WO2005029715A1

WO2005029715A1 - Method for controlling the gain of a signal in a transmitter

Info

Publication number: WO2005029715A1
Application number: PCT/IB2004/051786
Authority: WO
Inventors: Dominicus M. W. Leenaerts; Gerardus M. D. Jeurissen; Nenad Pavlovic
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2003-09-24
Filing date: 2004-09-17
Publication date: 2005-03-31
Anticipated expiration: 2006-03-24

Abstract

The present invention relates to a method a circuit for power amplification of an IF signal by means of a Gilbert mixer which is coupled to a power amplifier. The Gilbert mixer is directly coupled to a local oscillator which causes conversion gain variations of the Gilbert mixer. The conversion gain variations are compensated by controlling of the gain of the power amplifier correspondingly.

Description

METHOD FOR CONTROLLING THE GAIN OF A SIGNAL IN A TRANSMITTER

The present invention relates to the field of signal amplification, and more particularly without limitation, to radio transmitters having particular, but not exclusive, application in cellular communication networks. Transmitters are used in wireless communications to transmit electromagnetic waves in free space. The function of the transmitter is to modulate, up-convert and amplify signals for transmission via an antenna. A local oscillator provides a reference signal for the up-conversion performed in the transmitter; likewise a local oscillator is used in the receiver for down-conversion. Transmitters have a wide variety of applications ranging from low data rates wireless applications to medium data rate, such as bluetooth, and high data rate, such as wireless IEEE802.11 standards. Typically a mixer is used in a transmitter for the purposes of up-conversion. A commonly used mixer is the Gilbert mixer. This mixer is usually made of a differential transconductor and a pair of mixer cores. Mixer cores are each essentially a differential pair, and therefore switch the input current into a pair of differential outputs. The amount of current that goes to each output depends on the input voltage to the mixer core. In effect, the mixer core generates a differential output current that is approximately proportional to the input current multiplied by the differential signal at the mixer core gates. This current is usually driven into resistive loads to form an output voltage. Various designs for Gilbert mixers are known from the prior art (EPO998025A1, "Analysis and design of an integrated quadrature mixer with improved noise, gain and image rejection", Circuits and Systems, 2001. ISCAS 200; "The 2001 IEEE International Symposium " Harvey, J.; Harjani, R. Pages 786 -789 vol. 4; "Design of high performances Gilbert-cell mixers for GSM/DCS front-ends", Radio Frequency Integrated Circuits (RFIC) Symposium, 1998 IEEE, Colomines, S.; Amaud, T.; Plana, R.; Parra, T.; Graffeuil, J., Pages 143 -146). Usually the signal of the local oscillator (LO) which drives the mixer is buffered. Thereby the sinusoidal signal from the oscillator is turned into a square wave signal or a clipped sinusoidal signal. The present invention provides for a method of amplification of a signal, wherein the mixer is driven by a sinusoidal signal without buffering. Driving the mixer by the sinusoidal signal without buffering has the advantage that power dissipation can be minimized. In the case of quadrature signals any quadrature mismatches due to tolerance of the buffers which would otherwise degrade the performance of the transmitter are avoided. A further advantage of leaving out the buffering of the LO signals is that non linearity's of the buffer output signals with at least second and third harmonic distortion products are avoided. This way spurs in the output spectrum of the transmitter are eliminated. By driving the mixer with the sinusoidal signal without buffering conversion gain variation of the mixer results. The conversion gain variation is determined, such as by measuring the amplitude of the output signal provided by the mixer. The output of the mixer is coupled to a variable-gain amplifier (VGA) that acts as a power amplifier to drive an antenna. The gain of the VGA is controlled in order to compensate the variation of the conversion gain of the mixer. This way the VGA provides a high frequency signal with about constant amplitude for transmission by means of an antenna. This way maximum usage of the available amplification power is made under all circumstances. In accordance with a preferred embodiment of the invention an intermediate frequency (IF) signal is frequency converted by a Gilbert mixer to a high frequency (RF) signal. The RF signal is power amplified by the VGA and sent out via an air interface. The Gilbert mixer is directly coupled to a local oscillator which provides a sinusoidal signal. The level of the mixer output signal, i.e. the level of the RF signal, is measured in order to determine the conversion gain of the Gilbert mixer. Conversion gain variations of the Gilbert mixer are compensated by controlling the gain of the VGA correspondingly. For example, if a drop of the conversion gain of the Gilbert mixer is observed the gain of the VGA is increased correspondingly in order to compensate for the drop. Likewise if an increase of the conversion gain of the Gilbert mixer is observed the gain of the VGA is decreased correspondingly.

In the following preferred embodiments of the present invention will be explained by way of example by making reference to the drawings in which: Fig. 1 is illustrative of a flowchart of a preferred embodiment of a method of the invention, Fig. 2 is a block diagram of a preferred embodiment of an electronic circuit of the invention, Fig. 3 is a circuit diagram of an exemplary Gilbert mixer, and Fig. 4 is a circuit diagram of an exemplary variable gain control amplifier (VGA).

