JPH0362620A - Frequency synthesizer - Google Patents

Abstract

PURPOSE:To decrease a frequency switching time when a high frequency is outputted at a small channel interval or a small channel interval is provided by providing an oscillation section generating a signal of a local frequency and a mixing section. CONSTITUTION:A reference oscillator 11 generates a signal of a reference frequency and an oscillator 18 generates a signal with a local frequency fLO. Then a mixer 17 mixes a signal from an oscillator 15 with a signal from the oscillator 18, outputs the result to an LPF 19, where a high frequency component is eliminated and the result is outputted to a variable frequency divider 16. Moreover, a fixed frequency divider 12 frequency-divides the signal from the oscillator 11 and a phase comparator 13 compares the signal from the frequency divider 12 with the signal from the frequency divider 16, and outputs the signal proportional to the error to an LPF 14. The LPF 14 extracts a high frequency component of the signal from the comparator 13 and outputs the result to a voltage controlled oscillator 15 end the oscillator 15 based on the signal from the LPF 14 outputs a synthesizer output signal with a frequency fOUT to an output terminal 40. Thus, the reference frequency is increased to quicken the loop response speed, thereby decreasing the frequency switching time.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a frequency synthesizer using a digital phase-locked loop, so-called digital PLL.

[Conventional technology]

An example of this type of frequency synthesizer is shown in FIG. This conventional frequency synthesizer includes a reference oscillator 31, a fixed frequency divider 32, a phase comparator 33, a low-pass filter 34, a voltage controlled oscillator 35, and a variable frequency divider 36.

The connections of the frequency synthesizer having such a configuration are as follows. Reference oscillator 31 that generates a reference frequency signal
The output of is connected to a fixed frequency divider 32. The output of this fixed frequency divider 32 is connected to one input of a phase comparator 33. The output of this phase comparator 33 is connected to the input of a low-pass filter 34. The output of this low-pass filter 34 is connected to a control input of a voltage controlled oscillator 35. The output of this voltage controlled oscillator 35 is branched and connected to a variable frequency divider 36. The output of this variable frequency divider 36 is connected to the other input of the phase comparator 33.

The output signal of the voltage controlled oscillator 35 is given to the outside from an output terminal 40 as an output signal of the frequency synthesizer.

In such a frequency synthesizer, the phase comparator 33 compares the frequency and phase of the signal from the fixed frequency divider 32 with the oscillation frequency and phase of the voltage controlled oscillator 35, and calculates an average DC voltage proportional to the error. Output. This error voltage is passed through the low-pass filter 34 to the voltage controlled oscillator 3.
It is fed back to the control input of No. 5, forming a PLL and achieving a phase-locked state.

At this time, the frequency division number of the variable frequency divider 36 is Nv, the frequency of the signal output from the fixed frequency divider 32 to the phase comparator 33 is f, , and the frequency of the synthesizer output signal is f. u
If t, then the following equation holds true in the phase synchronization state.

Moreover, the following equation is established from this equation.

f,,t=f,·Nv Thus, the frequency f of the synthesizer output signal. uL is
The frequency f is fixed at .Nv. That is, the channel spacing is equal to f.

Therefore, when the channel spacing is small and the output frequency of the synthesizer is high, the frequency division number Nv of the variable frequency divider 36 is increased.

Furthermore, in order to reduce the channel spacing, the manual frequency fr of the phase comparator 33 is lowered.

[Problem to be solved by the invention]

When the frequency division number Nv of the conventional frequency synthesizer described above is increased, the following problem occurs.

That is, since the open loop gain of the frequency synthesizer of the digital PLL is inversely proportional to the frequency division number Nv, the loop gain becomes low, the frequency pull-in range is narrowed, and as a result, the loop response speed decreases. Therefore, the conventional frequency synthesizer has the disadvantage that it takes a long time to switch frequencies when outputting high frequencies with relatively small channel spacing.

Further, when the reference frequency f of the conventional frequency synthesizer is lowered, the following problems occur.

In other words, the cutoff frequency of the low-pass filter provided in the loop must be lowered in order to suppress unnecessary wave modulation (reference leak) due to frequency 11 that leaks from the phase comparator, and as a result, the loop response Speed decreases. Therefore, conventional frequency synthesizers have the disadvantage that frequency switching takes a long time when the channel spacing is small.

SUMMARY OF THE INVENTION An object of the present invention is to provide a frequency synthesizer that eliminates such drawbacks and can shorten frequency switching time when outputting a high frequency with a small channel spacing or when having a small channel spacing.

