JPH0338118A - Frequency synthesizer channel selection device - Google Patents

Abstract

PURPOSE:To increase the comparison reference frequency to be compared in phase by revising a cut-off frequency of a variable filter in response to the frequency of a broadcast wave. CONSTITUTION:When a microprocessor 4 sets a frequency division P in response to the frequency of a broadcast wave, the frequency division ratio of a programmable frequency divider 5 is 1/p. Then a signal of a frequency fL outputted from a local frequency oscillator 1 is inputted to a prescaler 2 and the frequency fL is divided into 1/M and inputted to a variable LPF 10, in which harmonic components are eliminated and the result is frequency-divided into 1/p by the programmable frequency divider 5 and the result is inputted to a phase comparator 6. An active filter 9 adjusts the phase difference voltage at a prescribed frequency characteristic and outputs a channel selection voltage and sets the cut-off frequency of the LPF 10 to a frequency near a fundamental wave 13 outputted from the prescaler 2.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a frequency synthesizer tuning device for receiving, for example, television broadcasting and selecting a specific channel.

[Conventional technology]

2. Description of the Related Art As a conventional frequency synthesizer tuning device for receiving a television signal and selecting a specific channel, one having the configuration shown in FIG. 4, for example, is known.

In the same figure, King is a local frequency oscillator composed of a voltage controlled oscillator (VCO), etc., and 2 is a local frequency oscillator 1. In order to easily handle the frequency of the signal output from the local frequency oscillator 1,
This is a prescaler that divides the frequency at a predetermined frequency division ratio (1/M). 3 is a low pass filter (LPF) that removes harmonic components generated during frequency division by the prescaler 2.

4 is the frequency division ratio (1/
A microprocessor 5 that determines the frequency division number P of P)
The frequency division number P determined by the microprocessor 4
This is a programmable frequency divider that divides the frequency of an input signal at a division ratio of 1 (2/P) based on .

8 is a reference frequency oscillator that generates a signal of a predetermined frequency;
7 is a frequency divider that divides the frequency of the signal output from the reference frequency oscillator 8 by a predetermined frequency division ratio (1/N). 6 is a phase comparator that compares the phases of the signal output from the programmable frequency divider 5 and the signal output from the frequency divider 7 and outputs a one-phase difference voltage. Reference numeral 9 denotes an active filter that adjusts the input phase difference voltage with predetermined frequency characteristics and generates a channel selection voltage that controls the frequency output from the local frequency oscillator 1.

Next, the operation of an example of the frequency synthesizer tuning device having the above configuration will be explained.

Now, when trying to receive broadcast waves of a certain frequency, it is necessary to output a local oscillation frequency corresponding to that frequency.

Therefore, when the microprocessor 4 sets the frequency division number P according to the frequency of the broadcast wave, the set value is input to the programmable frequency divider 5, and the frequency division ratio of the programmable frequency divider 5 becomes 1/P. Set.

Next, a signal with a frequency f output from the local frequency oscillator 1 is input to a prescaler 2, and after dividing the frequency f by 1/H, the signal is input to an LPF 3.

The signal output from prescaler 2 has a frequency fL/M
In addition to the fundamental wave, the signal contains harmonic components, so these harmonic components are removed by the LPF 3. The signal with the frequency fL/M from which harmonic components have been removed by the LPF 3 is input to the programmable frequency divider 5 whose frequency division ratio (1/P) has been set in advance, and the frequency fL/M is further increased by 1. /P. Thereafter, the signal of frequency f, /MP, which has been divided by 1/P, is input to the phase comparator 6.

On the other hand, the frequency of the reference signal generated by the reference frequency generator 8 is input to the frequency divider 7 and is divided into l/N. A signal having a frequency fa (comparison reference frequency) obtained by dividing the frequency of this reference signal by 1/N is input to a phase comparator 6, and its phase is compared with the frequency ft/MP. When a phase difference is detected by the phase comparator 6, a phase difference voltage corresponding to the phase difference is output.

Thereafter, the phase difference voltage is input to the active filter 9 and adjusted to a predetermined frequency characteristic.

Since the voltage output from the active filter 9 is inputted to the local frequency oscillator l as a tuning voltage, the local oscillation frequency changes depending on the tuning voltage.

In this way, the local oscillation frequency of the desired broadcast frequency is locked by the PLL circuit constituted by these circuits, and a signal with a stable oscillation frequency is sent to a mixer (not shown).

[Problem to be solved by the invention]

By the way, the frequency (Fo +
ax/M) needs to be sufficiently smaller than the lowest frequency (Fmin) of the broadcast wave. Otherwise, a beat will occur between the two, which is not desirable.

Therefore, the 1 frequency division ratio (1/M) is somewhat small (frequency division number M
must be set somewhat large).

Furthermore, in order to ensure stable operation of the digital AFC circuit, the width of change ΔfL in one local oscillation frequency fL when the division ratio 1/P of the programmable frequency divider 5 is changed by one step is usually equal to the intermediate frequency (ft F). 171,000 or less.

