Frequency synthesizer

Abstract: The present invention provides a frequency synthesizer which can be switched at a high speed and generates signals having a relatively high frequency with low power consumption; comprising a clock generator (21), a frequency setting circuit (10) in which phase increment for unit clock can be programmed, a phase accumulator (12) in which phase increment is accumulated, a ROM (14) which outputs a digital signal corresponding to cumulative phase output, a D/A convertor (15) which inverts polarity of output in each clock time, and a band-pass filter (22). The frequency synthesizer constitutes a direct digital synthesizer (DDS) whose output signal is supplied to a phase locked loop (PLL) comprising a 1 / N frequency divider. Because a bandpass filter (22) is connected to the output of the D/A converter (15) instead of a low-pass filter, a higher reference frequency can be supplied to the PLL.

Description: The present invention relates to a frequency synthesizer according to the preamble of claim 1, i.e. as disclosed in Electronics World and Wireless World, Vol. 97, No. 1678, September 92, Surrey, GB, pages 746 to 748. In a conventional type of frequency synthesizer, as shown in Fig. 6, a signal 2 indicating a reference frequency f 0 generated from a reference signal generator 1 is supplied as input to a phase comparator 3, and a signal 7 obtained by dividing output from a voltage control oscillator (VCO) 5 with a frequency divider circuit 6 is connected to another input terminal of a phase comparator 3 for comparison of the phases. Output 8 from a phase comparator 3 is supplied as input to a loop filter 4 and then connected to the VCO 5 to control the VCO, thus a feedback loop being completed. Frequency division ratio N of the frequency divider circuit 6 is programmable, so that it is possible to obtain output from the VCO as a synthesizer output of N x f 0 due to convergence of the feedback loop. Generally a loop filter fully intercepts a reference frequency component, so that a time required for convergence of a feedback loop is limited to a value corresponding to a cycle of the reference frequency or below. For this reason, a conventional type of feedback loop can not be applied as it is to a synthesizer which requires a relatively high switching time. As means for solving the problem in the conventional type of frequency synthesizer, a direct digital synthesizer (DDS) as shown in Fig. 7 is available. In this type of synthesizer, a frequency setting circuit 10 programs a phase increment 11 for unit clock corresponding to a frequency to be generated, the phase increase 11 is supplied as input to a phase accumulator 12, and accumulation of phase for unit clock of a clock signal 23 from the clock generator 21 is computed. A cumulative phase 13 is supplied as input to a ROM in which a value of sinusoidal wave form corresponding to each phase is stored, and output from the ROM is connected to a D/A convertor 15 and provided as output as a sinusoidal wave 16 converted into analog voltage. Analog output 16 is voltage sampled at a clock cycle, so that the wave form is as shown in Fig. 8A and includes many harmonic components. So a required synthesizer output 18 having a wave form as shown in Fig. 8B is obtained after removing the higher harmonic components by inserting a low-pass filter (LPF) 17. By introducing the configuration as described above which is not a feedback loop configuration, high speed switching has become possible. However, the DDS shown in Fig. 7 uses a logic section ( consisting of a phase accumulator and a ROM) and a D/A convertor to generate a signal, so that an operating speed of the DDS is limited by the two components above, and can hardly generate a relatively high frequency. Also, when trying to generate a relatively high frequency, the clock frequency must be made higher, which in turn makes power consumption in the logic circuit and the D/A convertor disadvantageously higher. From the Electronics World and Wireless World, volumn 97, no. 1678, September 1992, Surrey, GB, pages 746 to 748 "Direct Digital Synthesis", which publication forms the preamble of claim 1, a frequency synthesizer is known, including besides a clock generator and a frequency setting circuit as well as a phase accumulator, a waveform referring circuit for providing amplitude data. These amplitude data are referring to the output of a cumulative phase from the phase accumulator. Furthermore a D/A converter converts amplitude data stemming from the referring circuit into an analog signal. Additionally a filter is arranged so that only components of an analog signal from the D/A converter are passed having frequency components higher than one half of the clock frequency of the clock generator. In view of this prepublished frequency synthesizer it is the object of the present invention to prevent the signal-to-noise-ratio at the synthesizer output from largely