Direct conversion transmitter and communication system utilizing the same

1. Technical Field

The present disclosure relates to communication techniques. In particular, the present disclosure relates to signal processing techniques in a direct-conversion transmitter.

2. Description of the prior art

Main trends for radio frequency (RF) communication systems nowadays are to simplify circuits, to reduce chip size, and to save in overall hardware cost. In comparison with a superheterodyne transceiver, a conventional direct-conversion transceiver has a simpler structure but is not prevalent due to immature manufacturing techniques. In recent years, with the advancements in techniques for manufacturing integrated circuits, a direct-conversion transceiver with good performance becomes practicable and accordingly draws much attention.

FIG. 1 illustrates one part of a conventional direct-conversion transmitter. The direct-conversion transmitter includes a phase lock loop 120 , a data generator 140 , and a transmitter 100 . The phase-locked loop (PLL) 120 includes a phase detector 121 , a voltage-controlled oscillator (VCO) 122 , and a frequency divider 123 . The oscillating signal generated by the voltage-controlled oscillator 122 is divided in frequency by the frequency divider 123 , so as to generate a carrier signal. Based on the carrier signal and the data signal provided by the data generator 140 , the transmitter 100 generates an output signal. For instance, the transmitter 100 modulates the carrier signal with the data signal in order to generate the output signal.

One problem of this direct-conversion transmitter is that the modulation process in the transmitter 100 usually causes disturbance to the VCO 122 . Taking the condition that the divisor of the frequency divider 123 is equal to two (2), the frequency band of the second harmonic of the output signal in the transmitter 100 is very close to the frequency of the oscillating signal in the VCO 122 . Similarly, if the divisor of the frequency divider 123 is equal to four (4), the frequency band of the fourth harmonic of the output signal is very close to the frequency of the oscillating signal in the VCO 122 . This interference caused by the transmitter 100 introduces phase noise to the oscillating signal in the VCO 122 (i.e. causes the oscillating signal to float) and then inevitably decreases the output signal quality of the transmitter 100 .

To solve the aforementioned problem, the disclosure provides a new direct-conversion transmitter. By inserting a well-designed filter between the oscillator and transmitter, the interference from the transmitter to the oscillator may be substantially reduced.

One embodiment according to the disclosure is a direct-conversion transmitter including an oscillator, a frequency divider, a transmitter, and a filter. The oscillator generates an oscillating signal with an original frequency. The frequency divider performs frequency dividing on the oscillating signal, so as to generate a carrier signal. The transmitter receives the carrier signal from the frequency divider and generates an output signal based on the carrier signal and a data signal. The filter is coupled between the frequency divider and the transmitter. The filter filters out an interference signal fed-back from the transmitter to the oscillator, wherein the interference signal may cause the oscillating signal to float.

Another embodiment according to the disclosure is a communication system including a direct-conversion transmitter. The direct-conversion transmitter includes an oscillator, a frequency divider, a transmitter, and a filter. The oscillator generates an oscillating signal with an original frequency. The frequency divider performs frequency dividing on the oscillating signal, so as to generate a carrier signal. The transmitter receives the carrier signal from the frequency divider and generates an output signal based on the carrier signal and a data signal. The filter is coupled between the frequency divider and the transmitter. The filter filters out an interference signal fed-back from the transmitter to the oscillator, wherein the interference signal may cause the oscillating signal to float.

The advantage and spirit of the disclosure may be understood by the following recitations together with the appended drawings.

FIG. 1 (Prior Art) illustrates one part of an exemplary conventional direct-conversion transmitter.

FIG. 2(A) and FIG. 2(B) show the block diagram of the direct-conversion transmitter in one embodiment according to the disclosure.

In FIG. 3(A)˜FIG . 3 (E), several examples of the filter according to the disclosure are illustrated.

In FIG. 4 , exemplary stop-bands of the filter according to the disclosure are illustrated.