Fig. 1 shows a flow chart being illustrative of a method for amplification of a signal. In step 100 a mixer is driven with a sinusoidal signal which is provided by a local oscillator (LO). The mixer is used for up-conversion of an intermediate frequency signal (step 102). As the mixer is directly coupled to the local oscillator the conversion gain of the mixer varies. The conversion gain variation is determined in step 104 by monitoring of the RF signal output level provided at the output of the mixer. For example, this is done by measuring of the RF signal amplitude by means of a peak detector. In step 106 the gain of a variable-gain amplifier (VGA) is controlled in order to compensate the conversion gain variation. This aims to provide an about constant amplitude of the amplified RF signal. In step 108 the RF signal is amplified by the VGA. The power amplified RF signal is sent out via an antenna which is coupled to the VGA in step 110. As the gain of the VGA is controlled in order to compensate conversion gain variation of the mixer the amplitude of the power amplified RF signal is about constant. This way maximum usage of the available amplification power is made. Fig. 2 shows a block diagram of an electronic circuit 200. Electronic circuit 200 has Gilbert mixer 202 which is coupled to variable gain amplifier (VGA) 204. Further Gilbert mixer 202 has its output coupled to peak detector 206 which in turn is coupled to controller 208. Controller 208 has a control output which is coupled to a control input of VGA 204 for gain control. Output of VGA 204 is coupled to antenna 210. Gilbert mixer 202 is directly coupled to oscillator 212 which provides a sinusoidal signal to Gilbert mixer 202. This sinusoidal signal provides a frequency reference for frequency up-conversion of intermittent frequency (IF) signal which is inputted into Gilbert mixer 202. The resulting RF signal which is provided at the output of Gilbert mixer 202 is inputted both into VGA 204 and peak detector 206. Peak detector 206 determines the amplitude of the RF signal that is provided at the output of Gilbert mixer 202. The signal amplitude of the RF signal at the output of Gilbert mixer 202 is a measure for the gain variation of the Gilbert mixer which is due to its direct coupling to oscillator 212. A corresponding data value which is indicative of the actual conversion gain of Gilbert mixer 202 is provided from peak detector 206 to controller 208. Controller 208 determines the gain of VGA 204 which compensates a conversion gain variation of Gilbert mixer 202. The controller 208 issues a corresponding output signal which is provided to the control input of VGA 204 to set the gain of VGA 204. As a result the amplified RF signal which is provided at the output "RF out" of VGA 204 has an about constant amplitude. The amplified RF signal is sent out by means of antenna 210. Peak detector 206 can be a diode or any other circuit which can be used for peak detection, such as the peak detector disclosed in European provisional application EP01202614.2. A preferred application of electronic circuit 200 is for a transmitter within a cellular communication network, such as GSM, UMTS, WLAN, and in particular IEEE802.1 la. The transmitter can be implemented both in the network components of such a network, i.e. the so called base stations, node-Bs or access points, as well as in the corresponding mobile terminals, user equipments or data points. Fig. 3 shows an implementation example for Gilbert mixer 202 of Fig. 2. Gilbert mixer 202 has a differential input pair for receiving of the local oscillator (LO) signal from local oscillator 212. Further Gilbert mixer 202 has a differential input pair for receiving of IF signal. Gilbert mixer 202 has a differential output pair for outputting of the frequency up-converted RF signal. Fig. 4 shows an implementation example for VGA 204 of Fig. 2. At its input "in" VGA 204 receives RF signal which is outputted by Gilbert mixer 202 (cf. Fig. 3). The RF signal is power amplified and outputted at output "out" of VGA 204. The gain of VGA 204 is controlled by signal Vcontrol which is applied to the control input of VGA 204. LIST OF REFERENCE NUMERALS:

200 electronic circuits

202 gilbert mixer

204 variable gain amplifier (VGA)

206 peak detector

208 controller

210 antenna

212 oscillator

Claims

CLAIMS:

1. A method of amplification of a signal, the method comprising the steps of: driving a mixer (202) by a sinusoidal signal, transforming of the signal to a high frequency signal by means of the mixer, determining a conversion gain variation of the mixer, providing the high frequency signal to a power amplifier (204), controlling a gain of the power amplifier to compensate the conversion gain variation.

The method of claim 1, wherein the signal is an intermediate frequency (IF) signal.

3. The method of claim 1 or 2, wherein the mixer is a Gilbert mixer.

4. The method of claim 1, 2 or 3, wherein the determination of the conversion gain variation is performed by measurement of a level of the high frequency signal.

5. The method of any one of the preceding claims 1 to 4, wherein the determination of the conversion gain variation is performed by peak detection of the high frequency signal.

6. An electronic circuit of amplification of a signal (IF), the electronic circuit comprising: a mixer (202), the mixer being adapted to receive a sinusoidal signal for frequency up-conversion of the signal, means (206) for determining a conversion gain variation of the mixer, means (208) for controlling the gain of an amplifier (204) being coupled to the mixer in order to compensate the conversion gain variation.

7. The electronic circuit of claim 6, wherein the mixer is a Gilbert-type mixer. PHNL031153 _{pcτ/| B2}004/051786

8. The electronic circuit of claim 6 or 7, wherein the means for determining the conversion gain variation comprise peak detection means (206).

9. A transmitter comprising: a mixer (202), an oscillator (212) being coupled to the mixer for providing a sinusoidal signal to the mixer, means (206) for determining a conversion gain variation of the mixer, - a power amplifier (204) being coupled to an output of the mixer, means (208) for controlling the gain of the power amplifier in order to compensate the conversion gain variation.

10. A network component of a cellular communication network comprising: - a mixer (202), the mixer being adapted to receive a sinusoidal signal for frequency up-conversion of the signal, means (206) for determining a conversion gain variation of the mixer, means (208) for controlling the gain of an amplifier (204) being coupled to the mixer in order to compensate the conversion gain variation.

11. A mobile terminal for usage in a cellular communication network, the mobile terminal comprising: a mixer (202), the mixer being adapted to receive a sinusoidal signal for frequency up-conversion of the signal, - means (206) for determining a conversion gain variation of the mixer, means (208) for controlling the gain of an amplifier (204) being coupled to the mixer in order to compensate the conversion gain variation.