JPH0923158A - Frequency synthesizer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the spurious of a frequency synthesizer, to widen the band of output frequency and to speed up frequency switching by using a direct digital synthesizer. SOLUTION: When an output 2 from the direct digital synthesizer(DDS) 1 is included within a frequency range capable of reducing the generation of spurious, a switch 14 inputs the output 2 to a phase synchronizing loop 10 as a reference signal, and in the case of controlling the output 2 to a frequency range capable of increasing the spurious, the switch 14 seiects an output from a mixer 12 for mixing the output 2 in the frequency range for reducing the spurious and a signal with prescribed frequency outputted from an oscillator 13 and outputs the selected signal as a reference signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency synthesizer using a direct digital synthesizer (hereinafter referred to as "DDS") which has a low spurious frequency, a wide band, and a high speed.

[0002]

2. Description of the Related Art FIG. 7 shows, for example, Japanese Patent Laid-Open No. 63-296.
522, US Pat. No. 4,965,533 or 1981 IEEE 35th Annual Frequent
ncyControl Symposium Dige
It is a structural example of the conventional frequency synthesizer shown on pages 406 to 414 of st. In the figure, 1 is DDS, 2 is an output terminal of DDS1, 3 is a reference clock signal oscillator, 4 is a filter for removing spurious of DDS1 and harmonics due to a mixer, 5 is a phase comparator, 6 is a loop filter, 7 is VCO which is a voltage controlled oscillator, 8 is a coupler, 9 is a frequency divider, 10 is a phase locked loop composed of a phase comparator 5, a loop filter 6, a VCO 7, a coupler 8 and a frequency divider 9, and 11 is a frequency synthesizer. Output terminal.

Next, the operation will be described with reference to the drawings. In the conventional frequency synthesizer shown in FIG. 7,
A part of the output (fo) of the VCO 7 is taken out by the coupler 8,
The signal is frequency-divided by the frequency divider 9 and the output signal (f
r) and the phase comparator 5 perform phase comparison. By passing the output signal of this phase comparator through the loop filter 6, fo
The loop operates so that there is no phase difference between f and fr.

Next, the DDS 1 will be described. It generates a digitally controlled frequency from a single reference signal and is composed of a phase accumulator, a phase-amplitude converter and a D / A converter as shown in FIG. A phase accumulator is a device that produces a phase increase in the output waveform,
After that, the phase-amplitude converter converts the phase information into amplitude data, and the final stage D / A converter converts the phase information into an analog signal. The frequency of the output signal is expressed by the following equation. f _out = K · f _clk / 2 ⁿ where f _clk is a single reference clock signal and n is the word length of the phase accumulator.

[0005]

The waveform of the reference signal input to the phase comparator in the phase locked loop greatly affects the output waveform of the frequency synthesizer. That is, spurious signals having frequencies away from the carrier of the reference signal appear in the output signal at a level represented by Ps.20.logN without reducing spurious signals in the vicinity of the carrier by the loop filter 4. Here, Ps is a spurious level included in the reference signal. Fig. 9 shows the output spectrum of DDS1.
_The frequency spurious represented by _clk ± B · f _out (A, B = 0, 1, 2, 3 ...) is included. Therefore DDS
The spurious signal of 1 may appear in the vicinity of the carrier depending on the output frequency of the DDS 1, and if it cannot be removed by the filter 8 or the loop filter 4, the output waveform of the synthesizer deteriorates. Therefore, in the conventional synthesizer using the DDS1, there is a problem that the output waveform of the synthesizer deteriorates at a frequency where a high level spurious is generated near the carrier of the DDS1 output.

In order to widen the output frequency of the frequency synthesizer, it is necessary to widen the reference signal, that is, the DDS1 output. For example, when the output frequency range of the synthesizer is ΔF, the frequency range of the reference signal is represented by ΔF / N, so there is a problem that the output frequency range of the synthesizer is limited by the frequency range of DDS1.

