JPS5952852B2 - frequency synthesizer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency synthesizer suitable for application to various communication devices.

Conventionally, this type of frequency synthesizer has been configured as shown in FIG. 1, for example.

First, let us explain Figure 1 below.

20 is a phase locked loop (hereinafter abbreviated as PLL).

In this PLL 20, a part of the output signal of the variable oscillator 4 is supplied to the mixer 5, where it is mixed with the frequency signal supplied thereto to lower its frequency, resulting in so-called beatdown.

This beatdown signal is supplied to a programmable counter 6 as a variable frequency divider, divided by an appropriate value, and supplied to a phase comparator 2, where the phase is compared with the reference frequency signal from the reference oscillator 1.

This comparison output is supplied to the active filter 3, where it is converted into a direct current, and supplied to the variable oscillator 4 as a signal for controlling its oscillation frequency.

By the way, when the frequency is locked by this PLL 20, the frequency spectrum of the output signal of the phase comparator 2 is as shown in Fig. 2, which includes the fundamental wave signal of l0KH2, which is the reference oscillation frequency, and its harmonics. Contains signals.

By the way, as will be described later, the active filter 3 is composed of a resistor and a capacitor connected in series to a feedback circuit of an operational amplifier. A pass filter is configured.

The harmonic signal component contained in the output signal of the phase comparator 2 is reduced through the active filter 3, but since the frequency characteristics of the active filter 3 are determined by determining the natural frequency and damping factor of the PLL 20, the harmonic signal component contained in the output signal of the phase comparator 2 is reduced. There is a limit to the degree of component attenuation.

In view of the above, the present invention aims to propose a frequency synthesizer that has a simple circuit configuration and can effectively suppress the harmonic components of the fundamental wave signal contained in the output signal of the phase comparator.

An embodiment of the frequency synthesizer of the present invention will be described below with reference to FIG.

In FIG. 3, parts corresponding to those in FIG. 1 are designated by the same reference numerals, and redundant explanation thereof will be omitted.

That is, FIG. 3 shows a configuration in which, in FIG. 1, for example, a Chebyshev type reduced-pass filter is inserted as an auxiliary filter 8 in series with the active filter 3 between the phase comparator 2 and the variable oscillator 4.

This auxiliary filter 8 blocks the reference frequency signal and its harmonic signals supplied to the phase comparator, and also blocks the maximum of the comparison frequency signals output from the phase comparator when the frequency division ratio of the variable frequency divider changes. Its characteristics are set to pass frequency signals.

By the way, by adding this auxiliary filter 8, P
Since the frequency and damping factor of the LL 20 system must not be affected, the characteristics of this auxiliary filter 8 are determined by the following design.

The design of this auxiliary filter 8 will be examined below.

In the following description, a case will be described in which the frequency synthesizer of the present invention is applied to, for example, a 40-channel civil band (CB band) transceiver.

In FIG. 3, the output signal of the voltage controlled oscillator 4 is 162
The frequency is 70KHz to 16710KHz, and in the so-called beatdown mixing circuit 5, the frequency is 1536KHz from one input terminal.
A 0KHz signal is supplied and the programmable counter 6
A signal of 910 to 1350KH7 is supplied to .

This signal is frequency-divided by a programmable counter 6, and a phase comparator 2 compares the phase of this signal with a 10 KHz reference signal from a reference oscillator 1.

This comparison output is then supplied to the voltage controlled oscillator 4 as an oscillation frequency control signal via the active filter 3 and the auxiliary filter 8.

By the way, this transceiver is equipped with 40 channels, and the value of N in the frequency division ratio l/N is changed from 91 to 135 by the programmable counter 6.

In this programmable counter 6, the largest change in the value of N occurs when N changes from 91 to 135 and from 135 to 91.

N from 91 to 135 in programmable counter 6
At the moment when it changed to 16270KHz in PLL20 system
The signal with the frequency remains locked, and the programmable counter 6 divides 910 KHz by 135, so the 6.74 KHz output signal is sent to the phase comparator 2.
10:tG from the oscillator 1 at the phase comparator 2.
(When the phase is compared with the oscillation output signal of z, a signal with a high frequency of 3.26 KHz is extracted.

Similarly, in the programmable counter 6, N
At the moment when changes from 135 to 91, the signal output to 16710 remains locked in the PLL 20 system, and the signal output from programmable counter 6 is 1350KI (z is divided by 91, so the output of 14.84KH2 is in phase. The signal is supplied to the comparator 2, and a signal with a frequency as high as 4.84 & is taken out from the phase comparator 2.

Therefore, as for the characteristics of the auxiliary filter 8, it is sufficient to add a low-pass filter that transmits the frequency encountered at 4.84 without attenuation and attenuates the components beyond that, and by doing this, the initial characteristics of the PLL 20 Spurious and noise can be reduced without affecting.

