JP4737682B2 - High frequency signal generator - Google Patents

Description

  The present invention relates to a high-frequency signal generator that generates a high-frequency signal over a range from MHz to GHz.

Conventionally, a frequency synthesizer using a PLL is known as a high-frequency signal source (that is, a high-frequency signal generator) that generates a high-frequency signal.
FIG. 6 is a configuration diagram of a conventional frequency synthesizer. In this figure, 1 is a high-frequency oscillator (for example, a crystal oscillation circuit), 2 is a phase comparator, 3 is a low-pass filter, 4 is an amplifier, 5 is a voltage controlled oscillator (hereinafter referred to as “VCO”), and 6 is a minute. This is a frequency dividing circuit with a frequency ratio n.
A loop composed of the phase comparator 2, the low-pass filter 3, the amplifier 4, and the VCO 5 is called a phase lock loop (hereinafter referred to as “PLL”). In the PLL, the frequency dividing circuit 6 may not be provided.

When there is no reference signal Vr, the VCO 5 oscillates at a specific frequency (free running frequency) f ₀ and generates a voltage V ₀ . If you enter a reference signal of very close frequency fr to f 0 _{/ n,} the phase comparator 2, outputs an error signal V _e which is proportional to the phase difference between V _r and V _0. Unnecessary high frequency components are removed from V _e by the low-pass filter 3, and the signal V _c amplified by the amplifier 4 becomes the control voltage of the VCO 5, and the oscillation frequency f ₀ of the VCO ₅ is changed in a direction in which the error signal decreases, the f ₀ to match the n × _{f r.}
The fact that f ₀ matches n × f _r is called “PLL locks”. Once in the locked state _{f 0} is to follow the n × _{f r.} Thus, in this figure, the oscillation frequency _{f 0} of the VCO _{becomes f} 0 = n × _{f r.}

Conventional high-frequency signal sources are generally signal sources using the above-described VCO (voltage controlled oscillator), PLL (phase-locked loop), and frequency divider (hereinafter referred to as “synthesizer-type high-frequency signal source”). .
Such synthesizer type high frequency signal sources are disclosed in Patent Documents 1 to 3, Non-Patent Documents 1 and 2, and the like.

  As a high-frequency signal source other than the synthesizer method, a frequency multiplier using the nonlinear characteristic of the step recovery diode from a low frequency and a frequency dividing method from a higher frequency are known.

JP-A-11-112333, "PLL synthesizer" JP 2001-069003, “PLL control circuit and control method thereof” JP-A-2005-191684, “PLL Reference Frequency Phase Control Circuit”

Agilent Technology Co., Ltd. Catalog, 2004 edition, 136 pages Paul Horitz and Windfield Hill, “The art of electronics”, 2nd Edition, Cambridge, 1989, p653.

Signal sources such as accelerators for free electron lasers, femto, mode-locked signal sources for attosecond pulse lasers, and signal sources that provide time references such as GPS have a frequency stability of ^10-11 comparable to atomic clocks, Sub-second time stability is required to make the device function stably.

There is no signal source having such a high stability and low SSB noise (single side band noise) at present. For example, the SSB noise that guarantees the time accuracy (time jitter) of femtoseconds needs to be −120 dBc / Hz or less at 1 kHz away from the signal carrier in the signal source (about 6 GHz) of the accelerator for the free electron laser. .
This is a case where the phase relationship of generated noise is coherent and the use bandwidth is assumed to be 1 kHz. If there is a random noise component, the sum of the components is canceled out, so a higher SSB noise is sufficient.
Furthermore, accelerators for free electron lasers need to oscillate several synchronized frequencies simultaneously. There has been no signal source with low SSB noise and high frequency stability as described above with synchronized multi-frequency simultaneous output.

