored signal leaving the amplified, received signal substantially free from distortion. A preferred way of removing such unwanted DC offsets by means of digitizing the time derivatives of the I and Q waveforms will now be described. After digitizing the derivatives, the digital values are re-integrated in an I and a Q accumulator to restore the I,Q values. The re-integration process introduces arbitrary constants of integration into the I and Q values which are now however of comparable magnitude to the wanted signal and can be estimated and removed according to the aforementioned patent. Errors in the digitizing process can lead additionally to the re-integrated I and Q values exhibiting a systematic increase or decrease, and this unwanted slope is now removed at the same time as removing the unwanted arbitrary constants of re-integration by estimating both the constants and the slopes and subtracting these systematic errors from the I and Q waveforms respectively. The I and Q waveforms are then processed by numerical signal processing algorithms to demodulate and decode the information.
However, problems still remain even for the above identified methods. Rate of change of the DC offset or signal slope still causes problems. Therefore, it is desirable to provide a method for compensating for the rate of change or signal slope so that decoded information modulated on the radio input signal is substantially unimpaired.
SUMMARY OF THE DISCLOSURE
It is an object of the present invention to provide a method for compensating for the rate of change or signal slope so that decoded information modulated on the radio input signal is substantially unimpaired. A radio receiver according to one embodiment of the present invention receives a signal via an antenna and mixes it down to the complex baseband using a local reference oscillator. The complex baseband signal comprises a real part (I waveform) and an imaginary part (Q waveform) that are corrupted by DC offsets arising from mixer imperfections or from reference oscillator leakage radiation being received at the antenna as coherent self-interference.
According to one embodiment of the present invention, previously estimated errors are fed back to the digitization process to reduce errors in digitizing the derivatives of the I,Q signals. A preferred digitizing technique uses high bit-rate delta modulation with variable stepsize. The variable stepsize is obtained by switching positive and negative current sources of different current values to charge a principal integrator capacitor. Slope errors occur when a positive and negative current source pair do not produce equal current magnitudes. According to one embodiment of the present invention, the unequal current source values are compensated by using correspondingly unequal digital increment/decrement values that are applied to the re-integrating I,Q accumulators, the values being set by a calibration procedure or being updated by feedback calculated from the estimated slope errors.
According to one embodiment of the present invention, an improved radio receiving apparatus for direct conversion of signals to a baseband for processing is disclosed. The radio receiving apparatus comprises direct conversion means for converting a radio input signal to a complex baseband signal having a real waveform and an imaginary waveform. Slope and offset compensation means estimate offsets and systematic drifts in the real and imaginar

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