The present invention has been made to solve the above problems, and an object thereof is to obtain a wide band frequency synthesizer which is not affected by spurious near the carrier of the DDS1.

[0008]

A frequency synthesizer according to the present invention is a direct digital synthesizer for inputting a frequency setting signal and a reference clock, performing frequency synthesis by digital operation, and outputting a signal of a first frequency, First
Frequency conversion means for inputting a signal of a frequency of 1 to output a signal of a second frequency different from the first frequency, and phase synchronization for inputting a reference signal of a predetermined frequency and outputting a signal of a required frequency. A loop, and a signal of the first frequency and a signal of the second frequency according to the frequency of the reference signal of the predetermined frequency.
And a reference signal switching means for switching to the phase locked loop as a reference signal of the predetermined frequency.

Further, in the frequency synthesizer according to the next invention, the frequency converting means inputs and mixes an oscillator that oscillates a predetermined frequency, a signal of the first frequency and a predetermined frequency that the oscillator oscillates. It is provided with a mixer and outputs a signal of the second frequency.

Further, in the frequency synthesizer according to the next invention, the frequency conversion means outputs a plurality of oscillators that oscillate different frequencies, and a signal output by any one of the plurality of oscillators and a signal of the first frequency. A mixer for inputting and mixing and an oscillator switching means for switching the plurality of oscillators and connecting to the mixer are provided, and a signal of the second frequency is output.

In the frequency synthesizer according to the next invention, the frequency converting means is a frequency multiplier.

Further, a frequency synthesizer according to the next invention comprises a direct digital synthesizer which inputs a frequency setting signal and a reference clock and outputs a signal of a first frequency which is frequency-synthesized by digital operation, and a predetermined reference signal. A phase-locked loop that inputs and outputs a signal of a required frequency, and a plurality of low-pass filters having different characteristics,
A required low pass filter among the plurality of low pass filters is directly connected to the digital synthesizer according to a frequency of the signal of the first frequency, and an output of the required low pass filter is a reference signal of the predetermined frequency. And a filter selecting means for outputting as.

A frequency synthesizer according to the next invention comprises a first direct digital synthesizer for inputting a frequency setting signal and a reference clock and outputting a signal of a first frequency that is frequency-synthesized by digital operation, and the frequency synthesizer. A second direct digital synthesizer that inputs a setting signal and the reference clock, frequency-synthesizes them by digital calculation, and outputs a signal of a second frequency different from the first frequency, and inputs the reference signal And a mixer for mixing the signal of the first frequency and the signal of the second frequency and outputting the reference signal of the predetermined frequency.

Further, a frequency synthesizer according to the next invention comprises a first direct digital synthesizer which inputs a frequency setting signal and a reference clock and outputs a signal of a first frequency synthesized by digital operation, and the frequency setting. A second direct digital synthesizer that inputs a signal and the reference clock, synthesizes them by digital operation, and outputs a signal of a second frequency different from the first frequency, and a signal of the second frequency and a later-described signal. A mixer for mixing the feedback signal to be output, a phase comparator for phase-comparing the output signal of the mixer and the signal of the first frequency, and the output of the phase comparator is input through a loop filter to obtain N times ( N is a positive number) and has a voltage controlled oscillator that outputs a signal of a frequency, and a frequency divider that divides the output of the voltage controlled oscillator into 1 / N and outputs the feedback signal. It is obtained by a phase locked loop for outputting a frequency signal of a main.

[0015]

In the frequency synthesizer according to the present invention, the direct digital synthesizer can input the frequency setting signal and the reference clock and can output the signal of the first frequency with less spurious. A signal of a first frequency is input and a signal of a second frequency different from the first frequency is output, and the reference signal switching means changes the first signal according to the frequency of the reference signal input to the phase locked loop. The frequency signal and the second frequency signal are output to the phase locked loop as a switching reference signal.