For example, 1/p=1. When fx F=32Milz.

Δf, ≦f−y F/ 1000”32KHz, so if 1/M=1/128, f*=32/12
8" becomes +lz at 0.25. That is, the comparison reference frequency f becomes a relatively small value.

FIG. 5 is a characteristic diagram showing typical loop gain characteristics of the PLL circuit in the frequency synthesizer tuning device shown in FIG. 4.

FIG. 5 is a characteristic diagram when the active filter 9 is a double integral type, which speeds up the response when switching channels.
In addition, the response is determined to increase the loop gain when it is static (when operating near a DC component).

As is clear from the figure, if the comparison basic frequency flI is set low, the response will be relatively large, about -Xd8.

For this reason, it becomes difficult to suppress the reference standard frequency fR, and as shown in FIG. 6, a sideband occurs at a position offset by ±fR from the center frequency in the local frequency oscillator 1 composed of a VCO, etc. , its spectral purity decreases.

Therefore, by reducing the dividing number M, for example, the comparison J, u 161! It is also possible to increase the frequency f.

However, in doing so, the double value (Fm
ax/M) increases, and the cutoff frequency fc of the low-pass filter 3 increases. As a result, as shown in FIG. 7, not only the predetermined fundamental wave 13.

The 3rd, 5th, 7th, 9th, etc. harmonics 14 to 17 pass through the low-pass filter 3.

If the spatial distance between the prescaler 2 and the programmable frequency divider 5 is relatively large, there is a risk that the radiation will be radiated to the outside.

Furthermore, instead of increasing the comparison reference frequency f, it is also possible to limit the frequency characteristics of the loop gain.

However, if this is done, there is a risk that the pull-in operation at the time of channel switching may take time or malfunction may occur.

The present invention has been made in consideration of the above points, and is a frequency synthesizer that can solve the above problems by varying the cutoff frequency of a low-pass filter (1, PF) according to the frequency of broadcast waves.・The purpose is to provide a channel selection device.

[Means to solve the leg problem]

The frequency synthesizer tuning device of the present invention includes a base 48 frequency oscillator that generates a base frequency signal, a local frequency oscillator that generates a frequency signal corresponding to one channel to be tuned, and a base frequency oscillator that generates a frequency signal output from the base frequency oscillator. (A first frequency divider that divides the frequency of the i number, a second frequency divider that divides the frequency of the signal output from the local frequency oscillator, and a signal output from the second frequency divider. A variable filter that blocks high frequency components, a programmable frequency divider that divides the output of the variable filter at a frequency division ratio according to the selected channel, and a programmable frequency divider that divides the output of the programmable frequency divider and a first frequency component. It is characterized in that it has a phase comparator that detects the phase difference between the outputs of the frequency generator and controls the local frequency oscillator, and changes the cutoff frequency of the variable filter depending on the selected channel.

[Effect]

In the frequency synthesizer tuning device d having the above configuration,
The cutoff frequency of a variable filter, such as a low-pass filter, is changed depending on the frequency of the broadcast wave. As a result, the comparison base 7 of the signal output from the first frequency divider
(jH frequency can be set high.

Therefore, the spectrum purity of a local frequency oscillator such as a VCO is improved, the loop response speed when switching channels can be increased, and malfunctions of the loop can be prevented. Furthermore, unnecessary harmonic radiation can be suppressed.

〔Example〕

Hereinafter, a first embodiment of the frequency synthesizer tuning device of the present invention will be described with reference to FIG.

In FIG. 1, the same requirements as in FIG. 4 are given the same reference numerals.

In FIG. 1, reference numeral 10 denotes a variable low-pass filter (variable LPF) that changes its cutoff frequency in accordance with the channel selection voltage from the active filter 9.

The other configurations are the same as in FIG. 4.

Next, the operation of an embodiment having one or more configurations will be described.

When a certain broadcast wave is to be received, the microprocessor 4 sets a frequency division number P according to the frequency of the broadcast wave, and the set value is input to the programmable frequency divider 5.
(Programmable) The frequency division ratio of frequency unit 5 becomes 17p.

Next, a signal with a frequency f output from the local frequency oscillator 1 is input to the prescaler 2, and after the frequency fL is divided into l/H, the signal is input to the variable LPFIO.

The signal of frequency fL/H from which harmonic components have been removed by the variable LPFIO is input to the programmable frequency divider 5, and the frequency f*/M is further divided into 1/P. Thereafter, the signal of frequency f t /MP, which has been frequency-divided by l/P, is input to the phase comparator 6.

On the other hand, the signal generated by the reference frequency oscillator 8 is transmitted to the frequency divider 7
By inputting the signal to 1, the frequency is divided into 1. A signal of frequency flI (comparison reference frequency) whose frequency is divided by l/N is input to the phase comparator 6, and the signal of frequency fL/M is input to the phase comparator 6.
The signal of P and its phase are compared.

When a phase difference is detected by the phase comparator 6, a phase difference voltage corresponding to the phase difference is output.

