KR100550629B1 - Direct Digital Frequency Synthesise Apparatus and Method for eliminating spurious signal using feed-forward scheme - Google Patents

Abstract

1. TECHNICAL FIELD OF THE INVENTION

The present invention relates to a direct digital frequency synthesizing apparatus and method for improving spurious characteristics using a feedforward scheme.

2. The technical problem to be solved by the invention

According to the present invention, a spurious signal generated by a direct digital synthesizer (DDS) for driving a phase locked loop (PLL) frequency synthesizer is removed using a feedforward method, thereby providing a phase noise characteristic, It is an object of the present invention to provide a direct digital frequency synthesizing apparatus and method for improving spurious characteristics using a feedforward method, which can improve spurious characteristics, settling time and frequency resolution.

3. Summary of the Solution of the Invention

A direct digital frequency synthesizing apparatus for driving a phase locked loop (PLL) frequency synthesizer, comprising: direct digital frequency synthesizing means (DDS) for generating two signals having a predetermined frequency according to an external reference signal; Power distribution means for distributing the output signal of the direct digital frequency synthesizing means (DDS) into two in-phase signals; Time delay one of the signals distributed by the power distribution means, and change the phase and magnitude of the output signal of the direct digital frequency synthesizing means (DDS), and then the time delayed signal and the phase and magnitude A spurious signal extracting means for synthesizing the converted signal and extracting a spurious signal included in the output signal of the direct digital frequency synthesizing means (DDS); And inverting the phase of the spurious signal extracted by the spurious signal extracting means, synthesizing with another signal of the power distribution means, and removing the spurious signal included in the output signal of the direct digital frequency synthesizing means (DDS). Including spurious signal removal means.

4. Important uses of the invention

The present invention is used to improve the spurious properties.

Spurious, Receiver, Frequency Synthesizer, DDS, PLL, Feedforward.

Description

Direct Digital Frequency Synthesis Apparatus and Method for Eliminating Spurious Signal Using Feed-forward Scheme for Improving Spurious Characteristics by Using Feed Forward Method             

1A is a block diagram of a frequency synthesizer driven by a conventional reference frequency generator.

1B is a block diagram of a conventional DDS driving PLL frequency synthesizer.

2A and 2B are schematic diagrams of a DDS driving PLL frequency synthesizing apparatus for improving a conventional spurious characteristic.

3 is an explanatory diagram of a conventional DDS spurious improvement and dither signal cancellation method.

4 is a block diagram of an embodiment of a direct digital frequency synthesizing apparatus for improving spurious characteristics using a feedforward scheme according to the present invention.

5A to 5G are explanatory diagrams of the frequency spectrum at each point of the direct digital frequency synthesizing apparatus for improving the spurious characteristics by using the feedforward method according to the present invention.

6 is a flow diagram of an embodiment of a direct digital frequency synthesis method for improving spurious characteristics using a feedforward scheme according to the present invention.

* Explanation of symbols on the main parts of the drawing

40: reference signal generator 41: direct digital frequency synthesizer

42: PLL Frequency Synthesizer 401: Direct Digital Frequency Synthesizer (DDS)

402: power divider 403: time delay line

404, 407: phase shifter 405: variable attenuator

406 and 408: signal synthesizer 410: spurious signal extractor

411: spurious signal canceller

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct digital frequency synthesizing apparatus and method for improving spurious characteristics, and more particularly, to a direct digital frequency synthesizer (DDS) for driving a phase locked loop (PLL) frequency synthesizer. The present invention relates to a direct digital frequency synthesizing apparatus and a method for improving spurious characteristics by using a feedforward method, which removes a spurious signal generated by a feedforward method.

In a receiver that analyzes the spectrum of an arbitrary signal, such as a spectrum analyzer, the phase noise and spurious characteristics of the local oscillator (LO) in addition to the noise figure of the receiver have a great influence on the spectrum analysis performance.