A high-frequency signal source such as Agilent's E8241A disclosed in Non-Patent Document 1 is the highest level known at present, highly stable and extremely low in SSB noise (high signal purity) It is used as a time reference for GPS satellites and a signal source for laser mode lock.
However, even with this high-frequency signal source, the SSB noise is around −98 to −106 dBc / Hz in the frequency region of 3 to 10 GHz at a distance of 1 kHz from the signal carrier, and is less than the desired −120 dBc / Hz. More than 20 dBc / Hz, SSB noise is high.
In other words, the above-described conventional synthesizer-type high-frequency signal source cannot achieve a high signal stability and a low SSB noise level that guarantees, for example, femtosecond time accuracy (time jitter).

  Other than the synthesizer method, frequency multipliers that use the nonlinear characteristics of step recovery diodes and signal sources that use only frequency division have inherently low signal purity and poor frequency stability. Accordingly, it has been impossible to achieve high signal stability and low SSB noise level with these types of high-frequency signal sources.

  The present invention has been developed to solve such problems. That is, an object of the present invention is to provide a high-frequency signal generator capable of stably generating a high-frequency signal over a frequency range of several MHz to several GHz with very low SSB noise that could not be achieved conventionally. .

According to the present invention, a high-frequency oscillator (n is a positive integer) 1 / n of the predetermined carrier frequency f ₁ for oscillating a first frequency signal of the first oscillation frequency f _r which corresponds approximately to,
A voltage controlled oscillator for oscillating a second high-frequency signal having a second oscillation frequency f ₀ substantially corresponding to the carrier frequency f ₁ ;
A frequency division circuit with a frequency division ratio n that divides the output frequency f of the voltage controlled oscillator into 1 / n;
A phase comparator for outputting an error signal proportional to the phase difference of the frequency of 1 / n of the first high-frequency signal and the output frequency f is a second high-frequency signal of the first oscillator frequency f _r,
A VCO control circuit that outputs a control voltage according to the error signal and inputs the control voltage to a voltage controlled oscillator;
The VCO control circuit outputs a control voltage proportional to the error signal in a frequency band in which the SSB noise of the first high-frequency signal is smaller than the SSB noise of the second high-frequency signal,
There is provided a high frequency signal generator characterized in that the control voltage is set to 0 in a frequency band in which the SSB noise of the first high frequency signal is larger than the SSB noise of the second high frequency signal.

According to a preferred embodiment of the present invention, the high frequency oscillator is a highly stable time base in which the SSB noise is lower than the second high frequency signal at a carrier frequency f _{1 of} less than about 10 kHz,
The voltage controlled oscillator is a signal source of 2856 MHz in which the SSB noise is lower than the first high frequency signal at a carrier frequency f ₁ or more from about 10 kHz.
The VCO control circuit outputs a control voltage proportional to the error signal when the carrier frequency f ₁ is less than about 10 kHz, and sets the control voltage to 0 when the carrier frequency f _{1 is} about 10 kHz or more.

    In addition, a frequency multiplier is provided that forms a high-frequency signal of multiple times from the output frequency f of the voltage controlled oscillator.

    Further, a digital frequency dividing circuit for forming a plurality of high frequency signals from the output frequency f of the voltage controlled oscillator is provided.

    In addition, a band-pass filter for generating a sine wave from a pulsed high-frequency signal is provided.

In addition, a stabilized DC power supply that forms a predetermined first DC voltage for driving the high-frequency oscillator, voltage controlled oscillator, frequency divider, phase comparator, and VCO control circuit from a predetermined second DC voltage higher than the predetermined first DC voltage. When,
A power storage unit for supplying power of the second DC voltage to the DC stabilized power supply;
A rectifier circuit that supplies power of the second DC voltage from a commercial AC power source to the power storage unit.

  The power reserve is preferably a battery or a high capacity capacitor.

According to the configuration of the present invention, a high frequency oscillator, a voltage controlled oscillator, a frequency divider, a phase comparator, and a VCO control circuit constitute a phase locked loop (PLL).
In a frequency band in which the SSB noise of the first high frequency signal is smaller than the second high frequency signal, the VCO control circuit outputs a control voltage proportional to the error signal, and this control voltage causes the error signal of the phase comparator to decrease. The oscillation frequency f ₀ of the controlled oscillator (VCO) can be changed to make f ₀ coincide with n × f _r .
Further, in this frequency band, the SSB noise of the output frequency f of the voltage controlled oscillator (VCO) matches the SSB noise of the first high frequency signal, and becomes smaller than the second high frequency signal.