Thereafter, the phase difference voltage is input to the active filter 9. The active filter 9 adjusts this phase difference voltage with predetermined frequency characteristics and outputs a channel selection voltage. The channel selection voltage is input to the local frequency oscillator 1. As a result, the frequency of the signal output by the local frequency oscillator 1 is
It is set to a value corresponding to the frequency of the broadcast wave to be tuned.

Further, the channel selection voltage is input to the variable LPFIO, and the third
As shown in the figure, the cutoff frequency is set to a frequency near the fundamental wave 13 output from the prescaler 2.

Next, a second embodiment of the frequency synthesizer tuning device of the present invention will be described with reference to FIG.

In FIG. 2, the same reference numerals are given to the same requirements as in FIG. 1.

In FIG. 2, reference numeral 11 denotes a variable voltage generator that generates a voltage according to the frequency division number P set by the microprocessor 4. The variable low-pass filter 10 is controlled in accordance with the output of the variable voltage generator 11. The other configurations are the same as in FIG. 1.

Next, the operation of the second embodiment having the above configuration will be explained.

When a certain broadcast wave is to be received, the microprocessor 4 sets a frequency division number P according to the frequency of the broadcast wave, and the set value is input to the programmable frequency divider 5.
The frequency division ratio of the programmable frequency divider 5 becomes 1/P.

The frequency division number P is also input to the variable voltage generator 11.

The variable voltage generator 11 generates a voltage corresponding to the frequency division number P, and this voltage is input to the variable LPFIO. variable LP
The FIO changes its cutoff frequency depending on the input voltage.

Hereinafter, the operation in which the frequency of the local frequency oscillator 1 is set corresponding to the frequency division ratio 1/P is the same as that in FIG. 1.

As described above, according to the first and second embodiments of the present invention, the cutoff frequency of the low-pass filter is changed depending on the frequency of the received broadcast wave. The frequency division ratio 1/M can be made larger than in the conventional case shown in FIG. Therefore, the comparison reference frequency fR can be made higher than in the conventional case shown in FIG.

For example, 1/P=1, fIF=32Ml (when z, Δ
Since fL≦flF/1000=32K)lz, 1
/M=1/8.

f = 4KHz Therefore, it can be made much higher than in the conventional case.

As shown in FIG. 5, when one frequency fR becomes higher, the response becomes -yda. Therefore, the spectrum purity of the local frequency oscillator (VCO, etc.) is improved compared to the response (-xdB) when the frequency f is low.

In addition, in the first and second embodiments described above, an example was shown in which a low-pass filter was used as the variable filter, but even if it is a variable band-pass filter, it may be replaced with a single filter, or a series of filters such as a trap. It is clear that the objects and effects of the present invention can be achieved.

〔Effect of the invention〕

As detailed above, according to the frequency synthesizer tuning device of the present invention, the cutoff frequency of the variable filter is changed according to the frequency of the broadcast wave, so the reference frequency for comparison for phase comparison can be made higher. I can do it.

Therefore, the following effects can be achieved.

l) The spectral purity of the frequency oscillator can be improved without changing the time constant of the active filter connected as necessary to the phase comparator.

2) Loop response speed when switching channels can be increased.

3) Malfunction of the loop can be prevented.

4) The width of the high frequency wave output from the prescaler can be suppressed.

5) The degree of freedom in design is improved.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a first embodiment of the frequency synthesizer tuning device of the present invention, FIG. 2 is a block diagram showing the configuration of the second embodiment of the frequency synthesizer tuning device of the present invention, and FIG. The figure is a characteristic diagram showing the filter characteristics of the variable low-pass filter of the embodiment shown in Figs. 1 and 2, Fig. 4 is a block diagram showing the configuration of an example of a conventional frequency synthesizer tuning device, and Fig. Characteristic diagrams showing the loop responses of the configurations shown in Figures 1, 2, and 4, Figure 6 shows the characteristics of the local frequency oscillator, and Figure 7 shows the filter characteristics of the low-pass filter with the configuration shown in Figure 4. It is a characteristic diagram. l... Local frequency oscillator, 2... Prescaler, 4
... Microprocessor, 5... Programmable frequency divider, 6... Phase comparator, 7... Frequency divider, 8...
Reference frequency oscillator, 9...active filter, IO
...Variable low-pass filter, 11...Variable voltage generator.

Claims (1)

[Claims] A reference frequency oscillator that generates a signal at a reference frequency, a local frequency oscillator that generates a signal at a frequency corresponding to a channel to be tuned, and a frequency divider for dividing the frequency of the signal output from the reference frequency oscillator. a first frequency divider that divides the frequency of the signal output from the local frequency oscillator; and a second frequency divider that divides the frequency of the signal output from the second frequency divider. a programmable frequency divider that divides the output of the variable filter at a frequency division ratio according to the channel to be selected; and a phase comparator that detects a phase difference between outputs and controls the local frequency oscillator, and changes the cutoff frequency of the variable filter depending on the channel to be tuned. Device.