JPS6150532B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention improves the interference signal removal characteristics and S/
The present invention relates to a frequency tracking circuit that can extract N in an improved state.

The circuit shown in FIG. 1 has been proposed as an FM demodulation circuit for an FM receiver.

This is the FM to be demodulated from the input terminal 11.
On the other hand, the modulated signal is passed through the tracking filter 12
is supplied to a phase comparator 14 through a limiter 13 and, on the other hand, to a 90° phase shifter 15.
The output voltage of the phase comparator 14 is supplied through the loop filter 17 and the loop amplifier 18 to the tracking filter 12 as a control voltage at its center frequency, and the output voltage from the loop amplifier 18 is supplied to the phase comparator 14 through the limiter 16. For example, as shown in FIG. 2, the tracking filter 12 has an input terminal 21 connected to one end of a coil 23 via a resistor 22, and a variable capacitance diode 2 connected in parallel with the coil 23.
4 and 25 are connected in series, the intermediate point of the coil 23 is grounded, a capacitor 26 is connected in parallel with the coil 23 as required, and a control voltage is applied to the connection point 27 between the variable capacitance diodes 24 and 25. , the output signal is taken out from both ends of the coil 23.

The amplitude characteristic of the tracking filter 12 is as shown by the curve 31 in FIG. 3, and the center frequency f _c
The phase characteristic when fixed is as shown by curve 32 in the figure. Also, set the phase of the input signal to ₀ ,
If the phase of the output signal of the tracking filter 12, that is, one input signal of the phase comparator 14 is ₁ , and the phase of the output signal of the phase shifter 15, that is, the other input signal of the phase comparator 14 is ₂ , then ₁ - ₂
=( _1-0 ) _- ( _2-0 ) _{= ( 1-0} )-
Since _the angle is 90°, the characteristic of phase difference _1-2 is the third one.
It becomes like curve 33 in the figure. The input/output characteristics of the phase comparator 14, that is, the characteristics of the output voltage for a phase difference _{of 1-2 ,} are as shown by the solid line in FIG. 4, and the loop amplifier 18 is an inverting amplifier. The characteristics of the output voltage of the amplifier 18 for a phase difference _{of 1-2} are as shown by the broken line in FIG.

Also, when the input FM modulation signal is unmodulated, that is, when it has a carrier frequency, the center frequency f _c of the tracking filter 12 is applied to the variable capacitance diodes 24 and 25 of the filter 12 so that it matches the carrier frequency. The control voltage is biased.

First, when the input FM modulation signal is unmodulated, that is, at a carrier frequency, the phase difference ₁ - ₀ is 0°, and the phase difference ₂ - ₀ is +90°, so
The phase difference _1-2 is -90 degrees _, and therefore, at this time, the output voltage of the phase comparator 14 and the output voltage of the loop amplifier 18 become zero.

When the input FM modulation signal is modulated and its instantaneous frequency becomes higher than the carrier frequency, the phase difference ₁ - ₀ becomes negative, and the phase difference ₂ - ₀ becomes +
Since the angle is 90 degrees, the phase difference ₁ - ₂ becomes smaller than -90 degrees (becomes negative), so the output voltage of the phase comparator 14 becomes negative, and the output voltage of the loop amplifier 18 becomes positive. Then, the voltage applied to the variable capacitance diodes 24 and 25 of the tracking filter 12 becomes larger than the bias voltage.
The center frequency f _c of the filter 12 moves higher. At this time, if the amount of movement of the center frequency f _c is too large and the moved center frequency f _c becomes higher than the instantaneous frequency, the phase difference ₁ - ₀ becomes positive and the phase difference ₂ - ₀ is +90°. , phase difference _1
−2 becomes larger than −90° (becomes positive), therefore, the output voltage of the phase comparator 14 becomes positive, the output voltage of the loop amplifier 18 becomes negative, and the tracking filter 12's variable The voltage applied to the capacitive diodes 24 and 25 becomes smaller than the bias voltage, and the center frequency f _c of the filter 12 moves lower. Therefore, in the end,
The center frequency f _c settles down when it becomes equal to the instantaneous frequency.

On the other hand, when the instantaneous frequency becomes lower than the carrier frequency, the center frequency f _c settles at a point where it becomes equal to the instantaneous frequency by the operation opposite to that described above.

In this way, according to this circuit, the center frequency f _c of the tracking filter 12 follows the instantaneous frequency of the input FM modulation signal.

