JP2531135B2 - Crystal oscillator circuit - Google Patents

Description

The present invention relates to a crystal oscillator circuit suitable for use in, for example, an APC circuit of a VTR.

[Outline of Invention]

According to the present invention, in a crystal oscillator circuit, when the oscillation frequency is fixed, the resistance value of the terminating resistance of the crystal filter is reduced, and when the oscillation frequency is variable, the terminating resistance of the crystal filter is increased to fix the oscillation frequency. In some cases, a stable oscillation output can be obtained, and when the oscillation frequency is changed, the frequency variable range can be widened.

[Conventional technology]

In the VTR, the luminance signal is FM-modulated and the chroma signal is converted to low frequency and recorded on the magnetic tape. In a so-called 8 mm video with a tape width of 8 mm, the low-frequency conversion subcarrier frequency is set to 743 kHz (47.25f _H (f _H : horizontal frequency)).

When a chroma signal is converted into a low frequency band and recorded during VTR recording, a crystal oscillation circuit forms a signal locked to a subcarrier of an input video signal, and this signal is used for low frequency conversion. At the time of reproduction, a crystal oscillation circuit forms a signal of a fixed frequency in order to remove a time-axis variation in the reproduction signal.

As described above, the crystal oscillator circuit in the VTR is designed so that the frequency can be changed in order to lock it on the subcarrier of the input video signal in the recording system of the chroma signal, and the frequency does not change in the reproducing system of the chroma signal.

Conventionally, a crystal oscillator circuit has an amplifier as shown in FIG.
The output of 101 is supplied to the crystal filter 104 via the phase shift circuit 102 and the limiter 103, and the output of the crystal filter 104 is fed back to the amplifier 101. And
When the frequency is fixed, the phase shift control signal with a constant voltage is supplied to the terminal 105, and when the frequency is varied, the phase shift control signal supplied to the terminal 105 is changed, and the oscillation frequency is changed accordingly. Was made variable.

[Problems to be solved by the invention]

As described above, the crystal oscillation circuit of the VTR needs to allow the oscillation frequency to be widely variable during recording and be fixed during reproduction. The variable frequency range during recording is ±
About 500Hz is required. However, the conventional crystal oscillation circuit has a problem that the oscillation frequency during reproduction becomes unstable when an oscillation frequency having such a wide variable frequency is obtained during recording. When trying to stabilize the oscillation frequency during reproduction, the frequency variable width cannot be set wide.

Therefore, an object of the present invention is to provide a crystal oscillation circuit in which the variable width of the frequency is wide in the variable mode and the oscillation frequency does not fluctuate in the fixed mode and a stable oscillation output can be obtained.

[Means for solving problems]

The present invention is characterized in that when the oscillation frequency is fixed, the resistance value of the terminating resistor of the crystal filter is reduced, and when the oscillation frequency is variable, the resistance value of the terminating resistor of the crystal filter is set large. It is an oscillator circuit.

[Operation] When the resistance value of the terminating resistor is reduced, the rate of change of the oscillation frequency with respect to the change of the phase is reduced. When the resistance value of the terminating resistor is increased, the rate of change in the oscillation frequency with respect to the change in phase increases. In the fixed mode, a low level is supplied to the terminal 23. As a result, no current flows through the resistor 16, and the resistor 34 becomes a terminating resistor. In variable mode, terminal 23
Is supplied with a high level. As a result, the series connection of the resistor 16 and the resistor 34 becomes a terminating resistor. As described above, the resistance value of the terminating resistor is set to a small value in the fixed mode, and the resistance value of the terminating resistor is set to a large value in the variable mode. As a result, a stable oscillation output can be obtained in the fixed mode, and the frequency variable width can be increased in the variable mode.

〔Example〕

An embodiment of the present invention will be described in the following order.

Relationship between terminal resistance and frequency change rate b. Configuration and operation of one embodiment c. APC circuit a. Relationship between terminal resistance and frequency change rate As shown in FIG. The phase shift circuit
A crystal oscillating circuit can be configured by supplying the crystal filter including the terminating resistor 55 and the crystal oscillator 56 via the 52, the limiter 53, and the buffer 54, and supplying the output to the differential amplifier 51 for feedback. . Reference numeral 57 is a resistance for level balancing. When the amount of phase shift of the phase shift circuit 52 is controlled by the control voltage supplied to the terminal 58, the oscillation frequency changes according to the phase characteristic of this crystal filter.

This crystal filter has the following characteristics.