Therefore, a receiver for analyzing the spectral characteristics of a signal in an arbitrary frequency band requires a frequency synthesizer having good phase noise characteristics, spurious characteristics, settling time and frequency resolution.

Among the performance of the frequency synthesizer, the settling time and frequency resolution have a trade-off characteristic, and DDS is used to increase the frequency resolution. In other words, considering the frequency resolution and the settling time, the use of DDS in a frequency synthesizer is essential.

However, when using a direct digital synthesizer (DDS), the frequency resolution and phase noise characteristics are good, but the spurious generated by itself is large.

Therefore, if the spurious characteristics generated in the DDS are improved, a frequency synthesizer with good frequency resolution, small settling time, good phase noise characteristics and spurious characteristics can be obtained.

Hereinafter, a conventional technique for improving the spurious characteristics is as follows.

1A is a configuration diagram of a frequency synthesizer driven by a conventional reference frequency generator, and FIG. 1B is a configuration diagram of a conventional DDS drive frequency synthesizer.

As shown in FIG. 1A, the sum-of-frequency growth value driven by the reference oscillator 100 has a limited frequency resolution that is variable by a divider inside the phase locked loop frequency synthesizer 101. There was a problem.

The limited frequency resolution of such a frequency synthesizer can be improved by using a PLL frequency synthesis scheme driven by the DDS 110 as shown in FIG. 1B.

However, the PLL frequency synthesizer (see FIG. 1B) driven by the DDS 110 also has a problem in that the spurious characteristics generated in the DDS are not filtered and appear at the output of the final frequency synthesizer.

2A and 2B are schematic diagrams of a DDS driving PLL frequency synthesizer for improving spurious characteristics of the related art.

As shown in FIG. 2A, a DDS-driven PLL frequency synthesizer for improving spurious characteristics is a technique proposed in US Pat. No. 5,028,887, which includes a limiter 202 that suppresses spurious components above a certain amplitude in order to reduce spurious noise. I use it.

The output signal of the DDS 200 is input to the limiter 202 and the band pass filter 201 to remove spurious noise and harmonic noise, and then to the reference signal of the PLL frequency synthesizer 203.

Limiter 202 is configured using two diodes or saturating amplifiers connected back-to-back.

However, in the DDS-driven PLL frequency synthesizer, since the inserted limiter 202 is composed of a solid electronic device, additional spurious noise may occur and it is difficult to remove the close-in spurious component.

Next, as shown in FIG. 2B, the DDS driving PLL frequency synthesizing apparatus for improving spurious characteristics is a technique proposed in US Pat. No. 5,757,239, and relates to a DDS driving frequency synthesizing apparatus which improves the problem of FIG. 1B.

In FIG. 2B, the clean-up PLL frequency synthesizer 211 functions as a bandpass filter. That is, since the output frequency of the DDS 210 and the frequency of the clean-up PLL frequency synthesizer 211 are almost the same, the frequency bandwidth of the loop filter of the clean-up PLL frequency synthesizer 211 can be reduced. Since the frequency bandwidth of the loop filter of the clean-up PLL frequency synthesizer 211 is small, it is possible to filter the spurious generated by the DDS 210.

However, when the frequency bandwidth of the loop filter is small, there is a problem that the settling time is increased.

Therefore, the DDS driving frequency sum growth value as shown in FIG. 2B solves this problem by directly using the output of the DDS 210 before the clean-up PLL frequency synthesizer 211 performs normal operation.

That is, before the clean-up PLL frequency synthesizer 211 operates normally, the output of the DDS 210 having poor spurious characteristics is directly used, and when the clean-up PLL frequency synthesizer 211 operates normally, clean-up is performed. The output of the PLL frequency synthesizer 211 is used as an input signal of the final frequency synthesizer. Therefore, such a DDS-driven PLL frequency synthesizer has a problem that spurious characteristics may not be good initially.

