Quadrature signal generation in an integrated direct conversion radio receiver

[0002] Disclosed embodiments of the present invention relate to circuits, and more particularly, to radio frequency communication circuits.

[0003] In a direct conversion radio receiver or transceiver, an RF (Radio Frequency) signal is directly down-converted to a baseband signal without first down-converting to an IF (Intermediate Frequency) signal. For many types of receivers, both the inphase and quadrature components of the baseband signal are employed for further demodulation or detection. A simplified functional diagram of a transceiver employing direct conversion is illustrated in FIG. 1.

[0004] In FIG. 1, antenna 102 receives an RF signal. When the transceiver is in its receive mode, the received RF signal is filtered by bandpass filter 104 and mixed with quadrature RF local oscillator signals cos(ω_ct) and sin(ω_ct) by mixers 106 and 108, respectively, where ω_cis the carrier frequency in radians/sec. The output signals of mixers 106 and 108 are filtered by low pass filters 110 and 112, respectively, to provide the inphase (I) and quadrature (Q) components. Detector 114 further processes the I and Q components to provide the final digital data to the end user. Detector 114 may employ matched filtering, error detection and correction, spread-spectrum filtering (de-spreading), and other forms of detection appropriate for the particular modulation and coding scheme that is employed. When the transceiver is in its transmit mode, digital data provided by the end user is encoded by encoder 116, modulated to a baseband signal by modulator 118, up-converted via mixer 120 by mixing with cos(ω_ct) and band pass filtered by filter 122. The quadrature RF local oscillator signals cos(ω_ct) and sin(ω_ct) may be obtained from VCO (Voltage Controlled Oscillator) 126 within PLL (Phase Lock Loop) 124.

[0005] If the oscillation frequency of PLL 124 is substantially the same as the carrier frequency, ω_c, then VCO 126 may be susceptible to “frequency pulling” during transmission of an RF signal. This may be caused by VCO 126 locking on to the frequency of the transmitted RF signal during transmission because of unintended feedback. If FSK (Frequency Shift Key) modulation is employed, then the transmitted RF signal has a varying instantaneous frequency, possibly centered around ω_c. VCO 126 cannot instantly return back to the carrier frequency ω_cwhen the transceiver enters its receive mode, in which case a received RF signal may not be accurately mixed down to its baseband I and Q components.

[0006] VCO 126 is less susceptible to frequency pulling if its oscillation frequency is substantially different from ω_c. Thus, one approach to mitigating frequency pulling of VCO 126 is to run VCO 126 at twice the carrier frequency, and then synthesize the quadrature mixing signals by a divide-by-two division circuit. This is illustrated in FIG. 2, where VCO 202 oscillates at 2ω_cand JK flip flop 204 is configured as a divide-by-two division circuit. One of the quadrature mixing components is taken from output port 206 of JK flip flop 204. The other quadrature mixing component is taken at output port 208 of XOR (exclusive-OR) gate 210.

[0007] The quadrature mixing signals in FIG. 2 are square waves, whereas in FIG. 1 the quadrature mixing signals are represented by analog sinusoids, e.g., cos(ω_ct) and sin(ω_ct). It is to be understood that FIG. 1 is a representation of a communication circuit at a functional level, and that for particular implementations, the quadrature mixing signals may be square waves of frequency ω_crather than analog sinusoids of frequency ω_c. It is to be understood throughout these letters patent that sinusoids may be interpreted as square waves, or other discrete-valued periodic waveforms.

[0008] As carrier frequencies are increased for communication circuits, running a VCO at twice the carrier frequency may add unwanted cost. For example, in a communication circuit for the consumer market, such as a transceiver circuit in a cordless phone, the carrier frequency may be on the order of 2.5 GHz, so that running a VCO at 5 GHz may add to its manufacturing cost.

[0009] FIG. 1 is a prior art high-level functional diagram of a transceiver.

[0010] FIG. 2 is a prior art divide-by-two division circuit and XOR gate for generating quadrature signals.

[0011] FIG. 3 is a high-level functional diagram of a quadrature generator circuit according to an embodiment of the present invention.

[0012] FIG. 4 is an embodiment of a differential signal mixer and filter used in the embodiment of FIG. 3.