The equivalent circuit of the crystal unit is represented by capacitors C ₀ and C ₁ , a coil L ₁ and a resistor R ₁ , as shown by the broken line in FIG. In FIG. 3, Vin is an input signal source. The Q of this crystal unit is calculated by Q = 2πf ₀ L ₁ / R ₁ ... When the terminating resistor is R, the Q when the terminating resistor R is connected is Q = 2πf ₀ L ₁ / (R ₁ + R). From the equation, Q increases as the terminating resistance R increases.

The gain G at the resonance frequency f ₀ of the crystal filter is calculated by G = 20 log (R / (R ₁ + R)). From the equation, the loss increases as the terminating resistance R ₁ increases.

The transfer function A of a second-order bandpass filter is generally It is represented by. The delay D at f = f ₀ is D = −dθ / dω = Q / πf ₀ [sec] ... And the delay per frequency is dθ / df = 2πD = 2Q / f ₀ [rad / Hz]・ ・ It becomes. When the formula is corrected to deg, dθ / df = 360Q / πf ₀ [deg / Hz] ... Substituting Q obtained by the equation into the equation and reciprocal yields df / dθ = (R ₁ + R) / 720L ₁ ... From the above equation, the frequency change rate per angle 1 deg at f = f ₀ can be obtained. When the phase is changed, the oscillation frequency of the crystal oscillation circuit is changed based on the formula.

FIG. 4 shows the amplitude characteristic and the phase characteristic when the terminating resistor R is 75Ω. In FIG. 4, G ₁ is the amplitude characteristic and D ₁ is the phase characteristic. FIG. 5 shows the amplitude characteristic and the phase characteristic when the terminating resistance R is 1 kΩ. In FIG. 5, G ₂ is the amplitude characteristic and D ₂ is the amplitude characteristic.
Is the phase characteristic. As can be seen by comparing the phase characteristic D ₁ shown in FIG. 4 and the phase characteristic D ₂ shown in FIG. 5, the larger the terminating resistance R, the larger the change in phase with respect to the change in frequency at the frequency f ₀ .

From the equation, the relationship between the terminating resistance R and the frequency change rate is expressed as shown in FIG. In FIG. 6, the horizontal axis represents the resistance value of the terminating resistor, and the vertical axis represents the frequency change rate. The sixth
In the figure, the equivalent resistance R ₁ of the crystal unit is 20Ω.

From the above, in the case of configuring a crystal oscillation circuit, the terminating resistance R is made small in order to fix the frequency and perform stable oscillation, and the terminating resistance R is made large in order to make the frequency widely variable. I see that it is better.

b. Configuration and Operation of One Embodiment FIG. 1 shows an embodiment of the present invention. In this embodiment, by increasing the terminating resistance in the variable mode and decreasing the terminating resistance in the fixed mode, the variable frequency range can be widened in the variable mode, and stable oscillation output can be obtained in the fixed mode. .

In FIG. 1, 1, 2 and 3 are NPN type emitter follower transistors. Emitter follower transistor 1,
Transistors 4,5,6 that act as current sources
The collector of is connected. The collectors of the emitter follower transistors 1, 2, 3 are connected to the power supply terminal 7 of the power supply voltage Vcc. The base of the transistor 4 is connected to the power supply terminal 14 of the reference voltage Vr ₁ . The bases of the transistors 5 and 6 are connected to the connection point between the emitter of the transistor 8 and the base of the transistor 9. The emitters of the transistors 4, 5 and 6 are connected to the ground terminal 13 via the resistors 10, 11 and 12, respectively.

The base of the transistor 1 is connected to the input terminal 15.
The emitter of the transistor 1 is connected to one end of the resistors 16 and 17, and the PNP type emitter follower transistor 18
Connected to the base of. The collector of the transistor 18 is connected to the ground terminal 13. The emitter of the transistor 18 is connected to the power supply terminal 7 via the resistor 19, the emitter of the transistor 18 is connected to the bases of the transistors 2 and 3, and these connection points are connected via the resistor 20 to the transistor.
Connected to 21 collectors. The emitter of the transistor 21 is connected to the ground terminal 13. The base of the transistor 21 is connected to the terminal 23 via the resistor 22 and the base of the transistor 24.

The emitter of the transistor 24 is grounded. The collector of the transistor 24 is connected to the collector of the transistor 9 and the base of the transistor 8, and these connection points are connected to the power supply terminal 7. The collector of the transistor 8 is connected to the base of the transistor 8. The emitter of the transistor 9 is connected to the ground terminal 13 via the resistor 25. The base of the transistor 9 and the emitter of the transistor 8 are connected, and this connection point is connected to the ground terminal 13 via the resistor 26.