3 is an explanatory diagram of a conventional DDS spurious improvement and dithering signal cancellation method, which is used as a means for improving spurious generated in a digital signal processing apparatus such as a DDS or a digital-to-analog (D / A) converter. A spurious improvement method using dithering, which is a general method, is shown, which relates to a technique disclosed in US Pat. No. 6,522,176 B1.

The phase / magnitude converter 300 converts the phase into a corresponding magnitude value, and the band-shaped dither generator 301 generates a band limited dither signal. The output of the D / A converters 306 and 307 after adding or subtracting a dither signal to the signal of the I channel (302) and then rounding (304, 305) to convert to a suitable digital value. When subtracted from each other (308), the output can prevent the increase of noise caused by the dither signal.

Signal processing for the Q channel is the same as the I channel. By adding and subtracting a dither signal in this way, an increase in noise caused by the dither signal can be eliminated.

In general, when the dithering signal is used, the spurious signal is improved, but the phase noise is increased by the dithering signal.

In order to solve this problem, the method shown in FIG. 3 first improves the spurious characteristics by using a dithering signal, and then increases the phase noise by using a method of removing an added dithering signal. Improved.

However, the conventional technique shown in FIG. 3 is a method for improving the spurious characteristics of the DDS itself, or a prior art that can be used when integrating the DDS, and has a problem in that the structure is complicated.

The present invention has been proposed to solve the above problems, by removing the spurious signal generated in the DDS (Direct Digital Synthesizer) driving the PLL frequency synthesizer by using the feed forward method, phase noise characteristics, spurious characteristics It is an object of the present invention to provide a direct digital frequency synthesizing apparatus and a method for improving spurious characteristics by using a feedforward method, which can improve settling time, frequency resolution, and the like.

The present invention for achieving the above object, in a direct digital frequency synthesizer for driving a phase locked loop (PLL) frequency synthesizer, generating two signals having a predetermined frequency according to an external reference signal Direct digital frequency synthesizing means (DDS); Power distribution means for distributing the output signal of the direct digital frequency synthesizing means (DDS) into two in-phase signals; Time delay one of the signals distributed by the power distribution means, and change the phase and magnitude of the output signal of the direct digital frequency synthesizing means (DDS), and then the time delayed signal and the phase and magnitude A spurious signal extracting means for synthesizing the converted signal and extracting a spurious signal included in the output signal of the direct digital frequency synthesizing means (DDS); And inverting the phase of the spurious signal extracted by the spurious signal extracting means, synthesizing with another signal of the power distribution means, and removing the spurious signal included in the output signal of the direct digital frequency synthesizing means (DDS). Spurious signal removal means.

On the other hand, the present invention, in the direct digital frequency synthesis method, the direct digital frequency synthesis means (DDS) generates two signals having a predetermined frequency according to the external reference signal (hereinafter referred to as DDS output signal) Generating a DDS output signal; A power distribution step of power distribution of the generated DDS output signal into two signals in phase; Time-delay any one of the signals distributed in the power distribution step, convert phase and magnitude of the generated DDS output signal, synthesize the time-delayed signal and the signal whose phase and magnitude are converted, and A spurious signal extracting step of extracting a spurious signal included in the DDS output signal; And a spurious signal removing step of inverting a phase of the extracted spurious signal and combining the other signal among the signals distributed in the power distribution step to remove the spurious signal.

The above objects, features and advantages will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

4 is a configuration diagram of a direct digital frequency synthesizing apparatus for improving spurious characteristics using a feedforward method according to the present invention, wherein the direct digital frequency synthesizing apparatus 41 for improving spurious characteristics is a direct digital frequency. A combiner (DDS) 401, a power divider 402, a spurious signal extractor 410, and a spurious signal remover 411.

Here, the spurious signal extractor 410 removes only the spurious signal from the signal of the specific frequency of the DDS 401 including the spurious signal, and the spurious signal remover 411 outputs the signal (ie, the spurious) of the DDS 401. The spurious signal extracted by the spurious signal extractor 410 is removed.