Further, in the frequency synthesizer of the next invention, the mixer inputs the output of the oscillator, the signal of the first frequency with less spurious and the predetermined frequency oscillated by the oscillator, and outputs the signal of the second frequency. To do.

Further, in the frequency synthesizer according to the next invention, among a plurality of oscillators that oscillate different frequencies, the oscillator switching means inputs the output of the connected oscillator and the signal of the first frequency with less spurious. And the mixer outputs a signal of the second frequency.

Further, in the frequency synthesizer of the next invention, the frequency multiplier inputs the signal of the first frequency with less spurious and outputs the signal of the second frequency.

Further, in the frequency synthesizer according to the next invention, the filter selecting means directly digitalizes a required low-pass filter among a plurality of low-pass filters in accordance with the frequency of the signal of the first frequency output directly from the digital synthesizer. Since it is connected to the synthesizer, the spurious that changes according to the frequency of the signal of the first frequency is removed by the required low-pass filter, and the signal of the first frequency from which the spurious is removed is input to the phase locked loop as the reference signal. To be done.

A frequency synthesizer according to the next invention is a signal of a first frequency output from the first direct digital synthesizer and a second direct digital synthesizer and a signal of a second frequency different from the first frequency. And are mixed by a mixer and output to the phase locked loop as a reference signal.

Further, in the frequency synthesizer according to the next invention, the first direct digital synthesizer outputs a signal of the first frequency, and the second direct digital synthesizer outputs a signal of the second frequency different from the first frequency. And the phase-locked loop outputs the output of the phase comparator for phase comparison between the output of the mixer that mixes the input signal of the second frequency and the feedback signal and the input signal of the first frequency. It is input to the voltage-controlled oscillator via and outputs a signal of a required frequency.

[0022]

【Example】

Embodiment 1 FIG. An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a frequency synthesizer according to an embodiment of the present invention. In the figure, 1 is a DDS, 3 is a DDS reference clock oscillator, and 4a is a filter for removing harmonics and spurious contained in a DDS output signal. 1
Reference numeral 2 is a mixer, 13 is a local oscillator, 14 is a switch for switching between a system passing through the mixer and a system bypassing the mixer, and 10 is a phase locked loop.

Next, the operation will be described with reference to the drawings. The frequency synthesizer according to the present invention normally outputs the output signal of DDS1 through the filter 4a to the phase locked loop 10 as the reference signal of frequency fr. The output frequency of the phase locked loop 10, that is, the output signal of the frequency synthesizer is N · fr. By the way, since the output signal of the DDS1 includes the spurious represented by A · f _clk ± B · f _out as described above, at a certain DDS1 output frequency, the low-order spurious of the frequency f _sp is near the carrier. May occur. Such low-order spurs cannot be removed by the filter 4a or the loop filter 6 and appear as high-level spurious in the output signal of the synthesizer. In such a case, the switch 14 is switched to select a system passing through the mixer 12, and the output frequency of the DDS 1 is controlled so that the desired fr is obtained at the input of the phase comparator. Since the spurious of the DDS1 is determined by the value of the DDS1 output frequency, the spurious of low order is generated in the vicinity of the carrier without changing the output frequency of the DDS1 drastically. Therefore, the fr of the reference signal is largely changed by the system passing through the mixer 12, and the frequency is changed within a range in which the low-order spurious is not included in the output frequency of the DDS1.
Thus, the fr of the reference signal is finely adjusted so that low-order spurious is not generated near the carrier. At this time, it is necessary to select the frequency of the oscillator 13 in advance so that the spurious signal does not occur near the carrier of the DDS1 output signal. In this way, there is an effect of avoiding the influence of spurious generated near the carrier.

In the above description, the single oscillator 1 is used as the local of the mixer 12 for frequency converting the output signal of the DDS 1.
Although 3 is used, by providing a plurality of oscillators 13 so that the frequency of the oscillator 13 can be selected, the influence of spurious f _{sp in} many cases can be avoided and a wideband frequency synthesizer can be realized.