27 and 28 are transistors that form a differential amplifier. The emitters of the transistors 27 and 28 are connected to the collectors of the transistors 29 and 30, which operate as current sources, respectively, and the resistor 31 is connected between the emitters of the transistors 27 and 28. The bases of the transistors 29 and 30 are connected to the power supply terminal 14. The emitters of the transistors 29 and 30 are connected to the ground terminal 13 via the resistors 32 and 33, respectively.

The base of the transistor 27 is connected to one end of the resistor 34 and the terminal 36. A crystal oscillator 37 is connected to the terminal 36. The other end of the resistor 34 is connected to the connection point between the other end of the resistor 16 and the emitter of the transistor 3. Transistor
The base of 28 is connected to one end of resistor 35. The other end of the resistor 35 is connected to the connection point between the emitter of the transistor 2 and the resistor 17.

The collector of the transistor 27 is connected to the power supply terminal 7 via the resistor 38 and the output terminal 40. The collector of the transistor 28 is connected to the power supply terminal 7 via the resistor 39 and the output terminal 41.

The output terminals 40 and 41 are connected to the input terminals of the phase shift circuit 42. The amount of phase shift of the phase shift circuit 42 is changed by the control voltage supplied to the terminal 43. The output terminal of the phase shift circuit 42 is a limiter
Connected to 44 input terminals. The output terminal of the limiter 44 is connected to the input terminal 15.

The transistors 27 and 28 form a differential amplifier. The gain of this differential amplifier is set by the resistor 31. The output of this differential amplifier is supplied to the emitter follower transistor 1 via the phase shift circuit 42 and the limiter 44.
The output of the emitter follower transistor 1 is the terminating resistor 34,
Alternatively, it is fed back to the differential amplifier composed of the transistors 27 and 28 through the crystal filter composed of 16 and 34 and the crystal oscillator 37.

In the fixed mode, the low level is supplied to the terminal 23, the phase shift control voltage of a constant voltage is supplied from the terminal 43 to the phase shift circuit 42, and the phase shift amount of the phase shift circuit 42 is made constant.

When the low level is supplied to the terminal 23, the transistor 21
Turns off and the transistor 24 turns off. Transistor 24
When is turned off, a constant voltage Vr ₂ is supplied to the bases of the transistors 5 and 6 from the connection point between the base of the transistor 9 and the emitter of the transistor 8, and the transistors 5 and 6 operate as a constant current source. Since the transistor 21 is off and the transistors 5 and 6 are operating as constant current sources, the transistor 18 and the transistors 2 and 3 are operating as emitter follower transistors. As a result, an output of a level lower than the output of the emitter of the emitter follower transistor 18 by V _BE (base-emitter voltage) appears from the emitter of the emitter follower transistor 3. The emitter output of the emitter follower transistor 18 is higher than the emitter output of the emitter follower transistor 1 by V _BE . Therefore, the output of the emitter of the emitter follower transistor 3 is in phase with the force of the emitter of the emitter follower transistor 1. Since the output of the emitter of the emitter follower transistor 3 and the output of the emitter of the emitter follower transistor 1 are in phase, no current flows through the resistor 16 at this time.

Therefore, at this time, the resistor 34 becomes a terminating resistor. At this time, the output of the emitter of the emitter follower transistor 1 and the output of the emitter of the emitter follower transistor 2 are in phase. Therefore, no current flows through the resistor 17, and the DC balance of the differential amplifier is maintained.

The values of the resistors 34 and 35 are, for example, 100Ω, and the values of the resistors 16 and 17 are, for example, 2 kΩ. In the fixed mode, thus, the resistor 34 of, for example, 100Ω becomes the terminating resistor. As described above, when the terminating resistance is small, the frequency change rate becomes small. Therefore, in the fixed mode, the oscillation frequency hardly changes even if a phase variation occurs, and a stable oscillation output can be obtained.

In the variable mode, a high level is supplied to the terminal 23, and a control voltage is supplied from the terminal 43 to the phase shift circuit 42 to control the phase shift amount of the phase shift circuit 42.

