JPS6348997Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a frequency synthesizer using a phase-locked loop (also referred to as PLL).

[Prior art]

An example of a frequency synthesizer conventionally used in radio equipment is shown in FIG. In the same figure, 1
is a phase comparator that detects the phase difference between two input signals, and the phase comparator 1 compares the phase of the reference input signal fi and the divided output fed back from the frequency divider 6,
Generates a voltage proportional to the phase difference. The output is inputted to a voltage controlled oscillator 3 after removing harmonics by a low-pass filter 2, and this oscillator 3 generates a signal with a frequency determined by the input voltage. Then, a part of the output of the voltage controlled oscillator 3 is mixed with a local oscillator signal _L from a variable oscillator (not shown) in a mixer 4.
Converted to intermediate frequency _IF . The mixed output of the mixer 4 passes through a bandpass filter 5 having characteristics tuned to the intermediate frequency _IF to remove unnecessary signals, and then is frequency-divided by a frequency divider 6, and its frequency divided output _IF /N (N is a frequency division number) is input to the phase comparator 1 as a feedback signal, thereby forming a phase locked loop.

A frequency synthesizer configured in this way is
The output frequency of the voltage controlled oscillator 3 is changed by converting the frequency of the output of the voltage controlled oscillator 3 with a mixer 4, passing it through a bandpass filter 5, and dividing the frequency with a frequency divider 6.
Since _L + N・_i is obtained as _T , the frequency divider 6
If the frequency division number N is constant, any output frequency _T can be obtained by varying the local frequency _L as the local oscillator signal. However, in such conventional devices, when switching the frequency by switching the local frequency _L for channel switching, for example, if the switching width of the frequency is large,
Since the selectivity of the characteristics of the bandpass filter 5 is fixed, the signal passing through it is greatly attenuated, making it impossible to drive the frequency divider 6, resulting in a disadvantage that synchronization cannot be achieved.

[Purpose and structure of the invention]

The present invention was devised to eliminate these drawbacks, and its purpose is to provide a frequency synthesizer that prevents the phase-locked loop from becoming out of synchronization when switching frequencies due to channel switching or the like.

In order to achieve this purpose, the present invention
The output of the phase comparator is input to a voltage controlled oscillator through a reduction filter, the output of this voltage controlled oscillator is mixed with the local oscillator signal by a mixer, the mixed output is input to a frequency divider through a band pass filter, and the In the frequency synthesizer, the frequency synthesizer feedbacks a frequency output to the phase comparator to form a phase-locked loop and generates an output frequency synchronized with a reference input signal input from the voltage controlled oscillator to the phase comparator. A bandpass filter having selectivity switchable characteristics is used as a filter, and the reference input signal of the phase comparator and the divided output of the frequency divider are input, and the synchronization state of the phase locked loop is determined based on the phase difference between them. A phase synchronization detection circuit is provided for detecting the phase synchronization detection circuit, and the selectivity of the bandpass filter is switched according to the synchronization state of the phase synchronization loop based on the output of the phase synchronization detection circuit. Embodiments of the present invention will be described below with reference to the drawings.

〔Example〕

FIG. 2 is a block diagram of a frequency synthesizer according to an embodiment of the present invention. In the figure, the same or corresponding parts as those in FIG. The difference from FIG. 1 is that a coil 11, a capacitor 12, a resistor 13, and a switch element 14 are used as a bandpass filter to remove unnecessary signals included in the mixed output of the mixer 4 inserted into the feedback system of the phase-locked loop. A bandpass filter 10 is used, which has a characteristic that the selectivity can be switched and is configured by a parallel circuit consisting of a series circuit. A phase synchronization detection circuit 15 is provided which inputs the reference input signal input to the phase comparator 1 and the divided output of the frequency divider 6 and detects the synchronization state of the phase locked loop based on the phase difference between them. Synchronization detection circuit 1
The switch element 14 is turned on by the output of 5;
By controlling off, the bandpass filter 10
The configuration is such that the selectivity of .

In this way, by controlling the switch element 14 of the bandpass filter 10 on and off using the output of the phase lock detection circuit 15, the switch element 14 is turned on (closed) when the phase lock loop is asynchronous due to frequency switching, etc. , the coil 11, capacitor 12, resistor 13 and switch element 14
Since the selectivity of the bandpass filter 10 configured by is lowered, its passband is equivalently widened. Therefore, even if the frequency switching width is large, the frequency divider 6 can be reliably driven by the mixed output of the mixer 4 that passes through the bandpass filter 10. Furthermore, if the switch element 14 is turned off (open) when the phase-locked loop is synchronized, the selectivity of the bandpass filter 10 composed of the coil 11 and the capacitor 12 becomes relatively large. As with the conventional method, unnecessary signals can be removed, and spurious signals will not appear on the output side.

Note that the above bandpass filter 10 is not limited to one consisting of a coil 11, a capacitor 12, a resistor 13, and a switch element 14, but may also be configured to switch the selectivity by combining arbitrary circuit elements. .

[Effect of idea]

As explained above, according to the present invention, a circuit is provided to detect the synchronization state of the phase-locked loop, and the selectivity of the bandpass filter is reduced while the phase-locked loop becomes asynchronous due to frequency switching or the like. Therefore, even if the frequency switching width is large, the frequency divider can be reliably driven, and a wideband frequency synthesizer can therefore be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional frequency synthesizer, and FIG. 2 is a block diagram of a frequency synthesizer according to an embodiment of the present invention. 1... Phase comparator, 2... Low pass filter, 3...
... Voltage controlled oscillator, 4... Mixer, 6... Frequency divider, 10... Band pass filter, 15... Phase synchronization detection circuit.

Claims (1)

[Scope of utility model registration request] The output of the phase comparator is input to a voltage controlled oscillator through a low-pass filter, the output of this voltage controlled oscillator is mixed with a local oscillator signal by a mixer, and the mixed output is input to a frequency divider through a band pass filter. In a frequency synthesizer that generates an output at a frequency synchronized with a reference input signal input from the voltage controlled oscillator to the phase comparator by feeding back the divided output to the phase comparator to form a phase locked loop. , a bandpass filter having a selectivity switchable characteristic is used as the bandpass filter, and the reference input signal of the phase comparator and the divided output of the frequency divider are input, and phase synchronization is performed based on the phase difference between them. A frequency synthesizer comprising: a phase synchronization detection circuit for detecting a synchronization state of the loop; and an output of the phase synchronization detection circuit to switch the selectivity of the bandpass filter according to the synchronization state of the phase synchronization loop. .