Embodiment 2 FIG. 2 is a block diagram showing another embodiment of the frequency synthesizer of the present invention. In the figure, the same reference numerals as those in FIG. 1 designate the same or corresponding parts. In the figure, a switch 14a switches the output signal of the DDS 1 between a system passing through the mixer 12 and a system bypassing it, and inputs a reference signal into the phase synchronization loop 10, and 14b indicates a plurality of oscillators 13a, b, c. It is a switch to change.

Next, the operation will be described with reference to the drawings. Example 1
Similarly to the above, by switching the switch 14a at f _sp such that spurious is generated in the vicinity of the carrier in the output signal of DDS1, the system passing through the mixer 12 is switched,
A frequency at which the DDS1 output frequency does not generate spurious near the carrier is selected. At this time, if the number of output frequencies of the synthesizer is large, a large number of f _sp will be generated, and if the oscillator 13 is a single unit, f _sp should be avoided even if the system bypassing the mixer 12 and the system passing through the mixer 12 are switched. Is impossible. In such a case, by providing a plurality of oscillators 13 and switching the oscillators 13a, 13b, 13c according to the spurious near the carrier of the DDS1 output, it is possible to cope with multi-channel and wide band. Further, by having a plurality of oscillators 13, the fr of the reference signal becomes a wide band and a wide band frequency synthesizer can be realized.

Embodiment 3 FIG. FIG. 3 is a block diagram showing another embodiment of the frequency synthesizer of the present invention. In the figure, the same symbols as in FIG. 1 indicate the same or corresponding parts. In the figure, 14
Reference characters c and d are switches for switching between a system bypassing the multiplier and a system passing through the multipliers 15a and 15b for multiplying the frequency, and inputting a reference signal to the phase synchronization loop 10.

Next, the operation will be described with reference to the drawings. As described above, the output signal of the DDS 1 contains spurious signals, but as the output frequency increases, low-order and high-level spurious signals are generated near the carriers. In order to avoid such a high level spurious, the output frequency of DDS1 must be kept low, and the band of fr of the reference signal is limited. As shown in FIG. 3, by switching the switches 14c and 14d, there is an effect of widening the reference signal in a wide band while suppressing the output frequency of the DDS 1, so that a wide band frequency synthesizer can be realized. For example, when the DDS 1 can use only the band of 5 to 10 MHZ for the reference signal due to the spurious problem, the multiplier 15a can adjust the frequency to 10 depending on the output frequency.
To 20HZ, and 20 to 30 by the multiplier 15b.
By multiplying to HZ, the reference signal of 5 to 30 HZ can be made to avoid generation of low-order spurious.

Embodiment 4 FIG. FIG. 4 is a block diagram showing another embodiment of the frequency synthesizer of the present invention. In the figure, the same symbols as in FIG. 1 indicate the same or corresponding parts. In the figure, switches 14e and 14f switch filters having different pass band frequencies according to the output frequency of the frequency synthesizer.

Next, the operation will be described with reference to the drawings. DDS
As described above, the output signal of 1 is f _sp = f _clk ± f
_Although spurious represented by _out is included, this spurious has no difference in phase noise and level compared to the basic output signal of DDS1, and when used as reference signals, no difference is seen in the synthesizer output. In addition, some spurious signals of the DDS1 have a higher frequency than the fundamental wave, and selecting the band frequencies of the filters 4a, 4b, and 4c that prevent the passage of these spurious signals has the effect of broadening the frequency band of the reference signal fr. A frequency synthesizer can be realized.