Furthermore, the VCO control circuit sets the control voltage to 0 in the frequency band where the SSB noise of the first high-frequency signal is larger than that of the second high-frequency signal. In this frequency band, the voltage-controlled oscillator (VCO) unlocks the phase control. Thus, the free running frequency f ₀ is obtained, and the SSB noise of the output frequency f of the voltage controlled oscillator (VCO) coincides with the SSB noise of the second high-frequency signal and becomes smaller than the first high-frequency signal.

Therefore, according to the present invention, in the wide frequency band to be used, the smaller one of the SSB noises of the first high-frequency signal and the second high-frequency signal is selected by locking / unlocking the voltage controlled oscillator (VCO). Small noise can be maintained.
In addition, a very stable signal source can be provided by a phase-locked loop (PLL).

  Furthermore, according to the present invention, by providing a frequency multiplier and / or a digital frequency divider, a high-frequency signal over a frequency range of several MHz to several GHz can be stably generated with very low SSB noise that could not be achieved conventionally. Can be generated.

  Therefore, according to the present invention, it is possible to realize a time reference with higher accuracy than the current state and a high-accuracy signal source of a mode-locked laser, and an accelerator for an ultrashort pulse laser or a free electron laser having a pulse width whose time fluctuation is lower than that of the current state in a femtosecond region.・ A measuring device can be realized. In addition, it is possible to improve the accuracy of the GPS time reference signal, and thus the long-baseline interferometer VLBI.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

In order to realize the above-described highly stable and low SSB noise signal source, a signal source of about 10 MHz using rubidium or the like is used in the present invention. This signal source has low SSB noise from the oscillation carrier to about 100 Hz, and has a frequency of 10 ⁻¹¹ to 10 ⁻¹³ .
In the present invention, this signal source is combined with a source oscillator having a phase-locked PLL applied to its output. This source oscillator has a very low SSB noise above about 1 kHz from a high frequency (eg 2856 MHz) carrier.
By combining a high-stable frequency source with low SSB noise at a frequency close to the carrier and a signal source with low SSB noise at a high frequency away from the carrier, the entire frequency range from low to high is seen from the carrier. Low SSB noise was realized.
Of course, the frequency response of the PLL is adjusted to be low noise.

  In addition to the above, in the present invention, a signal source has been developed by combining a digital n divider circuit in which the SSB noise of the source oscillator becomes 1 / n and a low noise power source. This low noise power source had a noise floor of −140 dBV / √Hz or less at about 100 Hz with respect to a 10 V reference.

  The low noise power source was rectified from a commercial power source such as 50 Hz to create a DC stabilized power source with a series regulator, and its output was connected to a battery or a large-capacity capacitor. Furthermore, by adding the output of the battery to a series regulator using a highly stable reference power supply (3 ppm / ° C), a low-noise power supply has been realized.

FIG. 1 is an overall configuration diagram of a high-frequency signal generator according to the present invention. This figure shows a block diagram of a multi-frequency signal source for a free electron laser.
In FIG. 1, the high-frequency signal generator 10 of the present invention includes a high-frequency oscillator 11, a phase comparator 12, a VCO control circuit 13, an amplifier 14, a voltage-controlled oscillator 15, and a frequency divider circuit 16.

High-frequency oscillator 11, 1 / n of the predetermined carrier frequency f ₁ (n is a positive integer) for oscillating a first frequency signal a first oscillation frequency f _r which corresponds approximately to. In this example, a predetermined carrier frequency _{f 1} is 2856 MHz, the first oscillator frequency _{f r} of about 10 MHz, n is, for example, 285 or 286. Any frequency can be used as long as the phase can be compared.
This high-frequency oscillator 11 has a rubidium high-stable time base (stability of 10 ⁻¹¹ or less) in which the SSB noise is lower than a second high-frequency signal e (described later) of the voltage-controlled oscillator 15 below about 10 kHz from the oscillation carrier (carrier frequency f ₁ ). ).