First, the overall operating process of the present invention will be described.

The direct digital frequency synthesizer (DDS) 401 generates two signals having a specific frequency according to the reference signal generated by the reference generator 40.

The power divider 402 directly distributes the output signal of the digital frequency synthesizer (DDS) 401 into two in-phase signals.

The spurious signal extractor 410 then time-delays any one of the signals distributed by the power divider 402, and directly changes the phase and magnitude of the output signal of the digital frequency synthesizer (DDS) 401. Thereafter, both signals are synthesized to directly extract the spurious signal included in the output signal of the digital frequency synthesizer (DDS) 401.
In more detail, the time delay line 403 of the spurious signal extractor 410 has the same time period as the two input signals of the signal synthesizer 1 406 (a signal whose time is delayed and a phase shifted and variable attenuated signal) are the same. Used to be entered into.
In other words, the time delay line 403 is configured by the time delay line 403 by the time that the signal passing through the phase shifter 1 404 and the variable attenuator 405 of the output signal of the direct digital frequency synthesizer (DDS) 410 undergoes. Delaying the time of the remaining signals of the output signal of the (DDS) 410 to be input to the signal synthesizer 1 (406) at the same time.
In this case, the time delay line 403 may be generally implemented by connecting an RF cable. By measuring the time delay between the phase shifter 1 404 and the variable attenuator using an RF signal measuring device (network analyzer) and connecting the RF cable accordingly, a time delay line 403 for delaying the time of the signal is implemented. .

The spurious signal remover 411 inverts the phase of the spurious signal extracted by the spurious signal extractor 410 and synthesizes the signal with the other signal of the power divider 402 to directly convert the digital frequency synthesizer (DDS) 401. Remove the spurious signal included in the output signal of.

Hereinafter, the present invention will be described in detail with reference to FIG. 4.

A reference oscillator 40 provides a reference signal of the DDS driving frequency synthesizer, which can be linked with other devices.

The signal of the reference signal generator 40 is input as a drive signal of the direct digital synthesizer (DDS) 401, and the DDS 401 has a frequency resolution according to data information (ie, information on frequency setting) applied from the outside. It produces a specific frequency signal that has good and good phase characteristics but poor spurious characteristics.

The DDS 401 can produce two outputs at the same frequency, which contain spurious noise components. One of the two output signals of the DDS 401 is divided into two in-phase signals through a power divider 402, one of which is applied to a combiner2 408, and the remaining ones. The signal is applied to the signal combiner 1 406 via the delay line 403. Here, the time delay line 403 delays the time to reach the signal synthesizer 1 (406) at the same time as the output signal of the variable attenuator 405, as described above.

Another signal of the output of the DDS 401 is applied to the signal synthesizer 1 406 via the phase shifter 1 404 and the variable attenuator 405.

During the two paths between power divider 402 and signal synthesizer 1 406, if the magnitude of the signal is the same and the phases are reversed, the specific frequency signal of the DDS is removed from the output of signal synthesizer 1 406 and the DDS Only the spurious signal generated by X will be present.

The phase shifter 2 407 reverses the phase of this spurious signal, and the spurious signal converted by the phase shifter 2 407 out of phase in the signal synthesizer 2 408 and the output of the DDS distributed in the power divider 402. When the signal is synthesized, the spurious signal generated in the DDS 401 is removed.

The signal generated by the DDS 401 is passed through a feed-forward circuit and the spurious characteristic is improved, and then is applied as a driving signal of the PLL frequency synthesizer 42.

The PLL frequency synthesizer 42 converts a signal received from the DDS 401 into a signal of a specific frequency in accordance with externally applied data information (ie, information on frequency setting). Since the spurious characteristic of the reference signal applied to the PLL frequency synthesizer 42 is improved, the spurious characteristic is also improved at the output of the PLL frequency synthesizer 42 as a matter of course.