When a high level is supplied to the terminal 23, the transistor 21
Is turned on and the transistor 24 is turned on. When the transistor 24 is turned on, the constant voltage Vr ₂ is not applied to the bases of the transistors 5 and 6, and the transistors 5 and 6 are turned off. Since the transistors 5 and 6 are turned off, the emitter follower transistors 2 and 3 are turned off. Further, since the transistor 21 is turned on, the emitter follower transistor 18 is turned off. Therefore, the output of the emitter follower transistor 1 is supplied to the base of the transistor 27 via the resistors 16 and 34.

Therefore, the terminating resistance at this time is the sum of the resistance 16 and the resistance 34. At this time, the output of the emitter follower transistor 1 is supplied to the base of the transistor 28 via the resistors 17 and 35 for level balancing, and the DC level balance of the differential amplifier is maintained.

In variable mode, the termination resistance is thus 2 kΩ, for example.
It becomes the sum of the resistance 16 and the resistance 34 of 100Ω, for example. If the terminating resistance is large, the frequency change rate with respect to the phase change becomes large. Therefore, when the phase shift amount is changed by the phase shift circuit 42, the oscillation frequency changes widely, and the frequency variable width can be widened.

c. APC circuit FIGS. 7 and 8 show the APC circuit at the time of recording and reproducing the VTR. The present invention is suitable for use in a crystal oscillator circuit in a VPC APC circuit.

FIG. 7 shows an operation state during reproduction. In FIG. 7, the reproduction output of the rotary head 61 is a low pass filter.
It is supplied to 62 and the chroma signal is separated. This reproduction chroma signal has the subcarrier frequency converted to the low frequency band of 743 kHz, for example. During reproduction, the reproduction chroma contains a time axis fluctuation component ± Δf caused by uneven rotation of the head, expansion and contraction of the tape, and the like.

The output of the low pass filter 62 is supplied to the ACC circuit 63,
The burst level is constant. The output of the ACC circuit 63 is supplied to the main converter 64. The main converter 64 is supplied with an output having a frequency of 4.32 MHz from the sub converter 65 via the bandpass filter 66. Frequency 4.32MHz passed through bandpass filter 66 in main converter 64
Sub-converter 65 output and frequency 743 from ACC circuit 63
A difference frequency signal from the reproduced low frequency conversion chroma signal of kHz is formed, and a chroma signal having a subcarrier frequency of 3.58 MHz is formed. The output of the main converter 64 is a comb filter 67
It is output from the output terminal 73 via and is also supplied to the phase comparison circuit 68.

On the other hand, the phase comparison circuit 68 is supplied with a reference signal of, for example, a frequency of 3.58 MHz output from the crystal oscillation circuit 69.
The output of the crystal oscillator circuit 69 is fixed at a frequency of 3.58 MHz. The phase comparison circuit 68 performs a phase comparison between the output of the comb filter 67 and the output of the crystal oscillation circuit 69, and the error voltage is passed through a low pass filter 70 to a VCO (voltage controlled oscillator) 7
Supplied to 1. The output of the VCO 71 is supplied to the sub converter 65 via the 1/8 frequency divider counter 72. Sub converter 65
The crystal oscillator circuit 69 supplies an output with a frequency of 3.58 MHz to the sub converter 65, and the sum frequency signal of the output of the crystal oscillator circuit 69 and the output of the VCO 71 via the 1/8 frequency division counter 72 causes the frequency of 4.32 MHz. An output signal of MHz is formed. The output signal having the frequency of 4.32 MHz is supplied to the main converter 64 via the bandpass filter 66.

If the reproduced low frequency conversion chroma signal output from the ACC circuit 63 includes a time-axis variation of ± Δf, the same time-axis variation of ± Δf is also included in the output of the sub-converter 65. Be done. Main converter 64 and sub converter 65
Of the output of the ACC circuit 63 and the output of the ACC circuit 63 are formed, and this time axis fluctuation is removed.

FIG. 8 shows an operation state during recording. In FIG. 8, a chroma signal having a subcarrier frequency of 3.58 MHz is supplied to the input terminal 81, and this chroma signal is supplied to the main converter 84 via the bandpass filter 82 and the ACC circuit 83.

The sub converter 85 supplies the main converter 84 with an output having a frequency of 4.32 MHz. Output signal of frequency 4.32MHz output from sub converter 85 in main converter 84 and sub carrier frequency 3.5 output from ACC circuit 83
A difference frequency signal with the 8 MHz chroma signal is formed. From this difference frequency signal, a low frequency conversion chroma signal having a frequency of 743 kHz is formed. This low-frequency converted chroma signal is supplied from the main converter 84 to the addition circuit 88 via the low-pass filter 87, and is supplied to the rotary head 89 together with the luminance signal Y.