degrading. This object is solved by the frequency synthesizer including the features set forth in claim 1. Further advantageous embodiments of the present invention are defined in the dependent claims. Furthermore, a frequency synthesizer according to one embodiment of the invention comprises in addition to the components of the synthesizer above, a phase comparator, to one input terminal of which output from the filter is supplied as input, a loop filter into which output from the phase comparator is supplied as input, a voltage control oscillator which is controlled by output from this loop filter, and a frequency divider circuit which divides output from this voltage control oscillator and supplies the divided output to another input terminal of the phase comparator to form a feedback loop, wherein output from the voltage control oscillator is signal output. A feature of the present invention is that unnecessary frequency components generated by the DDS in Fig. 7 are positively utilized to generated a relatively high frequency. Analog output underwent D/A conversion by the DDS in Fig. 7 can be expressed by the convolution integration as shown below (EXPRESSION 1) with assuming f 0 as a clock frequency, T 0 as the periodic time, and f s as a sinusoidal wave frequency to be generated: Herein x(t) is a single pulse having a pulse width of T 0 . Fourier transformation of convolution integration is expressed by the product of Fourier transformation of each function for convolution integration. A wave form in a time domain and that in a frequency domain of each function are shown in Fig. 4. As shown in this figure, in a spectrum after D/A conversion, in addition to frequency components generally outputted in association with frequency f s , frequency components of nf 0 + f s exist (n: an integral number). These frequencies can be programmed by the frequency setting circuit 10 in the DDS shown in Fig. 7 using the equation above. From the sampling theorem, the maximum value of the frequency f s is 1/2 T 0 , and synthesizer output having a single frequency can be obtained by providing a band-pass filter which passes frequency components in a range from nf 0 - f 0 /2 to nf 0 or a band-pass filter which passes only frequency components in a range from nf 0 to nf 0 + f 0 /2 in the output side of the D/A convertor. Moreover, these frequencies are higher as compared to conventional ones. The synthesizer output thus obtained is advantageous in that a higher frequency can be obtained with the same power consumption as that in the conventional technology, but the output level is lower. Especially in an domain around nf 0 , amplitude comes closer to zero as clearly shown in Fig. 4, so that signal vs noise ratio (S/N) in the synthesizer output is sacrificed. As a method of preventing the S/N ratio in synthesizer output from largely degrading, in the present invention, polarity of output is inverted in each clock cycle when D/A conversion is executed. By inverting polarity in each clock cycle, the D/A-converted output as shown in Fig. 5A can be obtained. This output signal has a wave form obtained by multiplying a sinusoidal wave having a frequency of f s with a continuous rectangular waves for each having a cycle of 1/2 T 0 . This spectrum is expressed by convolution integration for wave forms obtained by Fourier transformation of the sinusoidal wave and continuous rectangular waves respectively, and has frequency components of nf 0 /2 + fs (n: odd number) as shown in Fig. 5B. The frequency components at this level are not affected by the frequency f s , so that large drop of output level due to frequency never occurs. For this reason, synthesizer output can be obtained by connecting a band-pass filter which passes only frequency components in a range from nf 0 /2 - f 0 /2 to nf 0 (n: odd number) or a band-pass filter which passes only frequency components in a range from nf 0 to nf 0 + f 0 /2 (n: odd number) behind the D/A convertor. In the present invention, the DDS according to the present invention is used as a reference frequency signal source based on a feedback loop or so-called PLL realized by phase comparison for the purpose to generate signals having a higher frequency than that generated by the synthesizers described above. As described before, the PLL switching time largely depends on characteristics of a loop filter in a feedback loop, and this is largely affected by a reference frequency. Namely a shorter switching time as compared to that for a conventional type of PLL can be realized by using a relatively higher frequency generated by the DDS according to the present invention as a reference frequency. A frequency generated by this frequency synthesizer is decided by a set frequency for the DDS and a frequency division ratio by a feedback loop frequency divider, so that a very wide range of frequencies can be generated. Other objects and features of this invention will become understood