Embodiment 5 FIG. FIG. 5 is a block diagram showing another embodiment of the frequency synthesizer of the present invention. In the figure, the same symbols as in FIG. 1 indicate the same or corresponding parts. In the figure, 1a
Is a first DDS, 1b is a second DDS, 4a and 4b are filters for removing spurious contained in the output signals of the first DDS 1a and the second DDS 1b, and the first DDS
The output signal of the DS1a is frequency-converted by the mixer 12 and used as the reference signal of the phase locked loop 10. 4C is the mixer 12
Is a filter that removes spurious contained in the output signal of.

Next, the operation will be described with reference to the drawings. As described above, when the output frequency of the first DDS1a is increased, low-order high-level spurious appears in the vicinity of the carrier. Therefore, when the output signal of the first DDS1a is used as the reference signal, the bandwidth is widened. Is difficult. Therefore, as shown in FIG. 5, frequency conversion is performed by the mixer 12 and the second DDS 1b, and both DDSs are frequency-controlled, so that there is an effect of widening the frequency band of the reference signal fr. In this case, since the phase of the reference signal continuously changes even if the frequency is changed, unlike the case of the second embodiment, there is an effect that the phase lock loop 10 does not lose synchronization and the frequency is smoothly switched. Therefore, a wideband and high-speed frequency synthesizer can be realized.

Embodiment 6 FIG. 6 is a block diagram showing another embodiment of the frequency synthesizer of the present invention. In the figure, the same reference numerals as those in FIG. 5 designate the same or corresponding parts. In the figure, 1b
Is a second DDS, 16 is a mixer for frequency-converting the output signal of the VCO 7, and a filter 4d for removing its harmonics. By controlling the respective DDS 1a, 1b, the output frequency of the synthesizer is controlled.

Next, the operation will be described with reference to the drawings. When the output frequency of the frequency synthesizer is changed, the first DDS 1a that outputs the reference signal is controlled and the VC
The mixer 16 and the second DDS 1b are used in the system that frequency-converts the output signal of O7 to the phase comparison frequency, and the second DDS 1b is also controlled. When the reference signal of the phase locked loop 10a is input from the first DDS 1a, the first DSS
When the output frequency of 1a is largely shaken, low-order spurious is generated near the output carrier of the first DSS 1a, and therefore the output frequency cannot be greatly shaken.

Therefore, this embodiment realizes a wide band frequency synthesizer by changing the frequency at the frequency dividing point of VC07. In the frequency synthesizer shown in FIG. 7 described in the conventional example, for example, VC
The frequency division number of O7 is fixed to 100 and the DSS1 output is 10M.
When the output frequency of DSS1 is changed by ± 5 MZ as HZ, the output of this frequency synthesizer is 1 GHZ ± 500 MHZ.
Becomes However, if the frequency division number of the VCO 7 is variable from 90 to 110, the output frequency of the DSS1 is 5 MHZ and V
When the CO7 sub-frequency is 90, the output frequency of the frequency synthesizer is 450 MHZ, and when the DSS1 output frequency is 15 MHZ and the VCO7 sub-frequency is 110, the frequency synthesizer output frequency is 1650 MHZ.
And becomes a wide band synthesizer from 450 MHZ to 1650 MHZ.

However, in such a configuration, when the frequency division number of the VCO 7 is changed, the phase becomes instantaneously discontinuous and the tuning speed of the frequency synthesizer becomes slow. Therefore, as shown in FIG. 6, by continuously oscillating the frequency division frequency of the VCO 7 by the second DSS 16, the phase changes continuously, so that the phase-locked loop 10a can be smoothly switched without losing synchronization. .

Further, instead of the second DDS 1b, a plurality of oscillators may be used and switched. However, DD
When S is used, when the output frequency of the synthesizer is changed, the phase-locked loop 10a switches smoothly without losing synchronization, which is advantageous for speeding up.