5A to 5G are explanatory diagrams of the frequency spectrum at each point of the DDS driven PLL frequency synthesis apparatus for improving the spurious characteristics by using the feedforward method according to the present invention.

The spectrum of the reference frequency signal, which is the signal of point A of FIG. 4, is represented by F _osc in FIG. 5A. This reference frequency signal is applied to the DDS 401, and the spectrum at points B and C, which are the outputs of the DDS 401, is spurious at a specific DDS output signal F _DDS as shown in FIG. 6B. It consists of a signal.

The DDS output signal is applied to a path having the time delay line 403 of FIG. 4 and a path composed of the phase shifter 1 404 and the variable attenuator 405. The same applies to the use of a time delay in place of the time delay line.

During these two paths, the magnitude change of the signal is the same, the phases are reversed (see Fig. 5c), and because of the time delay difference between the two paths, the narrowband signal (i.e., the Frequency signal) is removed (see FIG. 5D).

Therefore, at the point F of FIG. 4, as shown in FIG. 5D, only information about the spurious signal and the phase noise of the DDS output can be obtained, and the phase of the signal is reversed by the phase shifter 2 (407) (see FIG. 5E). By applying to the signal synthesizer 2 (408), at the point I, an F _DDS signal having improved spurious signals and phase noise signals existing around a specific frequency signal of the _DDS can be obtained as shown in FIG. 5F.

That is, in the signal synthesizer 2 408, the output signal of the power divider 402 (see FIG. 5B) and the output signal of the phase shifter 2 407 (see FIG. 5E) are combined to remove the spurious signal.

By applying the output signal of the signal synthesizer 2 408 to the PLL frequency synthesizer 42, a desired frequency component F _PLL , as shown in Fig. 5G, can be obtained.

6 is a flowchart illustrating an embodiment of a direct digital frequency synthesis method for improving spurious characteristics by using a feedforward scheme according to the present invention.

First, the direct digital frequency synthesizer 401 (DDS) generates two signals having a specific frequency (hereinafter, referred to as a DDS output signal) according to the reference signal generated by the reference signal generator 40 (600).

When the power divider 402 power-distributes the DDS output signal generated at " 600 " into two in-phase signals (603), the time delay line 403 time-delays any one of the two divided signals ( 604).

On the other hand, if the phase shifter 1 404 shifts the phase 180 degrees with respect to the DDS output signal generated at " 600 ", the variable attenuator 405 adjusts the magnitude of the phase-inverted DDS output signal (602).

Subsequently, the signal synthesizer 1 406 synthesizes a signal whose phase and magnitude are converted by the phase shifter 1 404 and the variable attenuator 405 and the time delayed signal by the time delay line 403 and includes it in the DDS output signal. The extracted spurious signal is extracted (605).

When phase shifter 2 407 inverts the phase of the extracted spurious signal by 180 degrees (606), signal synthesizer 2 408 combines with one of the signals distributed at " 603 " to remove the spurious signal (607). ).

As described above, the method of the present invention may be implemented as a program and stored in a recording medium (CD-ROM, RAM, ROM, floppy disk, hard disk, magneto-optical disk, etc.) in a computer-readable form.

The present invention described above is capable of various substitutions, modifications, and changes without departing from the spirit of the present invention for those skilled in the art to which the present invention pertains, and the above-described embodiments and accompanying It is not limited by the drawings.

The present invention as described above has the effect of improving the overall phase noise characteristics, spurious characteristics, settling time and frequency resolution by improving the spurious characteristics of the DDS driving frequency synthesizer.

In addition, the present invention can be effectively used to quickly and accurately measure the spectral characteristics of a signal in any band, and there is an effect that can be usefully used not only in communication systems but also in measurement equipment.

