JP2757801B2 - Direct digital synthesizer phase-locked oscillation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase locked oscillator having a direct digital synthesizer, and more particularly to a phase locked oscillator used as a local oscillator in a microwave band.

[0002]

2. Description of the Related Art A direct digital synthesizer is typically determined in correspondence with an output frequency (ROM) for holding a value at each phase angle of an output waveform in a look-up table format and an output frequency. An accumulator that accumulates the value in synchronization with the reference frequency and uses the accumulated value as an address to the ROM; and a D connected to a data output of the ROM.
/ A (digital / analog) converter. Compared to a PLL (Phase Locked Loop) type or phase-locked oscillation type frequency synthesizer, a direct digital synthesizer does not require a feedback loop, and therefore has a relatively low frequency band and a small amount of frequency hopping. Even in this case, it is possible to switch the frequency at high speed. However, it is difficult to generate a signal of a high frequency, for example, a microwave band, using only the direct digital synthesizer. Configure an oscillation circuit,
It has been practiced to obtain an output signal in a desired frequency band. When frequency switching is performed only by the phase-locked oscillation circuit, the phase comparison frequency is a frequency corresponding to the frequency difference of one step. Even if a phase-locked oscillation circuit is used, high-speed frequency switching can be performed.

FIG. 2 is a block diagram showing an example of the configuration of a conventional direct digital synthesizer phase-locked oscillation circuit. This circuit includes a voltage controlled oscillator (VCO) 21 whose output frequency changes according to a control voltage, a programmable counter (PC) 22 for dividing the output of the voltage controlled oscillator 21 at a desired frequency division ratio, a reference frequency A reference signal source 25 for generating a signal, and a direct digital synthesizer (DDS) 24 that operates according to a reference frequency signal
A phase frequency comparator (PFD) 23 that compares the phase of the output signal of the programmable counter 22 with the output signal of the direct digital synthesizer 24; and a voltage controlled oscillator that amplifies the comparison output of the phase frequency comparator 23 and controls the voltage as a control voltage. And an amplifier 26 for feeding back to the control signal 21. The output of the voltage controlled oscillator 21 is supplied to the outside as the output of the entire circuit.

Further, for example, Japanese Patent Application Laid-Open No. 4-260219,
Japanese Unexamined Patent Publication Nos. 5-55949 and 5-336181 disclose techniques for reducing the power consumption of a direct digital synthesizer phase-locked oscillation circuit and suppressing spurious signals. In the circuits disclosed in these publications, similarly to the circuit shown in FIG. 2, the output signal of the direct digital synthesizer is directly input to the phase comparator in the phase locked oscillation loop.

[0005]

The direct digital synthesizer is inevitably accompanied by quantization noise. In the conventional direct digital synthesizer phase-locked oscillation circuit described above, since the output of the direct digital synthesizer is directly input to the phase comparator,
The quantization noise is also multiplied in the phase-locked oscillation loop, which causes a problem that an unnecessary wave is generated near a desired output frequency. As a method of reducing this unnecessary wave, it is conceivable to extremely narrow the loop band of the phase-locked oscillation loop. However, narrowing the loop band causes a problem that the pull-in time of the phase-locked oscillation circuit becomes longer, Further, since the influence of the phase noise of the voltage controlled oscillator cannot be ignored, it is necessary to use a voltage controlled oscillator having a small phase noise, which is disadvantageous in terms of price and size.

SUMMARY OF THE INVENTION An object of the present invention is to provide a phase-locked oscillation circuit that can perform frequency switching in small steps at high speed, suppress generation of unnecessary waves near the output frequency, and reduce noise. .

[0007]

According to the present invention, there is provided a direct digital synthesizer phase-locked oscillation circuit, comprising: a voltage-controlled oscillator whose output frequency changes in response to a control voltage; and an output signal of the voltage-controlled oscillator. A 90 ° hybrid that outputs two signals that are 90 ° out of phase, and a direct digital system with sine wave output and cosine wave output
Synthesizer and output of the 90 ° hybrid 2
A single sideband mixer that performs frequency conversion by using a signal and a sine wave output and a cosine wave output of the direct digital synthesizer as inputs, and divides an output signal of the single sideband mixer by a desired frequency division ratio. A programmable counter that circulates, a frequency divider having a fixed frequency division ratio, a reference signal source that generates a reference frequency signal and supplies the signal to the direct digital synthesizer and the frequency divider; A phase frequency comparator for comparing the output with the output of the frequency divider, wherein the output of the phase frequency comparator is fed back to the voltage controlled oscillator as the control voltage.

