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I - Q BALANCE METHOD FOR DIRECT CONVERSION RECEIVERS

IP.com Disclosure Number: IPCOM000004650D
Original Publication Date: 2001-Mar-16
Included in the Prior Art Database: 2001-Mar-16
Document File: 3 page(s) / 28K

Publishing Venue

Motorola

Related People

Michael N. Pickett: AUTHOR

Abstract

This paper describes a method for balancing the intermediate frequency (IF) signals for use in a direct conversion receiver where the incoming signal is converted to an in-phase and quadrature (I and Q) IF signal at a very low frequency (near DC). The IF signals in the direct conversion receiver are then demodulated to recover the base-band information. The demodulators can be implemented with conventional analog circuits or the IF can be converted to digital signals and demodulated using software algorithms in a digital signal processor (DSP) or other micro-processor. In either case the amplitude and phase balance of the I and Q IF signals is important to properly carry out the demodulation process and minimize distortion of the base-band signal. Good amplitude and phase tracking are difficult to achieve across the frequency band in a wide band receiver. This is a method of calibration that matches the I and Q amplitude and tracks the phase across a wide RF frequency band.

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This is the abbreviated version, containing approximately 39% of the total text.

I Q BALANCE METHOD FOR DIRECT CONVERSION RECEIVERS

by Michael N. Pickett

ABSTRACT

This paper describes a method for balancing the intermediate frequency (IF) signals for use in a direct conversion receiver where the incoming signal is converted to an in-phase and quadrature (I and Q) IF signal at a very low frequency (near DC). The IF signals in the direct conversion receiver are then demodulated to recover the base-band information. The demodulators can be implemented with conventional analog circuits or the IF can be converted to digital signals and demodulated using software algorithms in a digital signal processor (DSP) or other micro-processor.

In either case the amplitude and phase balance of the I and Q IF signals is important to properly carry out the demodulation process and minimize distortion of the base-band signal. Good amplitude and phase tracking are difficult to achieve across the frequency band in a wide band receiver. This is a method of calibration that matches the I and Q amplitude and tracks the phase across a wide RF frequency band.

THEORY OF OPERATION

When the IF signals of a direct conversion receiver are demodulated and the amplitudes of the I and Q signals are not equal or the phase between the I and Q signals is not exactly 90 (the resulting baseband signal contains two times the IF frequency). This technique makes use of this fact to minimize the amplitude and phase errors between the I and Q IF signals.

Figure 1 is a simplified block diagram of a direct conversion receiver of the type where this technique would be used. In normal operation of the receiver the incoming RF signal is filtered and amplified and split into two equal amplitude signals of the same relative phase. The mixers convert the RF signal to I and Q IF signals at DC or near DC using LO signals that are in quadrature (90 degrees apart). The IF signals are filtered, converted to digital signals and sent to a Digital Signal Processor (DSP) for demodulation. The demodulator executes the necessary algorithm for the type of modulation. The output of the demodulator is the recovered base-band signal. If the I and Q IF signals are not equal in amplitude and 90° apart in phase the demodulated base-band output will contain two times the IF frequency for most demodulation algorithms.

The 2xIF product is an undesirable distortion product. Since the exact IF frequency is due to the frequency difference between the incoming RF signal and the LO frequency of the receiver it could be anywhere from 0 Hz to several thousand Hz depending on the operating frequency and the frequency accuracy of the receiver LO and incoming signal. The frequency of the 2xIF distortion product can be anywhere from DC to two times the IF frequency. This can result in an audible whistle in voice communications or degradation of the BER in a data system.

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