Claims (8)

In a direct digital frequency synthesizer for driving a phase locked loop (PLL) frequency synthesizer, Direct digital frequency synthesizing means (DDS) for generating two signals having a predetermined frequency in accordance with an external reference signal; Power distribution means for distributing the output signal of the direct digital frequency synthesizing means (DDS) into two in-phase signals; Time delay one of the signals distributed by the power distribution means, and change the phase and magnitude of the output signal of the direct digital frequency synthesizing means (DDS), and then the time delayed signal and the phase and magnitude A spurious signal extracting means for synthesizing the converted signal and extracting a spurious signal included in the output signal of the direct digital frequency synthesizing means (DDS); And Inverts the phase of the spurious signal extracted by the spurious signal extracting means, synthesizes it with another signal of the power distribution means, and removes a spurious signal included in the output signal of the direct digital frequency synthesizing means (DDS). Signal rejection means Direct digital frequency synthesizing apparatus for improving the spurious characteristics using a feed forward method comprising a. The method of claim 1, The spurious signal extraction means, Time delay means for time-delaying any one of the output signals of said power distribution means; Phase shifting means for inverting the phase of the other of the output signals of the direct digital frequency synthesizing means (DDS); Variable attenuation means for adjusting the magnitude of the signal inverted by the phase shifting means; And First signal synthesizing means for synthesizing a signal delayed by said time delay means and an output signal of said variable attenuation means for extracting a spurious signal; Direct digital frequency synthesizing apparatus for improving the spurious characteristics using a feed forward method comprising a. The method of claim 2, The time delay by the time delay means, And a time delay to reach the first signal synthesizing means at the same time as the output signal of the variable attenuation means. The direct digital frequency synthesizing apparatus for improving the spurious characteristics by using a feedforward method. The method of claim 2, Adjusting the signal magnitude of the variable attenuation means, And a direct digital frequency synthesizing apparatus for improving the spurious characteristics by using a feedforward method, characterized in that the signal has the same magnitude as the output signal of the time delay means. The method according to any one of claims 1 to 4, The spurious signal removing means, Second phase shifting means for inverting the phase of the spurious signal extracted by the spurious signal extracting means; And Second signal synthesizing means for removing the spurious signal included in the output signal of the direct digital frequency synthesizing means (DDS) in combination with another signal among the signals distributed by the power distribution means; Direct digital frequency synthesizing apparatus for improving the spurious characteristics using a feed forward method comprising a. In the direct digital frequency synthesis method, A DDS output signal generating step of direct digital frequency synthesizing means (DDS) generating two signals having a predetermined frequency (hereinafter referred to as DDS output signals) according to an external reference signal; A power distribution step of power distribution of the generated DDS output signal into two signals in phase; Time-delay any one of the signals distributed in the power distribution step, convert phase and magnitude of the generated DDS output signal, synthesize the time-delayed signal and the signal whose phase and magnitude are converted, and A spurious signal extracting step of extracting a spurious signal included in the DDS output signal; And A spurious signal removing step of inverting the phase of the extracted spurious signal and then removing the spurious signal by combining with one of the signals distributed in the power distribution step; Direct digital frequency synthesis method for improving the spurious characteristics using a feed forward method comprising a. The method of claim 6, The spurious signal extraction step, A time delay step of time-delaying any one of the signals distributed in the power distribution step; A phase shifting step of inverting the phase of another signal among the DDS output signals generated in the DDS output signal generating step; A variable attenuation step for adjusting the magnitude of the phase inverted signal in the phase shifting step; And A first signal synthesis step of synthesizing a signal delayed in the time delay step and a signal whose signal size is adjusted in the variable attenuation step to extract a spurious signal included in the DDS output signal; Direct digital frequency synthesis method for improving the spurious characteristics using a feed forward method comprising a. The method of claim 7, wherein The spurious signal removing step, A second phase shift step of inverting the phase of the spurious signal extracted in the first signal synthesis step; And A second signal synthesizing step of synthesizing the spurious signal phase-inverted in the second phase shifting step with another one of the signals distributed in the power distribution step to remove the spurious signal included in the DDS output signal; Direct digital frequency synthesis method for improving the spurious characteristics using a feed forward method comprising a.

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