The phase comparator 12 is an error proportional to the phase difference between the first high-frequency signal a having the first oscillation frequency f _r (about 10 MHz) and the second high-frequency signal g having 1 / n the output frequency f of the voltage controlled oscillator 15. The signal _Ve is output.

The VCO control circuit 13 outputs a control voltage according to the error signal V _e output from the phase comparator 12, and inputs the control voltage V _c amplified by the amplifier 14 to the voltage controlled oscillator 15.

The VCO control circuit 13 outputs a control voltage proportional to the error signal Ve in a frequency band where the SSB noise of the first high-frequency signal a is smaller than the second high-frequency signal b, and the SSB noise of the first high-frequency signal a is the second high-frequency signal. The control voltage is set to 0 in a frequency band greater than e.
That is, in this example, VCO control circuit 13 outputs a control voltage proportional to the error signal _{V e} in less than about 10kHz from the carrier frequency _{f 1,} the control voltage to 0 at about 10kHz or more from the carrier frequency _{f 1.} As a result, in this frequency band, a voltage controlled oscillator (VCO) is a free-running frequency f ₀ is unlocked for phase control.

The voltage controlled oscillator 15 oscillates a second high-frequency signal e having a second oscillation frequency f ₀ (2856 MHz in this example) substantially corresponding to the carrier frequency f ₁ (2856 MHz in this example).
This voltage-controlled oscillator 15 is a signal source of 2856 MHz whose SSB noise is lower than the first high-frequency signal a at about 10 kHz or more from an oscillation carrier (carrier frequency f1) obtained by applying a phase-locked PLL to a time base (high-frequency oscillator 11).

    The frequency dividing circuit 16 is a frequency dividing circuit with a frequency dividing ratio n that divides the output frequency f of the voltage controlled oscillator 15 into 1 / n. For example, n is 285 or 286 described above.

FIG. 2 is a frequency characteristic diagram of each signal source. The horizontal axis represents the frequency (deviation) from the carrier frequency f ₁ (2856 MHz in this example), and the vertical axis represents SSB noise. 2A shows the SSB noise of the high-frequency oscillator 11 (first high-frequency signal a) and the voltage-controlled oscillator 15 (second high-frequency signal e), and FIG. 2B shows the high-frequency signal generator 10 of the present invention. The SSB noise of the output frequency f is shown.

According to the configuration of the present invention described above, the high-frequency oscillator 11, the voltage controlled oscillator 15, the frequency divider 16, the phase comparator 12, and the VCO control circuit 13 constitute a phase-locked loop (PLL).
In a frequency band where the SSB noise of the first high-frequency signal a is smaller than the second high-frequency signal e, the VCO control circuit 13 outputs a control voltage proportional to the error signal Ve, and this control voltage causes the error signal V of the phase comparator 12 to be output. By changing the oscillation frequency f ₀ of the voltage controlled oscillator (VCO) in the direction of decreasing _e , f ₀ can be matched with n × f _r .

    As shown in FIG. 2B, in this frequency band (less than about 10 kHz), the SSB noise of the output frequency f of the voltage controlled oscillator 15 (VCO) matches the SSB noise of the first high-frequency signal a. The SSB noise is smaller than the second high-frequency signal e.

    Further, the VCO control circuit 13 sets the control voltage to 0 in the frequency band where the SSB noise of the first high-frequency signal a is larger than the second high-frequency signal e. Therefore, in this frequency band (about 10 kHz or more), the voltage controlled oscillator (VCO ) Is released and becomes a free running frequency, and the SSB noise of the output frequency f matches the SSB noise of the second high-frequency signal e and becomes smaller than the first high-frequency signal a.