In the present invention, as a single sideband mixer, for example, a first multiplier to which a sine wave output of a direct digital synthesizer and one output signal of a 90 ° hybrid are inputted, a cosine wave output and a 90 ° A circuit composed of a second multiplier to which the other output signal of the hybrid is input, and an adder that adds and outputs the output of the first multiplier and the output of the second multiplier is used. can do. In the present invention, it is preferable that the oscillation frequency of the voltage-controlled oscillator be higher than the output frequency of the direct digital synthesizer.

[0009]

[Function] Rather than directly inputting the output of the direct digital synthesizer to the phase frequency comparator, it is mixed with the output of the voltage controlled oscillator, divided by a programmable counter, and then input to the phase frequency comparator. Therefore, the quantization noise included in the output of the direct digital synthesizer is not multiplied in the phase locked oscillation loop. If the output of the voltage-controlled oscillator is simply mixed with the output of the direct digital synthesizer to perform frequency conversion, an image (image) frequency wave is generated and causes a malfunction. -Since the mixer is used, generation of the image frequency wave is substantially suppressed. Therefore, without using a filter, etc.
It operates stably without malfunction.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram illustrating a direct communication system according to an embodiment of the present invention.
FIG. 2 is a block diagram illustrating a configuration of a digital synthesizer phase-locked oscillation circuit. The circuit of this embodiment is different from the conventional circuit shown in FIG. 2 in that a 90 ° hybrid (90 ° HYB) and a single sideband mixer (single-side frequency mixer, SSB mixer) are provided in a phase-locked oscillation loop. Insert the output of the direct digital synthesizer into the single sideband mixer instead of the phase frequency comparator, and supply the phase frequency comparator with the reference frequency signal from the reference signal source via the frequency divider. Is different.

That is, the direct digital synthesizer phase-locked oscillation circuit of this embodiment shown in FIG.
A voltage controlled oscillator 1 whose output frequency changes in response to a control voltage; and (b) a 90 ° hybrid 8 which receives an output signal of the voltage controlled oscillator 1 and outputs two signals whose phases are shifted from each other by 90 °. (c) Sine wave output (sin) and cosine wave output
Direct digital synthesizer 4 with (cos)
(D) a single sideband mixer 9 for performing frequency conversion using two signals output from the 90 ° hybrid 8 and a sine wave output and a cosine wave output of the direct digital synthesizer 4 as inputs, and (e) a single sideband mixer. A programmable counter 2 for dividing the output signal of the sideband mixer 9 by a desired dividing ratio, and (f) a fixed dividing ratio N
(G) a reference signal source 5 for generating a reference frequency signal and supplying it to the direct digital synthesizer 4 and the frequency divider 7, and (h) an output of the programmable counter 2 and frequency division A phase-frequency comparator 3 for comparing the phase of the output of the comparator 7, and (i) an amplifier 6 for amplifying the output of the phase-frequency comparator 3 and feeding it back to the voltage-controlled oscillator 1 as a control voltage.
It is constituted by. The output of the voltage controlled oscillator 1 is
The output of the entire circuit is supplied to the outside. The single sideband mixer 9 includes a first multiplier 11 to which a sine wave output of the direct digital synthesizer 4 and one output signal of the 90 ° hybrid are input,
The second multiplier 12 to which the cosine wave output of the direct digital synthesizer 4 and the other output signal of the 90 ° hybrid are input, the output of the first multiplier 11 and the output of the second multiplier 12 are added. And outputs the result.

As described in the section of "Prior Art", a direct digital synthesizer outputs a signal waveform using a look-up table having a value corresponding to a phase angle. By preparing a cosine wave table or adding a value corresponding to 90 ° to the output value of the accumulator, a sine wave and a cosine wave for the same frequency can be simultaneously generated.

Next, the operation of the circuit of this embodiment will be described. Here, the operating frequency band of the voltage controlled oscillator 1 is set to the 2.4 GHz band, and 2415 to 2445
And to obtain an output frequency f _o in the range of MHz, the range of the output frequency f _d of the direct digital synthesizer 4 and 2～3MHz. Obtained by frequency conversion by the single sideband mixer 9 frequency f _m is f _m
= When represented by f _o -f _d, the range of the frequency f _m is represented by 2413～2443MHz.

When a frequency of 1 MHz is supplied from the reference signal source 5 to the phase frequency comparator 23 via the frequency divider 7, the output of the single sideband mixer 9 is output according to the frequency division ratio n of the programmable counter 2. frequency f _m is 1MHz
The voltage controlled oscillator 1 operates so as to be n times as large as. After all, by changing the setting of the dividing ratio of the programmable counter 2, the output frequency f _o changes in steps of 1 MHz, and the output frequency f _{d of the} direct digital synthesizer 4 changes.
At 1 m at the output frequency f _o
Fractional part below Hz changes, for example, it is possible to vary the output frequency f _o in increments such 25 kHz. In this case, even if the frequency size is 25 kHz, the phase comparison frequency is 1 MHz.