from the following descriptions with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram illustrating a frequency synthesizer which is discussed only for the clarification of the present invention; Fig. 2 is a block diagram illustrating an embodiment of the present invention; and Fig. 3 is a block diagram illustrating an embodiment of the present invention. Fig. 4 is a view illustrating a time domain wave form as well as a frequency domain wave form in the DDS output; Fig. 5A is a view illustrating a wave form in an time domain for explanation of a principle of the present invention; Fig. 5B is a view illustrating a wave form in a frequency domain for explanation of a principle of the present invention; Fig. 6 is a block diagram illustrating configuration of a conventional type of frequency synthesizer; Fig. 7 is a block diagram illustrating configuration of a conventional type of direct digital synthesizer (DDS); Fig. 8A is a view illustrating a wave form provided as output from the DDS; Fig. 8B is a view illustrating a wave form provided as output from the DDS; Fig. 1 is a block diagram illustrating configuration of a frequency synthesizer wherein a band-pass filter (BPF) 22 is used in place of a conventional type of DDS low pass filter 17 as shown in Fig. 7. The same reference numerals are assigned to the same components as those in Fig. 7. It is to be noted that the embodiment of figure 1 does not belong to the invention and does not fall within the scope of the claims. As detailed in detail in relation to effects provided by the present invention, in a spectrum of output frequency from a D/A convertor 15 exist, in addition to frequency components generally output in association with the frequency f s , frequency components of nf 0 + f s (n: integral number). For this reason, synthesizer output having a single frequency of nf 0 - f s or nf 0 + f s can be obtained by using a band-pass filter which passes only frequency components from nf 0 - f 0 /2 to nf 0 or a band-pass filter which passes only frequency components from nf 0 to nf 0 + f 0 /2 in a band-pass filter 22. Next description is made for operations of a frequency synthesizer according to the present invention. A frequency set value input 20 is supplied as input to the frequency setting circuit 10 to program phase increment 11 for unit clock corresponding to a frequency to be generated, and the phase increment 11 is supplied as input to the phase accumulator 12 to obtain accumulation of phases for each unit clock. The phase increment for unit clock is a value expressed by 2 π f S T 0 . The clock signal 23 is generated by the clock generator 21 and supplied as input to the phase accumulator 12. The cumulative phase 13 is supplied as input to the ROM 14 in which a value of sinusoidal wave form corresponding to each phase is stored, and output from the ROM is connected to the D/A convertor and provided as output as the signal 16 converted into analog voltage. Signals from the frequency setting circuit 10 to the D/A convertor 15 are digital signals each consisting of a plurality of bits. For the necessity to process signals at a high speed, these digital signals are generally transferred in parallel through a plurality of lines. Analog output 16 is connected to the band-pass filter 22, which outputs a sinusoidal wave signal 18 having a single frequency component. Fig. 2 is a block diagram illustrating an embodiment of the present invention wherein the polarity of output from a D/A converted is inverted in each clock cycle. The frequency setting circuit 10, phase accumulator 12, clock generator 21, and ROM 14 are the same as those shown in Fig. 1. The frequency synthesizer according to this embodiment of the present invention includes the frequency divider circuit 24 which divides output from the clock generator 21 into two halves and supplied a clock signal divided into two halves to the D/A convertor 25. As the band-pass filter 26, a band-pass filter 26 which passes only frequency components from nf 0 /2 - f 0 /2 to nf 0 /2 (n:odd number) or a band-pass filter which passes only frequency components from nf 0 /2 to nf 0 /2 + f 0 /2 (n:odd number) is used. Operations of the frequency setting circuit 10, phase accumulator 12, and ROM 14 are the same as those in Fig. 1. To the D/A convertor is connected a clock signal divided into a half (1/2) by the frequency divider circuit 24 as described above, and polarity of output from the D/A convertor is inverted according to the logic of "1" or "0". The analog output 27 is connected to the band-pass filter (BPF) 26, which outputs the sinusoidal wave signal 28 having a single frequency component. The band-pass filter 26 can pick up a frequency component of f 0 /2 + f s by setting, for instance, a low band cut-off frequency to f 0 /2 and a high band cut-off frequency to f 0 . Fig. 3 is a block diagram illustrating an embodiment of the present invention in which a conventional type of