Therefore, according to the present invention, since the voltage control oscillator (VCO) is locked / unlocked in a wide frequency band to be used, the one having the smaller SSB noise of the first high frequency signal a and the second high frequency signal e is selected. It can always be kept at a smaller noise.
In addition, a very stable signal source can be provided by a phase-locked loop (PLL).

  In FIG. 1, the high-frequency signal generator 10 according to the present invention further includes a frequency multiplier 18, digital frequency dividing circuits 19, 20, and 21, and a bandpass filter 22.

  The frequency multiplier 18 forms a high frequency signal of multiple times from the output frequency f of the voltage controlled oscillator 15. In this example, the output frequency f (2856 MHz) is doubled to form a high frequency signal h of about 6 GHz (5712 MHz) suitable for a free electron laser.

    The digital frequency dividing circuits 19, 20, and 21 form a plurality of pulsed high frequency signals from the output frequency f of the voltage controlled oscillator 15. In this example, digital frequency dividers 19, 20, and 21 are provided in series downstream of the frequency multiplier 18, and 1/2 (2856 MHz), 1/6 (476 MHz), and 1/2 (238 MHz) are generated in order from 5712 MHz. To do.

    The band-pass filter 22 is a band-pass filter that generates a sine wave from the pulsed high-frequency signals h, i, j, and k.

FIG. 3 is a noise characteristic diagram of an output signal by the high-frequency signal generator 10 of the present invention. In this figure, the horizontal axis represents the frequency (deviation) from the carrier frequency, and the vertical axis represents SSB noise.
In this figure, 5712 MHz is obtained by converting the high-frequency signal h in FIG. 1 into a sine wave by the bandpass filter 22. From this figure, it can be seen that the signal source (approximately 6 GHz) of the free electron laser accelerator satisfies the required “−120 dBc / Hz or less at 1 kHz away from the signal carrier”.

Further, in this experimental data, the noise floor is lowered to 1 / n at the frequency division ratio n at each frequency as theoretically.
That is, in the digital n divider circuit (digital divider circuit), the SSB noise of the source oscillator becomes 1 / n. In this figure, the SSB noises of 2856 MHz, 476 MHz, and 238 MHz are all necessary areas beyond 5712 MHz. It was confirmed that it becomes smaller at.

FIG. 4 is a configuration diagram of a low noise power source used in the high frequency signal generator 10 of the present invention.
As shown in this figure, the low noise power source 30 includes a DC stabilized power source 32, a power reserve unit 34, and a rectifier circuit 36.

The DC stabilized power supply 32 is composed of a series regulator using a highly stable reference power supply (3 ppm / ° C.). The high frequency oscillator 11, voltage controlled oscillator 15, frequency divider circuit 16, phase comparator 12, VCO control circuit 13 shown in FIG. In addition, a predetermined first DC voltage (for example, DC 10V) for driving the other 18 to 22 is formed from a predetermined second DC voltage (for example, DC 12V) higher than this.
The power storage unit 34 is a battery or a high-capacitance capacitor, and supplies power of the second DC voltage to the DC stabilized power supply 32.
The rectifier circuit 36 includes a first-stage transformer, a rectifier circuit, and a series regulator, and supplies power of a second DC voltage from a commercial AC power supply (for example, AC 100 V, 50 Hz) to the power storage unit 34.

    FIG. 5 is a noise spectrum of the low noise power supply 30 in the present invention. From this figure, it is understood that a noise floor of −140 dBV / √Hz or less is realized when the 10V output voltage is used as a reference.

  As described above, according to the present invention, it is possible to provide a signal source that is very stable and has low SSB noise. As a result, a time reference with higher accuracy than the current situation and a high-accuracy signal source for mode-locked lasers are realized. An accelerator / measurement device for ultrashort pulse lasers and free electron lasers with pulse widths that have less time fluctuation than the current situation in the femtosecond range. Can be realized. In addition, it is possible to improve the accuracy of the GPS time reference signal, and thus the long-baseline interferometer VLBI.