[0038]

As described above, according to the present invention, the frequency synthesizer uses the signal of the first frequency as a reference in a frequency range in which spurious is less likely to occur in the signal of the first frequency output directly from the digital synthesizer. In the frequency range in which spurious occurs in the signal of the first frequency as a signal input to the phase locked loop, the reference signal switching means switches to the signal of the second frequency output by the frequency converting means,
If the reference signal is input to the phase-locked loop, spurious near the carrier of the reference signal can be reduced, and the spurious level is always small, so that a wideband frequency synthesizer can be provided.

According to another aspect of the present invention, the frequency synthesizer is configured so that the output of the oscillator, the signal of the first frequency with less spurious and the predetermined frequency at which the oscillator oscillates are input to the mixer. Even if the signal is output, the spurious near the carrier of the reference signal can be reduced, and the spurious level is always small, so that a wide-band frequency synthesizer can be provided.

According to the next invention, the frequency synthesizer receives the output of the oscillator to which the oscillator switching means is connected among the plurality of oscillators oscillating different frequencies and the signal of the first frequency with less spurious signals. Even if the mixer outputs the signal of the second frequency, the spurious near the carrier of the reference signal can be reduced, and the spurious level is always small, so that a wide-band frequency synthesizer can be provided.

Further, according to the following invention, even if the frequency synthesizer inputs the signal of the first frequency with less spurious to the frequency multiplier and outputs the signal of the second frequency, The spurious near the carrier can be reduced, and the spurious level is always small, so that a wideband frequency synthesizer can be provided.

According to the next invention, in the frequency synthesizer, the filter selection means directly selects a desired low-pass filter among the plurality of low-pass filters according to the frequency of the signal of the first frequency output from the digital synthesizer. Is directly connected to the digital synthesizer, the spurious that changes according to the frequency of the signal of the first frequency is removed by the required low-pass filter, and the signal of the first frequency from which the spurious is removed is phase-synchronized as the reference signal. Since it is input to the loop, the spurious near the carrier of the reference signal can be reduced, and the spurious level is always small, so that a wideband frequency synthesizer can be provided.

Further, according to the next invention, the frequency synthesizer includes a signal of the first frequency output from the first direct digital synthesizer and a signal of the first frequency output from the second direct digital synthesizer, and a second frequency different from the first frequency. When the mixer mixes the frequency signal and outputs it to the phase-locked loop as the reference signal, it becomes possible to set the frequency range of the first frequency signal to a range in which spurious generation is small, and the frequency synthesizer concerned. Since it is possible to switch the frequency regardless of the switch, it is possible to smoothly switch the frequency without losing the phase synchronization and to reduce the spurious near the carrier of the reference signal, so that the spurious level is always small and the frequency synthesizer has a wide band. The effect of being able to provide.

Further, according to the following invention, in the frequency synthesizer, the first direct digital synthesizer outputs the signal of the first frequency and the second direct digital synthesizer outputs the second frequency different from the first frequency. Output a frequency signal,
The phase-locked loop outputs the output of the mixer, which mixes the input signal of the second frequency and the feedback signal, and the output of the phase comparator, which compares the phase of the input signal of the first frequency, through a loop filter. By inputting it to the voltage controlled oscillator and outputting the signal of the required frequency, it is possible to switch the frequency of the frequency synthesizer without using a switch or variable frequency divider. There is an effect that the frequency can be switched, the spurious near the carrier of the reference signal can be reduced, the spurious level is always small, and a wideband frequency synthesizer can be provided.

[Brief description of drawings]

FIG. 1 is a block diagram showing a frequency synthesizer according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing a frequency synthesizer according to a second embodiment of the present invention.

FIG. 3 is a block diagram showing a frequency synthesizer according to a third embodiment of the present invention.

FIG. 4 is a block diagram showing a frequency synthesizer according to a fourth embodiment of the present invention.

FIG. 5 is a block diagram showing a frequency synthesizer according to a fifth embodiment of the present invention.

FIG. 6 is a block diagram showing a frequency synthesizer according to a sixth embodiment of the present invention.

FIG. 7 is a block diagram showing a frequency synthesizer having a conventional configuration.