Then, the variable capacitance diode 2 of the filter 12
Since the control voltages of the variable capacitance diodes 24 and 25 and the center frequency f _c of the filter 12 are proportional to each other, the control voltage of the variable capacitance diodes 24 and 25, that is, the voltage obtained at the output terminal 19, is the demodulated output of the input FM modulation signal. becomes.

By the way, the 90° phase shifter 15 in this circuit
A differential circuit is used for this purpose. For this reason,
If there is a high-frequency interference signal or noise in the input signal, this is amplified by the phase shifter 15 and causes loop disturbance.

Even if an integrating circuit is used as the 90° phase shifter 15 (expressed by -j) and the loop amplifier 18 is made into an in-phase amplifier, the same operation as described above is performed.
However, in this case, if there is a low-frequency interference signal or noise in the input signal, this will be amplified by the phase shifter 15 and will still cause loop disturbance.

In view of this point, the present invention is designed to prevent disturbance of the loop due to interference signals and noise in the high and low frequencies.

FIG. 5 shows an example of the frequency tracking circuit of the present invention, in which a signal from an input terminal 41 is supplied to a phase comparator 44 through a first tracking filter 42 and a limiter 43 on the one hand, and a phase comparator 44 on the other hand. , through a second tracking filter 45, through a 90° phase shifter 46, and through a limiter 47 to the phase comparator 44.
The output voltage of No. 4 is supplied through the loop filter 48 and the loop amplifier 49 to the first and second tracking filters 42 and 45 as control voltages at respective center frequencies. Note that in the case of an FM demodulation circuit, an output terminal 51 is derived from the loop amplifier 49.

The tracking filters 42 and 45 are each configured as shown in FIG. 2, for example, but their Qs and therefore the group delay times at the center frequency f _c are different from each other. For example, filter 42
The Q of the filter 45 is relatively high and the group delay time at its center frequency f _c is relatively large, and the Q of the filter 45 is relatively low and its group delay time at its center frequency f _c is relatively small. be done. Therefore, the amplitude characteristics of filters 42 and 45 are as shown in FIGS.
Curves 34 and 36 in the figure are shown, and the phase characteristics when the center frequency f _c is fixed are curve 35 in the figure.
and 37. Also, change the phase of the input signal.
₀ , the phase of the output signal of the tracking filter 42, that is, one input signal of the phase comparator 44.
₁ , the phase of the output signal of the tracking filter 45 is ₃ , and the phase of the output signal of the phase shifter 46, that is, the other input signal of the phase comparator 44, is ₂ .
₂ - ₀ = ( ₂ - ₃ ) + ( ₃ - ₀ ) = (
₃ − ₀ ) +90°, so the phase difference _{is 2} − ₀
The characteristic is as shown in curve 38 in Fig. 7, and ₁ −
₂ = ( ₁ - ₀ ) - ( ₂ - ₀ ), so the characteristic of phase difference ₁ - ₂ is shown by curve 39 in Figure 8.
become that way. The input/output characteristics of the phase comparator 44, that is, the characteristics of the output voltage with respect to a phase difference _{of 12 ,} are as shown by the solid line in FIG.
9 is made into an inverting amplifier, so the phase difference _{is 1} −
The characteristics of the output voltage of the _amplifier 49 with respect to the fourth
It will look like the dashed line in the figure.

According to this circuit, the center frequency f _c of the first and second tracking filters 42 and 45 follows the instantaneous frequency of the input signal by operating in accordance with the operation of the circuit shown in FIG. Thus, an FM demodulated output of the input signal is obtained at the output end 51.

A differential circuit is used as the 90° phase shifter 46. However, even if an integrating circuit is used (expressed as -j) and the loop amplifier 49 is made into an in-phase amplifier, the same operation is performed.

According to this circuit, even if there is a high-frequency or low-frequency interference signal or noise in the input signal, the two signals supplied to the phase comparator 44 will pass through the tracking filters 42 to 45. Since the high-frequency and low-frequency interfering signals and noise are removed, the loop is not disturbed by interfering signals and noise.

The output can also be derived from the limiter 43 or 47, and in this case, the input signal can be extracted with improved interference signal removal characteristics and S/N ratio.

The frequency tracking circuit of the present invention is suitable for use in a beam index type color television receiver.

A beam-indexed color television receiver has a single electron beam as a picture tube and a phosphor surface with horizontally arranged red, green and blue colored phosphor stripes; The index phosphor stripes arranged horizontally on the inner surface of the phosphor surface are used, and the colors can be switched based on the index signal obtained by scanning the index phosphor stripes with an electron beam. when the electron beam scans the red color phosphor stripe with a red primary color signal;
When scanning a green phosphor stripe, a green primary color signal is used, and when a blue phosphor stripe is scanned, a blue primary color signal is used for density modulation.