[0013] FIG. 3 provides a high-level functional diagram of a circuit for generating quadrature mixing signals according to an embodiment of the present invention. The quadrature mixing signals, denoted as cos(ω_ct+∂) and sin(ω_ct+∂), are provided at ports 302 and 304, respectively, where ∂ denotes some arbitrary phase. Another sinusoid, denoted as cos(ω_ct+ω), is provided at port 306, where ω denotes some arbitrary phase. For simplicity, the amplitudes of the sinusoids are taken as unity, but are not limited as such. Quadrature signals cos(ω_ct+∂) and sin(ω_ct+∂) are available for direct conversion of received RF signals, and the sinusoid cos(ω_cat+∂) is available for up-converting modulated signals to the carrier frequency ω_cfor subsequent transmission.

[0014] VCO 308 provides an output signal with oscillation frequency (⅔)ω_c. Divide-by-two division circuit 310 provides quadrature signals at ports 312 and 314 of frequency (⅓)ω_c, which are mixed by mixers 316 and 318 with a sinusoid having the same frequency as VCO 308, (⅔)ω_c. The lower frequency components of the resulting mixed output are filtered out by filters 320 and 322, so that the frequency of the output signals of filters 320 and 322 is given by the sum of the input frequencies to mixers 316 and 318, ω_c=(⅔)ω_c+(⅓)ω_c. In similar fashion, divide-by-two division circuit 324, mixer 326, and filter 328 provide the sinusoid cos(ω_ct+ω) at port 306.

[0015] An advantage of the embodiment illustrated in FIG. 3 is that VCO 308 need not provide an output sinusoid having a frequency greater than the carrier frequency, and yet frequency pulling is reduced because the output frequency of VCO 308 is substantially different from the carrier frequency. In the particular embodiment shown in FIG. 3, the output sinusoid of VCO 308 is at (⅔)ω_c, but other embodiments may be realized in which divide-by-two division circuits 310 and 324 are replaced with divide-by-N division circuits and VCO 308 is configured to provide an output sinusoid of frequency [N/(N+1)]ω_c. N may be an integer or ratio of integers. However, as N increases, [N/(N+1)]ω_capproaches ω_c, and frequency pulling may be a problem if N is too large.

[0016] Specific implementations of the high-level functional diagram of FIG. 3 may be realized using circuits for single-ended signals or circuits for differential signals. For differential signal circuits, output ports 312, 314, and 328 each provide a differential signal comprising two signals at frequency (⅓)ω_c, one signal having an amplitude below a common-mode voltage and one signal having an amplitude above the common-mode voltage. Port 330 also provides a differential signal at frequency (⅔)ω_c. These differential signals are then provided to a mixer. One embodiment for a combination of mixer and filter for differential signals is provided in FIG. 4.

[0017] In FIG. 4, differential signal notation is employed, where a differential signal x(t) means that the actual voltage is x(t) above a common-mode voltage. A differential signal at frequency (⅓)ω_cis applied to input ports 402 and 404, and a differential signal at frequency (⅔)ω_cis applied to input ports 406 and 408. A bias voltage is applied to input port 410 so that nMOSFET 412 sinks current from the pair of nMOSFETs 414 and 416, and nMOSFET 418 sinks current from the pair of nMOSFETs 420 and 422. Inductor 424 and capacitor 426 provide a reactive load to the pair of nMOSFETs 414 and 416, and inductor 428 and capacitor 430 provide a reactive load to the pair of nMOSFETs 420 and 422. Inductor 428 and capacitor 430, and inductor 424 and capacitor 426, are each resonant circuits with a resonant frequency of ω_c. The output differential signal is provided at output ports 432 and 434.

[0018] One of ordinary skill in the art will realize that various portions of the embodiment of FIG. 3 may be realized by various combinations of circuits for single-ended signals and differential signals. Similarly, once the quadrature mixing signals, whether differential or single-ended, are synthesized according to the embodiment of FIG. 3, various implementations of the transceiver of FIG. 1 may be realized with various combinations of circuits for single-ended signals and differential signals. Consequently, it is to be understood in the claims that a signal may be either a single-ended signal or a differential signal.

[0019] Various modifications may be made to the disclosed embodiments without departing from the scope of the invention as claimed below.