  Therefore, the high-frequency signal generator of the present invention, that is, the signal source is a geodetic technical field such as GPS, a time reference signal generating field of a radio clock, a short pulse, an accelerated scientific field for generating continuous wave radiation, a short pulse / frequency stable. It can be widely applied to fields that require high time standards such as lasers and optical combs.

  In addition, this invention is not limited to embodiment mentioned above, Of course, it can change variously in the range which does not deviate from the summary of this invention.

1 is an overall configuration diagram of a high-frequency signal generator according to the present invention. It is a frequency characteristic figure of each signal source. It is a noise characteristic figure of the output signal by the high frequency signal generator of the present invention. It is a block diagram of the low noise power supply used with the high frequency signal generator of this invention. It is a noise spectrum of the low noise power supply in this invention. It is a block diagram of the conventional frequency synthesizer.

Explanation of symbols

10 high frequency signal generator, 11 high frequency oscillator,
12 phase comparator, 13 VCO control circuit,
14 amplifier, 15 voltage controlled oscillator,
16 frequency divider, 18 frequency multiplier,
19, 20, 21 Digital frequency divider,
22 Bandpass filter,
30 Low noise power supply, 32 DC stabilized power supply,
34 Power reserve, 36 Rectifier circuit

Claims (7)

A high-frequency oscillator that oscillates a first high-frequency signal having a first oscillation frequency fr substantially corresponding to 1 / n (n is a positive integer) of a predetermined carrier frequency f1,
A voltage controlled oscillator for oscillating a second high frequency signal having a second oscillation frequency f0 substantially corresponding to the carrier frequency f1;
A frequency division circuit with a frequency division ratio n that divides the output frequency f of the voltage controlled oscillator into 1 / n;
A phase comparator that outputs an error signal proportional to the phase difference of 1 / n of the output frequency f that is the first high-frequency signal of the first oscillation frequency fr and the second high-frequency signal;
A VCO control circuit that outputs a control voltage according to the error signal and inputs the control voltage to a voltage controlled oscillator;
The VCO control circuit is configured such that the SSB noise of the first high-frequency signal causes the SSB noise of the second high-frequency signal.
Output a control voltage proportional to the error signal in a frequency band smaller than noise,
A high-frequency signal generator, wherein the control voltage is set to 0 in a frequency band in which the SSB noise of the first high-frequency signal is larger than the SSB noise of the second high-frequency signal. The high-frequency oscillator is a highly stable time base in which SSB noise is lower than the second high-frequency signal at a carrier frequency f1 of less than about 10 kHz,
The voltage controlled oscillator is a signal source of 2856 MHz in which SSB noise is lower than the first high frequency signal at about 10 kHz or more from the carrier frequency f1.
2. The VCO control circuit outputs a control voltage proportional to the error signal when the carrier frequency is less than about 10 kHz from a carrier frequency f1, and sets the control voltage to 0 when the carrier frequency is about 10 kHz or more from the carrier frequency f1. The high-frequency signal generator described.   The high-frequency signal generator according to claim 1, further comprising a frequency multiplier that forms a high-frequency signal having a multiple of the output frequency f of the voltage-controlled oscillator.   2. The high frequency signal generator according to claim 1, further comprising a digital frequency dividing circuit that forms a plurality of high frequency signals from an output frequency f of the voltage controlled oscillator. Comprising a bandpass filter to produce a sine wave from the pulse of the high frequency signal, the high-frequency signal generator according to claim 3 or 4, characterized in that. A DC stabilized power source that forms a predetermined first DC voltage for driving the high-frequency oscillator, voltage controlled oscillator, frequency divider, phase comparator, and VCO control circuit from a predetermined second DC voltage higher than the predetermined first DC voltage;
A power storage unit for supplying power of the second DC voltage to the DC stabilized power supply;
The high-frequency signal generator according to claim 1, further comprising: a rectifier circuit that supplies electric power of the second DC voltage from a commercial AC power source to the power storage unit.   The high-frequency signal generator according to claim 6, wherein the power reserve is a battery or a high-capacity capacitor.

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