Multistage modulation architecture and method in a radio

[0001] 1. Field of Invention

[0002] The present invention relates to modulation in radio frequency transmitters and more particularly to efficient provision of mixing frequency signals.

[0003] 2. Background in the Art

[0004] While the present invention has a wide range of utility, it finds particular application in wireless applications, particularly those embodied in silicon chips. A particularly important application for solid state transmitters is in mobile telephones. It is desirable to provide architecture for generating second generation or third generation mobile telephone operation. Second generation standards include CDMA (Code Division Multiple Access), TDMA (Time Division Multiple Access, IS-136), and GSM standard (Global Systems for Mobile). Third generation standards include WCDMA (Wide-band Code Division Multiple Access) and CDMA 2000.

[0005] A radio frequency transmitter must translate an analog input into a radio frequency signal and also provide for variable gain to provide for a selected input level to a power amplifier prior to transmission. In the modulation process, an analog signal is mixed with a mixing frequency signal to provide a higher frequency output signal. The mixing signal is commonly provided from a frequency synthesizer including a voltage controlled oscillator (VCO). Wireless transmitters in today's marketplace must be implemented cost-effectively in order to be competitive.

[0006] Current ways of addressing this need have particular drawbacks. In the well-known simple direct modulation transmitter, translation of the analog signal to a radio frequency signal is done in one stage, with one mixing step. CDMA and WCDMA systems require a dynamic range of 90 dB. Almost the whole range must be performed in amplifiers or attenuators working at the same frequency, nominally between 1 and 2 GHz. Consequently, the range of variable gain required of the transmitter must be accomplished in this frequency domain, which is quite difficult.

[0007] In the direct modulation transmitter, isolation between the power amplifier and VCO is minimized, and “cross-talk” between them may result, causing signal distortion. Additionally, carrier feedthrough may result in VCO signal leakage into the radio frequency band, also distorting the transmitted signal. Since the transmission frequency provided by the VCO is working on the same or on a harmonic of the transmitter output frequency, frequency pulling or inject locking of the VCO may result. There is also a risk for oscillation due to the high gain in direct modulation on the same frequency and feedback within an integrated circuit (IC) in which the direct modulator is embodied and outside the IC including a power amplifier between the modulator and a transmitting antenna and other coupling components.

[0008] The well-known double conversion transmitter addresses these problems found in the direct modulation transmitter. The initial modulation and mixing described above is done in a first stage providing an intermediate frequency output. A portion of the variable gain range is then implemented at the intermediate frequency. The intermediate frequency signal is the mixed with a second mixing frequency signal from a second frequency synthesizer and second VCO. Also, an extra intermediate frequency filter is required in the transmitter circuit to avoid production of spurious signals. The added circuits elements add significant expense to a transmitter embodied in a silicon chip. The degree of added expense could be sufficient to render such a transmitter uncompetitive in the marketplace.

[0009] It is therefore a general object of the present invention to provide a radio frequency transmitter having an efficient and reliable modulation scheme particularly suited for 2^nd and 3^rd generation wireless mobile transceiver operation.

[0010] It is a more specific object of the present invention to provide a transmitter for modulating and amplifying analog signals wherein a single signal frequency source is used as a source for mixing frequencies for a plurality of mixing stages.

[0011] It is a more particular object of the present invention to provide a transmitter of the type described in which a plurality of frequencies is produced from said frequency source.

[0012] It is a further object of the present invention to provide a transmitter of the type described in which dividers are provided to produce each of the plurality of frequencies.

[0013] It is also an object of the present invention to provide a transmitter of the type described in which a modulus of each divider is selected for minimizing production of spurious signals.

[0014] It is an additional object of the present invention to provide a method for supplying each mixing frequency in a transmitter of the type described.

[0015] Briefly stated, in accordance with the present invention there are provided a method and apparatus for transmitter transmitting a radio frequency signal in response to an analog signal input and having a plurality of frequency conversion stages. In the embodiment comprising two stages, a modulator includes a first mixer providing an intermediate frequency output and a second mixer providing a radio frequency output, said mixers being supplied with first and second mixing signals respectively. The mixing signals are provided from first and second frequency dividers each receiving an input from the same frequency synthesizer. Each of said first and second frequency dividers has a respective, selected modulus which in a preferred form may be selected for minimization of spurious signals.

[0019] Referring now to FIG. 1, a transmitter 10 constructed in accordance with the present invention is illustrated in block diagrammatic form. The transmitter 10 receives inputs from analog sources 1 and 2, illustrated here as digital to analog converters, at input ports 11 and 12 respectively. The input ports 11 and 12 feed in-phase and quadrature channels I and Q respectively. The use of the out of phase channels I and Q is a common technique in digital modulation.