FIG. 2(A) shows a direct-conversion transmitter 200 in one embodiment according to the present disclosure. The direct-conversion transmitter 200 includes an oscillator 210 , a frequency divider 220 , a filter 230 , a data generator 240 , and a transmitter 250 . In practice, this direct-conversion transmitter 200 may be independently manufactured or be integrated in various communication systems utilizing the direct-conversion structure.

The oscillator 210 generates an oscillating signal OSC with an original frequency F 0 . The frequency divider 220 is coupled to the oscillator 210 and performs frequency dividing on the oscillating signal to generate a carrier signal with a carrier frequency F C . Based on the carrier signal and the data signal provided by the data generator 240 , the transmitter 250 generates an output signal. As shown in FIG. 2(B) , in practice, the oscillator 210 is a voltage-controlled oscillator (VCO). VCO 210 , a phase detector 215 , and the frequency divider 220 form a phase-lock loop (PLL). The PLL generates the oscillating signal OSC and the carrier signal according to a reference signal REF. In the following paragraphs, the structure illustrated in FIG. 2(B) is taken as an example to explain the essence of the present disclosure, but the scope of the disclosure is not limited to this structure.

Several different embodiments of how to implement the filter in the direct-conversion transmitter in the present disclosure are illustrated in FIG. 3(A)˜FIG . 3 (E). In FIG. 3(A) , the filter 230 coupled between the frequency divider 220 and the transmitter 250 includes an inductance component L and a capacitance component C connected in serial between the ground node and the path for forwarding the carrier signal to the transmitter 250 . The inductance component L and capacitance component C are designed to allow the pass of signals with frequency equal to the carrier frequency F C and to block signals with frequency close to the original frequency F 0 , so as to meet the following requirements.

To be specific, with the filter 230 , the carrier signal provided by the frequency divider 220 may still pass to the transmitter 250 via the connection path; thus, the overall function of this direct-conversion transmitter is not affected. On the contrary, the interference with frequency close to the frequency F 0 that might be fed-back to the voltage-controlled oscillator 210 via this connection path is blocked by the filter 230 . Exemplary stop-bands (shown as shaded regions) of the filter 230 are illustrated in FIG. 4 . Even if the interference is not completely filtered out, the power level of the interference may be considerably attenuated. That is to say, the interference from the transmitter 250 to the voltage-controlled oscillator 210 is greatly reduced, and the output signal of the transmitter 250 may keep its shape. The aforementioned interference could be the harmonics of the output signal of the transmitter 250 and/or noises induced by other reasons.

It is to be noted that in other embodiments, the filter 230 may be disposed at other critical paths between the oscillator 210 and the transmitter 250 . For instance, the filter 230 may also be disposed at a power transmission line, a ground line, or any path that may introduce interference from the transmitter 250 to the oscillator 210 . The number of filters 230 is not limited to any specific embodiment. The circuit designer may plan the position and number of filters upon design choices.

Furthermore, FIG. 3(B) shows an embodiment of the filter 230 that includes one resistance component R and one capacitance component C connected in serial between the ground node and the path for forwarding the carrier signal to the transmitter 250 . As for the embodiment shown in FIG. 3(C) , the filter 230 includes one inductance component L and two capacitance components C 1 /C 2 . In FIG. 3(D) , another embodiment of the filter 230 includes two inductance components L 1 /L 2 and one capacitance component C. In FIG. 3(E) , the filter 230 includes one capacitance component C. In practice, the filter 230 may be designed as a low-pass filter or a notch filter; as long as the stop-band of the filter is located around the original frequency F 0 , the goal of blocking interference to the oscillator may be achieved.

Moreover, the filter 230 may have common power transmission lines (including power supply lines and/or ground lines) with the transmitter 250 while the phase-locked loop that includes the voltage-controlled oscillator 210 does not have common power transmission lines with the filter. This design may prevent the interference entering the ground node of the filter 230 from attacking the voltage-controlled oscillator 210 via power lines.

As described above, the disclosure provides a new direct-conversion transmitter. By inserting a well-designed filter between the oscillator and transmitter, the interference from the transmitter to the oscillator may be substantially reduced.

With the example and explanations above, the features and spirits of the disclosure will be hopefully well described. Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teaching of the disclosure. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.