Here, the principle of operation of the single sideband mixer 9 will be described. When the time is represented by t, the sine wave output and the cosine wave output of the direct digital synthesizer 4 are A · sin (2πf _d t) and A
Cos (2πf _d t). Similarly, the two output signals of the 90 ° hybrid 8 are respectively B · sin
(2πf _o t-φ), it is expressed as _{B · cos (2πf o t-} φ). As a result, the output of one multiplier 11 is A · B · s
in (2πf _d t) · sin (2πf _o t), and from the product of the trigonometric functions → sum formula, A · B ｛cos (2π (f _d −f
_{o) t + φ) -cos (} 2π (f d + f o) t-φ)} / 2 to become. Similarly, the output of the other multiplier 11 is A · B ｛co
_{s (2π (f d -f o} ) t + φ) + cos (2π (f d + f o) t
−φ)｝ / 2. Therefore, the output of the adder 13 having these as inputs is A · B · cos (2π (f _d −
f _o ) t + φ), and the output frequency f of the adder 13
_m is represented can be seen in f _m = f _d -f _o. That is, by using a single sideband mixer, generation of an image wave can be suppressed.

[0016] in the case of using a normal mixer is not a single sideband mixer, the output frequency f _o 24
15 MHz, when a 2MHz the output frequency f _d of the direct digital synthesizer, is from the mixer, f _o -
A signal of 2413 MHz corresponding to f _d is output, and an image wave of 2417 MHz corresponding to f _o + f _d is also output at the same level, so that the phase-locked oscillation loop may erroneously pull in the image wave. In addition, it is extremely difficult to attenuate an image wave of 2417 MHz with respect to the original frequency of 2413 MHz by a filter.

In this embodiment, the single side band
Since the generation of the image wave is suppressed by using the mixer 9, it operates without causing the erroneous pull-in,
It is possible to configure a circuit that does not multiply the quantization noise of the direct digital synthesizer. Further, since the generation of the image wave is suppressed irrespective of the filter, for example, the output frequency of the direct digital synthesizer can be lowered to about 2 to 3 MHz, and the reference frequency signal generated by the reference signal source 5 is generated. The frequency can be as high as about 10 MHz at most, and a low-cost direct digital synthesizer and a general-purpose crystal oscillator can be used.

[0018]

As described above, according to the present invention, the phase-locked oscillation is performed so that the output of the direct digital synthesizer and the output signal of the voltage controlled oscillator are input to the single sideband mixer to perform frequency conversion. By configuring a loop, the effect of the quantization noise of the direct digital synthesizer does not affect the output signal, the phase-locked oscillation circuit operates without malfunction, and the frequency changes quickly in small steps That is, there is an effect that a circuit that can perform the operation can be easily realized.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a direct digital synthesizer phase-locked oscillation circuit according to one embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a conventional direct digital synthesizer phase-locked oscillation circuit.

[Explanation of symbols]

 1,21 Voltage controlled oscillator 2,22 Programmable counter 3,23 Phase frequency comparator 4,24 Direct digital synthesizer 5,25 Reference signal source 6,26 Amplifier 7 Divider 8 90 ° hybrid 9 Single sideband・ Mixer 11,12 Multiplier 13 Adder

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H03L 7/18 H03L 7/16 H03B 28/00

Claims (3)

(57) [Claims] 1. A phase-locked oscillation circuit, comprising: a voltage-controlled oscillator whose output frequency changes in response to a control voltage; and two signals whose input signals are input and whose phases are shifted by 90 ° from each other. 90 ° hybrid with direct output and sine wave output and cosine wave output
A digital synthesizer, and a single signal for performing frequency conversion using two signals output from the 90 ° hybrid and a sine wave output and a cosine wave output of the direct digital synthesizer as inputs.
A sideband mixer, a programmable counter for dividing the output signal of the single sideband mixer by a desired frequency division ratio, a frequency divider having a fixed frequency division ratio, and a reference frequency signal The direct digital
A reference signal source to be supplied to the synthesizer and the frequency divider; and a phase frequency comparator for comparing the output of the programmable counter with the output of the frequency divider. A direct digital synthesizer phase-locked oscillation circuit that returns a voltage to the voltage controlled oscillator. 2. The single sideband mixer comprises: a first multiplier to which the sine wave output and one output signal of the 90 ° hybrid are input; and a single multiplier which receives the cosine wave output and the other of the 90 ° hybrid. 2. The circuit according to claim 1, further comprising: a second multiplier to which an output signal is input; and an adder that adds an output of the first multiplier and an output of the second multiplier and outputs the result. Direct digital synthesizer phase-locked oscillation circuit. 3. The direct digital synthesizer phase-locked oscillation circuit according to claim 1, wherein an oscillation frequency of said voltage controlled oscillator is higher than an output frequency of said direct digital synthesizer.