The auxiliary filter 8 is preferably a Chebyshev filter whose characteristics are shown in FIG. 4 or a low-pass filter that passes frequencies of, for example, 10 KHz or less.

This Chebyshev type filter has remarkable cutoff characteristics at a reference frequency of 10 KHz and a high frequency twice that frequency of 20 KHz.

FIG. 5 shows a specific example of the configuration of the active filter 3, the auxiliary filter 8, the impedance matching device 14, and the voltage control oscillator 4 when a Chebyshev type filter is used as the auxiliary filter 8.

The active filter 3 is composed of an operational amplifier 10, a resistor 11 connected in series to its feedback circuit, a capacitor 12, a load resistor 13, etc.
The frequency characteristics of the filter are determined by constants such as resistors 11 and 13.

The Chebyshev filter 8 is composed of a plurality of coils and capacitors, and its characteristics are shown in FIG. 4 as described above.

The filter 8 may be designed as a low-pass filter with infinite output impedance in order to eliminate transmission loss of signals of frequencies within the passband.

Then, the output of the low-pass filter is received by the field effect transistor 15 with high input impedance of the impedance matching device 14, and the impedance is lowered by the transistor 16, and then supplied to both ends of the variable capacitance diode 17 of the voltage controlled oscillator 4. Drive variable capacitance diode 1
7 to reduce the noise generated.

The case where a low-pass filter is used as the auxiliary filter has been described above, but the auxiliary filter may also be a notch filter.

The notch filter has the characteristics shown in FIG. 6, and is designed to attenuate by 159 dB at a harmonic frequency relative to the reference frequency 10.

When a low-pass filter is used as an auxiliary filter, the PLL 20 will not lock if the beat frequency of the output of phase comparator 2 is higher than the cut-off frequency of the low-pass filter, but when a notch filter is used, the output of phase comparator 2 If the frequency of the beat signal is other than several times the reference frequency, the beat signal will not be attenuated and the PLL 20 will lock, so the so-called lock range of the PLL 20 will be widened.

Next, the frequency spectra of the output signals of the variable oscillators of the conventional frequency synthesizer and the frequency synthesizer of the present invention will be compared and explained using a spectrum analyzer.

FIG. 7 is a frequency spectrum diagram of the output signal of the voltage controlled oscillator in a conventional frequency synthesizer without an auxiliary filter, and the level of the carrier signal is -32 dB,
Separated from the carrier signal by 1QKHz corresponding to the reference frequency, the level of the second harmonic level signal is approximately -70 dB, and the level of the third harmonic level signal is approximately -74 dB. include.

On the other hand, FIG. 8 is a frequency spectrum diagram of the output signal of the voltage controlled oscillator in the frequency synthesizer of the present invention with an auxiliary filter added, and shows that harmonic signals are well removed.
00dB or less.

Thus, according to the frequency synthesizer of the present invention, an auxiliary filter is inserted in series with the active filter between the phase comparator and the variable oscillator, and the auxiliary filter receives the reference frequency signal and its harmonics supplied to the phase comparator. The characteristic is set to block the signal and pass the maximum frequency signal among the comparison frequency signals output from the phase comparator when the frequency division ratio of the variable frequency divider changes. A pure output signal containing almost no harmonic signal components is obtained, and the second harmonic level component separated by 10 KTh corresponding to the reference frequency from the carrier signal is 100 dB.
I was able to do the following.

Note that the auxiliary filter is not a Chebyshev type low-pass filter, but may be a low-pass filter that combines a coil and a capacitor.

[Brief explanation of drawings]

FIG. 1 is a system diagram of an example of a conventional frequency synthesizer, FIG. 2 is a frequency spectrum diagram for explaining the operation of FIG. 1, FIG. 3 is a system diagram of an example of the frequency synthesizer of the present invention, and FIG. Circuit diagram showing the specific configuration of the main parts in Figure 3, No. 4
6 and 6 are frequency characteristic diagrams of an example of an auxiliary filter, FIG. 7 is a frequency spectrum diagram for explaining a conventional frequency synthesizer, and FIG. 8 is a frequency spectrum diagram for explaining the frequency synthesizer of the present invention. ' 2 is a phase comparator, 3 is an active filter, 4 is a variable oscillator, 6 is a variable frequency divider, and 8 is an auxiliary filter.

Claims (1)

[Claims] 1. An auxiliary filter is inserted in series with the active filter between the phase comparator and the variable oscillator, and the auxiliary filter blocks the reference frequency signal and its harmonic signals supplied to the phase comparator, and also blocks the variable frequency divider. A frequency synthesizer whose characteristics are set so as to pass the maximum frequency signal among comparison frequency signals output from a phase comparator when the frequency division ratio changes.