The output of the VCO 90 is supplied to the sub-converter 85 via the 1/8 frequency dividing counter 91, and the output of the frequency-variable crystal oscillation circuit 92 is supplied. For VCO90, from terminal 95 to AFC
A control voltage is supplied. The output of the crystal oscillator circuit 92 is supplied to the phase comparison circuit 93. The output of the ACC circuit 83 is supplied to the phase comparison circuit 93, and the phase comparison between the output of the ACC circuit 83 and the crystal oscillation circuit 92 is performed. This error voltage is supplied to the crystal oscillation circuit 92 via the low pass filter 94, and the oscillation frequency of the crystal oscillation circuit 92 is controlled by this error voltage. This allows the frequency 3.58 locked to the subcarrier.
An output signal of MHz is output from the crystal oscillation circuit 92.

The sub-converter 85 forms an output signal with a frequency of 4.32 MHz from the sum frequency of the output of the crystal oscillation circuit 92 and the output of the VCO 90 via the 1/8 frequency division counter 91. This output signal is supplied to the main converter 84 via the bandpass filter 86.

The crystal oscillation circuit 69 during reproduction oscillates at a fixed frequency. Therefore, at the time of reproduction, the above-described embodiment is operated in the fixed mode. As a result, a stable oscillation output can be obtained. The crystal oscillation circuit 92 during recording is
The frequency is variable to lock to the subcarrier. Therefore, at the time of recording, the above-described embodiment is operated in the variable mode. This makes it possible to obtain an oscillation output having a wide frequency variable range.

〔The invention's effect〕

According to the present invention, the resistance value of the terminating resistor is set to a small value in the fixed mode, and the resistance value of the terminating resistor is set to a large value in the variable mode. When the resistance value of the terminating resistor is small, the rate of change of the oscillation frequency with respect to the change of the phase becomes small. Therefore, in the fixed mode, even if phase fluctuation occurs,
The oscillation frequency becomes stable. When the resistance value of the terminating resistor is large, the rate of change in the oscillation frequency with respect to the change in phase increases. Therefore, in the variable mode, the frequency variable width can be widened.

For example, in the fixed mode, the resistance value of the terminating resistor is reduced and the rate of change of the oscillation frequency with respect to the phase change is set to 1 Hz / deg.
In the variable mode, the resistance value of the terminating resistor is increased and the rate of change of the oscillation frequency is set to 10 Hz / deg. in this case,
In the fixed mode, the change in oscillation frequency is suppressed to ± 5 Hz even if ± 5 ° phase fluctuation occurs. On the other hand, in the variable mode, if the amount of phase shift can be changed ± 45 °, ± 450Hz
The variable frequency range can be obtained.

[Brief description of drawings]

FIG. 1 is a connection diagram of one embodiment of the present invention, FIG. 2 is a block diagram used for explaining one embodiment of the present invention, and FIG. 3 is an equivalent circuit diagram used for explaining one embodiment of the present invention. FIGS. 4 and 5 are frequency characteristic diagrams used for explaining one embodiment of the present invention, FIG. 6 is a graph used for explaining one embodiment of the present invention, and the present invention is applicable to FIGS. 7 and 8. VTR APC
FIG. 9 is a block diagram of a conventional crystal oscillator circuit when reproducing and recording the circuit, and FIG. Description of main symbols in the drawings 1,2,3,18: Emitter follower transistor, 15: Input terminal, 16,34: Termination resistor, 23: Mode setting terminal, 27, 28:
Transistor constituting differential amplifier, 42: Phase shift circuit.

Claims (1)

(57) [Claims] 1. A crystal filter, a first differential input and a second differential input, wherein the crystal filter is connected to the first differential input side, oscillates the crystal filter, and outputs an output of the oscillation. A differential amplifier adapted to be taken out from the dynamic output side; a limiter for inserting the oscillation output into a feedback path for feeding back to the first differential input side to limit the output level of the oscillation below a predetermined level; A first load resistor connected to the first differential input side and serving as a load of the crystal filter; and a first load resistor connected to the second differential input side and having the same resistance value as the first load resistor. The load resistance of No. 2 and the values of the first and second load resistances are simultaneously decreased by the same resistance value when the oscillation frequency of the oscillation is fixed, and simultaneously increased by the same resistance value when the oscillation frequency is variable. With setting means for setting Crystal oscillator circuit, characterized in that.