[Means to solve the problem]

A first frequency synthesizer of the present invention includes a voltage-controlled oscillator that generates a frequency signal based on a control signal and outputs it to the outside as a synthesizer output signal, and a voltage-controlled oscillator that generates the control signal based on a mixed signal and a predetermined reference signal. A frequency synthesizer comprising: an oscillation section that generates a local frequency signal; and a frequency synthesizer that mixes and mixes down a synthesizer output signal from the oscillation section and a signal from the voltage controlled oscillation section. The apparatus is characterized in that it includes a mixing section that outputs a signal to the generation section.

A second frequency synthesizer of the present invention includes a voltage controlled oscillator that generates a frequency signal based on a control signal, and a generator that generates the control signal based on the signal from the voltage controlled oscillator and a predetermined reference signal. a frequency divider that divides the signal from the voltage controlled oscillator, and a frequency synthesizer that removes harmonic components from the signal from the frequency divider,
It is characterized by having a filter section that outputs the synthesizer output signal to the outside.

〔Example〕

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

FIG. 1 is a configuration diagram showing an embodiment of a first frequency synthesizer. This first frequency synthesizer includes a reference oscillator 11, a fixed frequency divider 12, a phase comparator 13,
It is composed of a low-pass filter 14, 19, a voltage controlled oscillator 15, a variable frequency divider 16, a mixer I7, and an oscillator 18.

In the frequency synthesizer having such a configuration, the reference oscillator 11 generates a signal at the reference frequency, and the oscillator 18 generates a signal at the local frequency fLo.

Mixer 17 combines the signals from voltage controlled oscillator 15 and oscillator 18.

A low-pass filter 19 removes high-frequency components of the signal from the mixer 17, and a low-pass filter 14 removes high-frequency components from the signal from the mixer 17.
Removes high-frequency components from the signal.

A fixed frequency divider 12 divides the signal from the reference oscillator 11,
A variable frequency divider 16 divides the signal from the low pass filter 19.

A phase comparator 13 compares the signal from the fixed frequency divider 12 and the signal from the variable frequency divider 16, and outputs a signal proportional to the error.

A voltage controlled oscillator 15 generates a frequency f based on the signal from the low pass filter 14. The synthesizer output signal of ut is output to the output terminal 40.

Next, the operation of this frequency synthesizer will be explained.

A reference oscillator 11 generates a signal at a reference frequency.

The frequency of the signal output from the reference oscillator 1 is divided by a fixed frequency divider 12 to a frequency f equal to the channel spacing, and is input to a phase comparator 13.

On the other hand, the oscillator 18 generates a signal at the local frequency rto, and the voltage controlled oscillator 15 generates the signal at the local frequency f based on the signal from the phase comparator 13 via the low-pass filter 14. It outputs the ut signal. Voltage controlled oscillator 15
The branched output frequency f. L is mixed with the frequency fLo outputted from the oscillator 18 by the mixer 17, passes through the low-pass filter 19, and only the mixed down frequency components are extracted and input to the variable frequency divider 16. Here, if the mixed-down frequency is r7 and the frequency division number of the variable frequency divider 16 is Nv, the following equation holds true in the phase synchronization state.

fot-fto+f,=fLo+fr-Nv1 Nv- f.

Therefore, if the frequency fm that has been downgraded is a relatively low frequency, even if the channel spacing rr is small, N
v does not become large.

In this way, in a phase-locked frequency synthesizer composed of a reference oscillator, a fixed frequency divider, a phase comparator, a low-pass filter, a variable frequency divider, and a voltage-controlled oscillator, the output of the voltage-controlled oscillator is downscaled. The mixer includes a mixer that generates a local frequency, an oscillator that generates a local frequency, and a low-pass filter, and the mixed-down frequency is input to a variable frequency divider. In other words, since the output frequency of the voltage controlled oscillator is mixed down by a mixer and fed back to the variable frequency divider, the frequency division number of the variable frequency divider can be reduced (
Loop gain can be increased. As a result, the loop response speed becomes faster and frequencies can be switched quickly.

FIG. 2 is a block diagram showing an embodiment of the second frequency synthesizer. This second frequency synthesizer includes a reference oscillator 21, a fixed frequency divider 22, 27, and a phase comparator 2.
3, low-pass filters 24 and 28, a voltage controlled oscillator 25, and a variable frequency divider 26.

In the frequency synthesizer having such a configuration, the reference oscillator 21 generates a signal at a reference frequency.