In this case, conventionally, the output signal of the photodetector is supplied to a bandpass filter with a fixed center frequency, and an index signal with a frequency determined by the pitch of the index phosphor stripe and the scanning speed of the electron beam is extracted. index signal
It is supplied to the PLL circuit and frequency multiplied, and this PLL
The output signal of the circuit is frequency-divided by a ring counter to obtain three-phase gate pulses whose phases are shifted by 120 degrees from each other to gate the primary color signals of each color.These gate pulses sequentially gate the primary color signals of each color. , the switched primary color signals are applied to the picture tube.

By the way, since the scanning speed of the electron beam usually changes due to poor linearity of horizontal deflection, the frequency of the index signal fluctuates. For this reason,
As a bandpass filter for extracting the above-mentioned index signal, one having a low Q value and a relatively wide bandwidth is used.

However, the output signal of the photodetector is cross-modulated by the luminance signal, the color signal, the above-mentioned gate pulse, etc., and contains high- and low-frequency interference signals other than the index signal. Contains noise generated by the photodetector itself. For this reason, as described above, if the bandwidth of the filter is wide, the interference signal removal characteristics and S/N will deteriorate, and the image quality will deteriorate.

However, if the frequency tracking circuit of the present invention is used instead of this bandpass filter, the bandwidth of the tracking filters 42 and 45 can be narrowed, and the center frequency of the filters 42 and 45 can be adjusted by following the frequency of the input index signal. Since f _c varies, the index signal can be extracted with improved interference signal removal characteristics and S/N ratio, resulting in improved image quality. In this case, the above-mentioned PLL circuit can also serve as a 90° phase shifter.

FIG. 9 shows an example of a beam index type color television receiver using the frequency tracking circuit of the present invention, in which 60 is a beam index type color picture tube, and a photodetector 70 is disposed outside the funnel. The output signal of this photodetector 70 is supplied to the above-mentioned frequency tracking circuit, and the index signal is taken out from the tracking filter 42 and supplied to the PLL circuit 80 through the limiter 43.

In the PLL circuit 80, the index signal passed through the limiter 43 is supplied to the phase comparator 81, and the output pulse of the voltage controlled oscillator 82 is supplied to the frequency divider 83 where the frequency is divided by 1/N. The output voltage of the phase comparator 81 is supplied to an oscillator 82 through a low-pass filter 84, and the output voltage from the oscillator 82 is outputted from the oscillator 82 at a frequency N times that of the index signal, that is, one of the color phosphor stripes.
A pulse with a frequency three times the so-called triplet frequency determined by the pitch of the set and the scanning speed of the electron beam is obtained.

Here, the output pulse of the frequency divider 83 of the PLL circuit 80 has the same frequency but a phase shift of 90 degrees with respect to the index signal passed through the limiter 43. is supplied to the phase comparator 44. Therefore, the 90° phase shifter 46 is not required on the output side of the tracking filter 45.

Then, the output pulse of the oscillator 82 of the PLL circuit 80 is supplied to a gate pulse generator 90 consisting of a ring counter, and the primary color signals of red, green and blue are to be gated under the condition regulated by the mode set pulse _PMS . A three-phase gate pulse is obtained, which is supplied to gate circuits 100R, 100G, and 100B to sequentially gate the red, green, and blue primary color signals E _R , E _G , and E _B , and the switched primary color signals are used as video signals. The signal is supplied to, for example, the first grid 61 of the picture tube 60 through the amplifier 110.

[Brief explanation of the drawing]

Fig. 1 is a system diagram of a conventional circuit, Figs. 2 to 4 are diagrams for explaining its operation, Fig. 5 is a system diagram of an example of the circuit of the present invention, and Figs. 6 to 8 are diagrams for explaining its operation. FIG. 9, which is a diagram for explaining its operation, is a system diagram of an example of a beam index type color television receiver using the circuit of the present invention. 42 and 45 are tracking filters, 44 is a phase comparator, and 46 is a 90° phase shifter.

Claims (1)

[Claims] 1 comprising first and second tracking filters to which an input signal whose frequency varies is commonly supplied, and a phase comparator that compares the phases of the output signals of the first and second tracking filters with each other, The group delay times at the center frequencies of the first and second tracking filters are made different from each other, and the output voltage of the phase comparator is applied to the first and second tracking filters at the respective center frequencies through the loop filter. A frequency tracking circuit that is supplied as a control voltage and is controlled so that the center frequencies of the first and second tracking filters respectively follow the frequency of the input signal.