[0020] In the I channel, the input port 11 provides the analog signal to an input filter 13 coupled to an input terminal 16 of a modulator 20. Similarly in the Q channel, the input port 12 provides the analog signal to an input filter 14 coupled to an input terminal 17 of the modulator 20. The modulator 20 has an output terminal 21. A first mixing frequency f_m1, further described below, is connected to a terminal 22 of the modulator 20. First and second mixers 24 and 26 in the I and Q channels respectively provide signal outputs to a signal adder 28 providing a signal to the output terminal 21 of the modulator 20. The first mixing signal input is provided to the mixer 26 from the mixing signal terminal 22. The terminal 22 is also coupled via a quadrature phase shifter 29 to a second input of the mixer 26. Alternatively, the quadrature phase shifter 29 can be coupled to the mixer 24. FIG. 1 is also illustrative of an embodiment where any kind of digital modulation is performed. The analog signals from the sources 1 and 2 are modulated in a well-known manner. The mixers 24 and 26 are comprised in a first stage 27. Due to the quadrature phase shifter 29, an out of phase components is supplied to the signal adder 28, whereby the signals from the sources 1 and 2 are quadrature modulated and added and provided to the output terminal 21 of the modulator 20. This output is an intermediate frequency signal at frequency f₁.

[0021] The output terminal 21 provides the intermediate frequency signal to a variable gain amplifier 36, which provides a part of the total dynamic range of the transmitter 10. The variable gain amplifier 36 provides the intermediate frequency signal to an intermediate frequency filter 38, which provides the intermediate frequency signal to a mixer 40. The mixer is comprised in a second stage 41. As further described below, the mixer 40 also receives an input which is a second mixing frequency signal, f_m2. The mixer 40 provides a radio frequency output at frequency f_RF. This radio frequency output is coupled to a variable gain amplifier 43 coupled to a radio frequency filter 45. The output of the radio frequency filter 45 is amplified by a power amplifier 46 and coupled for transmission by an antenna 48.

[0022] In accordance with the present invention, a single source from which both the first and second mixing frequency signals are produced is a frequency synthesizer 56 including a voltage controlled oscillator (VCO) 58 providing a signal at a frequency f_VCO at a terminal 60. First and second frequency dividers 64 and 66 are provided, each having an input connected to the terminal 60. The frequency divider has a first modulus, M₁, and provides an output at the frequency f_m1 to the terminal 22 of the modulator 20. The frequency divider 66 has a second modulus, M₂, and provides an output at a frequency f_m2 to the mixer 40. Consequently, frequency conversion at a plurality of stages, e.g. two stages, is accomplished with a single frequency synthesizer 56.

[0023] Since neither f_m1 nor f_m2 are in the transmission frequency f_RF, as in a direct modulator, this circuit architecture does not suffer from carrier feedthrough. In this structure, modulation frequencies and the transmitter output frequency are related as in equation 1.
f_RF=[(M1+M2)/(M1*M2)]*f_VCO  (1)
The RF transmission frequency is not a direct multiple of f_VCO. Consequently, interaction between the frequency synthesizer 56 and the power amplifier 46 is reduced. Based on equation (1), spurious signals, e.g., harmonics that can be produced by the two mixing stages 27 and 41, are multiples of the fundamental frequency of f_VCO/(M1*M2). This set of frequencies is easier to analyze than that produced by the conventional double conversion modulator whose corresponding spurious signal expression is M*f_m1±Nf_m2, where M and N are integers. This expression creates a solution set of spurious signals which is more difficult to work with due to its containing terms rather than factors only.

[0024] It is preferred to break down the frequency chain by which the modulated signal is translated to a radio frequency into not-harmonically related subfrequencies. The frequencies f_VCO, f_m1 and f_m2 should be selected so that it has no harmonic or subharmonic relation to f_RF. This is accomplished first by selection of f_VCO, and next by selecting moduli M1 and M2 to have different values. For example, M1 and M2 may be in the ratio of 4:3 or 8:7. M1 may conveniently be a number divided by 4 to provide for simple generation of a quadrature f_m1 for the modulator 20. In order to meet the above constraint relating to harmonics, M2 may be conveniently selected as M1±N, where N is a positive integer. However, M1 and M2 can be any combination of two different positive integers.

[0025] FIG. 2 is a flow chart illustrating the method of the present invention. The components referred to in connection with the method steps are illustrated in FIG. 1. At block 100, the local oscillator frequency f_VCO is provided by a single source, here the frequency synthesizer 56. The frequency f_VCO is divided by the modulus M1 of the frequency divider 62, as illustrated at block 102. At the same time, the local oscillator frequency f_VCO is also divided by frequency divider 66 with modulus M2, as seen at block 103. As seen at block 105, the mixing step is performed in the modulator 20. As seen at block 106, the mixing step is performed in the mixer 40. The moduli used in the frequency dividers 64 and 66 are preset when the frequency dividers are manufactured.

[0026] An efficiently constructed transmitter and a method are provided with the capability of minimizing spurious signal production. The above description will enable those skilled in the art to make many different forms of transmitters constructed in accordance with the present invention.