Fixed frequency divider 22 divides the signal from reference signal oscillator 22, and fixed frequency divider 27 divides the signal from voltage controlled oscillator 25 at frequency rvco. In addition, the variable frequency divider 26
divides the frequency of the signal from the voltage controlled oscillator 25.

A low-pass filter 24 removes high-frequency components of the signal from the phase comparator 23, and a low-pass filter 28 removes high-frequency components of the signal from the fixed frequency divider 27, resulting in a frequency f. u
The synthesizer output signal of t is output to the output terminal 40.

A phase comparator 23 compares the signal from the fixed frequency divider 22 and the signal from the variable frequency divider 26, and outputs a signal proportional to the error.

A voltage controlled oscillator 25 generates a frequency f based on the signal from the low pass filter 24. Outputs the VC signal.

Next, the operation of this frequency synthesizer will be explained.

A reference signal oscillator 21 generates a signal at a reference frequency. The frequency of the signal output from the reference oscillator 21 is divided into a frequency f by a fixed frequency divider 22, and the frequency of the signal output from the reference oscillator 21 is
3 is input.

On the other hand, the voltage controlled oscillator 25 outputs a signal of frequency f VCO based on the signal from the phase comparator 23 that has passed through the low-pass filter 24 . Voltage controlled oscillator 2
The output frequency rvco of 5 is branched, one is fed back to the variable frequency divider 26, and the other is fed back to the output frequency f of the synthesizer via the fixed frequency divider 27 and low-pass filter 28. U
, are applied to the outside from the output terminal 40. here,
Let the frequency division number of the variable frequency divider 26 be Nv, and the frequency division number of the fixed frequency divider 27 be Nv. ut+synthesizer output frequency as f. u
Assuming t, the following equation holds true in the phase synchronization state.

Also, from the 0 formula and ■ formula, the following formula stands out.

f.

in this way, Synthesizer output frequency f. U.

is the frequency, the isa output frequency f.t is 40MIIz±5 kHz, and the channel interval is 50Hz.Also, if the frequency division number N.ut of the fixed frequency divider 27 before output is 20, the voltage control is Output frequency f of oscillator 25 VCO is 800MH
z±100kHz, and from formula ■, the reference frequency r,
is 1 kHz.

In this way, the output of the voltage-controlled oscillator is divided into two parts in a phase-locked frequency synthesizer composed of a reference oscillator, a fixed frequency divider, a phase comparator, a low-pass filter, a variable frequency divider, and a voltage-controlled oscillator. The synthesizer includes a frequency divider that rotates the frequency, and a low-pass filter that removes harmonics of the output of the frequency divider, and uses the divided frequency as the output of the synthesizer.

That is, since the output frequency of the synthesizer is obtained by dividing the output frequency of the voltage controlled oscillator by the frequency divider, the reference frequency can be increased to some extent even when the channel spacing is small. As a result, the loop response speed becomes faster and the frequency switching speed can be reduced.

〔Effect of the invention〕

As described above, according to the present invention, there is an effect that the frequency switching time can be shortened when a high frequency is output with a small channel spacing or when a channel spacing is small.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of a first frequency synthesizer of the present invention. FIG. 2 is a block diagram showing an embodiment of a second frequency synthesizer of the present invention. FIG. 2 is a configuration diagram showing an example of a frequency synthesizer. 11... Reference oscillator 12... Fixed frequency divider 13... Phase comparator 14, 19... Low pass filter 15... Voltage controlled oscillator I6... Variable frequency divider 17... Mixer 18... Oscillator 40... Output terminal

Claims (2)

[Claims] (1) A voltage controlled oscillation unit that generates a signal with a frequency based on a control signal and outputs it to the outside as a synthesizer output signal, and a generation unit that generates the control signal based on a mixed signal and a predetermined reference signal. The frequency synthesizer includes an oscillator that generates a local frequency signal, mixes a synthesizer output signal from the oscillator and a signal from the voltage controlled oscillator, and outputs a mixed down mixed signal to the generator. A frequency synthesizer comprising a mixing section. (2) A frequency synthesizer comprising a voltage controlled oscillator that generates a signal with a frequency based on a control signal, and a generator that generates the control signal based on the signal from the voltage controlled oscillator and a predetermined reference signal, a frequency dividing section that frequency divides the signal from the voltage controlled oscillator, and removing harmonic components from the signal from the frequency dividing section;
A frequency synthesizer, comprising: a filter section that outputs a synthesizer output signal to the outside.