FIG. 8 is a block diagram showing a direct digital synthesizer used in a conventional frequency synthesizer and a frequency synthesizer of the present invention.

FIG. 9 is a spectrum diagram showing an output of a direct digital synthesizer used in a conventional frequency synthesizer.

[Explanation of Codes] 1 direct digital synthesizer, 3 reference clock oscillator, 4 filter, 5 phase comparator, 7 voltage controlled oscillator, 9 frequency divider, 10 phase locked loop, 11 frequency synthesizer output, 12 mixer, 13 oscillator, 1
4 switches, 15 multiplier.

Claims (7)

[Claims] 1. A direct digital synthesizer for inputting a frequency setting signal and a reference clock, performing frequency synthesis by digital operation and outputting a signal of a first frequency, and the first digital synthesizer.
The second frequency different from the first frequency by inputting the signal of the frequency of
Frequency conversion means for outputting a signal of a frequency, a phase-locked loop for inputting a reference signal of a predetermined frequency and outputting a signal of a required frequency, and the phase-locked loop according to the frequency of the reference signal of the predetermined frequency. A frequency synthesizer, comprising: a reference signal switching means for switching between a signal having a frequency of 1 and a signal having the second frequency and outputting the signal as the reference signal of the predetermined frequency to the phase locked loop. 2. The frequency conversion means includes an oscillator that oscillates a predetermined frequency, and a mixer that inputs and mixes a signal of the first frequency and a predetermined frequency that the oscillator oscillates. The frequency synthesizer according to claim 1, wherein the frequency synthesizer outputs a signal. 3. The frequency conversion means includes a plurality of oscillators that oscillate different frequencies, a mixer that inputs and mixes a frequency output from any one of the plurality of oscillators and a signal of the first frequency, 2. The frequency synthesizer according to claim 1, further comprising oscillator switching means for switching a plurality of oscillators to connect to the mixer, and outputting a signal of a second frequency. 4. The frequency synthesizer according to claim 1, wherein the frequency converting means is a frequency multiplier. 5. A direct digital synthesizer that inputs a frequency setting signal and a reference clock and outputs a signal of a first frequency that is frequency-synthesized by digital operation, and a predetermined reference signal that is input and outputs a signal of a desired frequency. A phase-locked loop, a plurality of low-pass filters having mutually different characteristics, and a required low-pass filter among the plurality of low-pass filters connected to the direct digital synthesizer according to the frequency of the signal of the first frequency. And a filter selecting means for outputting the output of the required low-pass filter as the reference signal of the predetermined frequency. 6. A first direct digital synthesizer that inputs a frequency setting signal and a reference clock and outputs a signal of a first frequency that is frequency-synthesized by digital operation, and inputs the frequency setting signal and the reference clock. A second direct digital synthesizer that outputs a signal of a second frequency different from the first frequency by performing frequency synthesis by digital calculation, and phase synchronization that inputs a reference signal and outputs a signal of a required frequency. A frequency synthesizer comprising: a loop; and a mixer that mixes the signal of the first frequency and the signal of the second frequency and outputs the reference signal of the predetermined frequency. 7. A first direct digital synthesizer for inputting a frequency setting signal and a reference clock and outputting a signal of a first frequency synthesized by digital operation; and inputting the frequency setting signal and the reference clock. A second direct digital synthesizer that outputs a signal of a second frequency different from the first frequency, and a mixer that mixes the signal of the second frequency with a feedback signal described later. And a phase comparator for phase-comparing the output signal of the mixer and the signal of the first frequency, and the output of the phase comparator is input through a loop filter to input N times (N is a positive number) frequencies. A voltage-controlled oscillator that outputs a signal, a phase-locked loop that outputs a signal of a required frequency, and a frequency divider that divides the output of the voltage-controlled oscillator into 1 / N and outputs the feedback signal, A frequency synthesizer characterized by having.