frequency synthesizer as shown in Fig. 6 is connected to the DDS shown in Fig. 2. A phase comparator 29, a loop filter 30, a VCO 31, and a 1/N frequency divider circuit are inserted behind the band-pass filter 26. In the present embodiment, a first frequency set value 33 is supplied as input to the frequency setting circuit 10, and a second frequency set value 34 is supplied as input to a frequency divider circuit 32. A sinusoidal wave signal output 28 of f 0 /2 + f s from the band-pass filter 26 described in relation to the embodiment of the present invention shown in Fig. 2 is connected as a reference frequency signal source to the phase comparator 29. A signal obtained by dividing output from the VCO 31 by N with the frequency divider 32 is supplied as input to another input terminal of the phase comparator 29. Output from the phase comparator is supplied as input to the loop filter 30 and then connected to the VCO to form a feedback loop. A frequency division ratio N in the frequency divider circuit 32 can be programmed according to a second frequency set value input 34, and the VCO 31 outputs a frequency of N x (f 0 /2 + f S ) as a synthesizer output 35. In the embodiment shown in Fig. 3, the synthesizer shown in Fig. 6 is connected to the DDS shown in Fig. 2, but also the synthesizer shown in Fig. 6 can be connected to the DDS shown in Fig. 1. As described above, the present invention advantageously provides a synthesizer which is based on a relatively simple circuit configuration, yet can switch a frequency at a high speed. Also the synthesizer can generate a wide range of frequencies without losing the advantage described above, when it is necessary to generate an extremely high frequency.

Claims: A frequency synthesizer comprising: a clock generator (21) for generating a clock signal (23); a frequency setting circuit (10) in which a phase increment (11) corresponding to a frequency to be generated is programmed according to an input frequency set value (20); a phase accumulator (12) which adds for each cycle of said clock signal (23) the phase increment (11) set by said frequency setting circuit (10) to a phase cumulative value in said phase accumulator (12) at the previous cycle of said clock signal to generate a new phase cumulative value; a waveform referring circuit (14) for providing as output amplitude data corresponding to said phase cumulative value output from said phase accumulator (12); a D/A converter (25) to which said amplitude data from said waveform referring circuit (14) is supplied as an input and which converts said amplitude data into an analog signal (27); and a filter (26) which passes, of components of said analog signal (27) from said D/A converter (25), only frequency components higher than half the frequency of said clock signal; characterized in that a frequency divider (24) is further provided for dividing said clock signal from said clock generator into two halves; the divided clock signal from said frequency divider (24) is further supplied as an input to said D/A converter (25) for inverting the polarity of said analog signal (27) in each clock cycle according to the logic level of said divided clock signal. A frequency synthesizer according to claim 1, wherein said filter (22; 26) is a band-pass filter, a high band cut-off frequency of said band-pass filter is less than an odd number excluding 1, this odd number times of 1/2 of said clock frequency and a low band cut-off frequency of said band-pass filter is higher than an integral number times of said clock frequency, and said integral number is a number obtained by dividing by two a value obtained by subtracting 1 from said odd number. A frequency synthesizer according to claim 1, wherein said filter (22; 26) is a band-pass filter, a low band cut-off frequency of said band-pass filter is not less than an odd number excluding 1, this odd number times of 1/2 of said clock frequency, the high band cut-off frequency being lower than an integral number times of said clock frequency, and said integral number is a value obtained by dividing by two a value obtained by adding 1 to said odd number. A frequency synthesizer according to one of claims 1 to 3, further comprising: a phase comparator (29), to one input terminal of which is supplied the output from said filter (26); a loop filter (30) into which an output from said phase comparator (29) is supplied as an input; a voltage control oscillator (31) controlled according to an output from said loop filter (30); and a frequency divider (32) which divides an output from said voltage control oscillator (31) and adds said divided output to another input terminal of said phase comparator (29) to form a feedback loop; wherein an output from said voltage control oscillator (31) is a signal output.

Inventors: MAEDA KAZUO, MAEDA, KAZUO
Applicants: UNIDEN KK, UNIDEN CORPORATION
Application year: 1993
Main classification: H03L7
IPC codes: G06F1, H03B28, H03L7