JPH0677730A - Variable frequency oscillator and fm demodulator using the same - Google Patents

Abstract

PURPOSE:To obtain the variable frequency oscillator which varies in oscillation frequency with an external signal over a wide range. CONSTITUTION:The output of a band-pass filter 1 which has a center frequency omegaB, is inputted to a high-pass filter 2 which has a cutoff frequency omegaH, and the output of this high-pass filter is fed back to the output side of the band-pass filter. When those frequencies are so set that omegaB<omegaH, the oscillation frequency is set to omega0, at which the phases of the output signals of both the circuits cancel each other. The CR time constant circuit which constitutes the high-pass filter is set by the emitter resistance of a transistor and a capacitor and then the variable frequency oscillator which has a high frequency and a wide variation range can be constituted and utilized as an FM demodulator.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The first invention of the present invention relates to a variable frequency oscillator whose oscillation frequency can be easily varied by a control signal from the outside. The second invention relates to an FM demodulator using this variable frequency oscillator.

[0002]

2. Description of the Related Art A voltage-controlled oscillator is commonly used as a variable frequency oscillator whose oscillation frequency is changed by a signal supplied from the outside.

In order to make the oscillation frequency variable, a conventional voltage control type oscillator generally uses a variable capacitance capacitor (VCC) forming a resonance circuit, and by applying a control voltage to this VCC, the oscillation frequency is increased. Is being made variable.

FIG. 9 shows an example of such a voltage control type oscillator. Positive feedback is provided from the output terminal A of the differential type transistors Q ₁ and Q ₂ to the input terminal B via capacitors C ₂ and C _3. A path is added, and a parallel resonant circuit including a coil L ₂ and a varicap VC is provided on the input terminal B side.
Incidentally, C ₁ is a capacitor whose other input terminal is grounded in an alternating current, and R ₁ and L ₁ are load impedances.

In this circuit, the oscillating condition is satisfied by the frequency in the vicinity where the impedance of the parallel resonance circuit provided in the positive feedback loop is maximum, and therefore the varicap VC
It is possible to easily change the oscillation frequency by controlling V by the voltage applied from the outside.

[0006]

However, since the oscillation circuit changes the oscillation frequency by changing the impedance of the resonance circuit, for example, the L component or the C component is used to set the center frequency. It is necessary to adjust the absolute value of, and there is a problem that the number of steps for adjustment is increased, skill is required, and cost is increased.

Further, when the common emitter current of the differential type amplifier is made variable, it is possible to positively utilize the changing collector capacitance of the transistor to make the oscillation frequency variable. In this case, the oscillation frequency is changed. There was a problem that the variable range of was narrowed.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and forms a variable frequency oscillator by combining a bandpass filter circuit and a highpass filter circuit. That is, the center frequency is ω
A high-pass filter circuit having a cut-off frequency of ω _H is connected to a stage subsequent to the band-pass filter circuit of _B, and the output of the high-pass filter circuit is fed back to the input side of the band-pass filter circuit. When the center frequency ω _B is set higher than the cutoff frequency ω _H , the oscillation frequency is ω
₀ can be obtained as − (ω _B / ω ₀ −ω ₀ / ω _B ) Q = ω _H / ω ₀ .

[0009]

When the cutoff frequency ω _H of the high pass filter circuit is made higher than the center frequency ω _B of the band pass filter circuit, the oscillation frequency ω ₀ becomes a frequency at which the phase shift amounts of both circuits are canceled. Therefore, if the cutoff frequency of the high-pass filter circuit formed by the CR time constant is made variable by an external signal, the oscillation frequency ω ₀ can be easily changed.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic view of the present invention.
Is a band-pass filter circuit whose center frequency is set to ω _B , 2 is a high-pass filter circuit whose cut-off frequency is ω _H, and whose output has a predetermined gain. It is fed back to the input side of 1.

2 and 3 show an example of the configuration of the bandpass filter circuit described above. Q ₁ and Q ₂ are transistors forming a differential amplifier, and each transistor Q
_A current source I is provided at the common emitter of ₁ and Q ₂ .

In the embodiment of FIG. 2, a resonance coil L ₁ and a capacitor C ₂ are provided between the collectors of the transistors Q ₁ and Q _{2 in} order to give a bandpass characteristic. Further, in the embodiment shown in FIG. 3, a coil L ₂ and a capacitor C ₂ are provided as a resonance circuit at the differential input terminals of the transistors Q ₁ and Q ₂ .

In the bandpass filter circuit of FIG. 2, ω = ω _B
The gains G ₃ and Q ₂ at the time of are: Q ₂ = √ (C ₁ / L ₁ ) × R ₁ ‥‥‥‥‥‥‥‥‥ 1-1 G ₂ = R ₁ / R _e ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ Although it is shown in ‥‥‥‥ 1-2, when the band-pass filter circuit is likewise omega = omega _B in FIG. _{3, Q 3 = √ (C} 2 / L 2) × 2R 2 ‥‥‥‥‥ 2- 1 (however, R ₂ << R ₃ ) G ₃ = R ₁ / R _e ‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 2-2. (However, R _e is a differential pair of transistors Q ₁ and Q
₂ emitter resistance)

[0014] Since the value of the collector resistor R ₁ in the embodiment of FIG. 2 affects the value of Q _2, G _2, Q _2, G ₂ and becomes difficult to design independently, in the case of FIG. 3 Has the advantage that the impedances for setting Q ₃ and G ₃ can be designed independently, the degree of freedom of Q is increased, and the oscillation frequency is set easily, as described later.

Next, FIG. 4 shows a conceptual diagram of the high-pass filter circuit 2 of FIG. The input signal is supplied to the base of the emitter follower transistor Q ₃ , and the output of the emitter is supplied to the emitter of the transistor Q ₄ for setting the time constant via the capacitor C _H. The transistor Q ₄ is of a grounded base type (biased ground), and is formed so that the emitter current i _e is controlled by the variable current source I (internal resistance: 100 KΩ).

When the emitter resistance of the transistor Q ₄ is R _e , the cutoff frequency ω _H of this high-pass filter circuit and the phase φ _H of the output signal at the frequency ω can be expressed by the following equation. _{ω H = 1 / (C H} × R e) ‥‥‥‥‥‥‥ 3-1 φ H = tan -1 (ω H / ω) ‥‥‥‥‥‥ 3-2 Note that emitter resistor R _e is As is well known, it can be shown by the following formula. _{R e = kT / (q ×} i e) ‥‥‥‥‥‥‥‥ 4 where, k = Boltzmann's constant T = absolute temperature q = electron charge _{Thus, R e (Ω) ≒ 26} / i e (mA) Can be

As described above, the bandpass filter circuit 1 and the highpass filter circuit 2 which constitute the variable frequency oscillator of FIG. 1 have the center frequency ω _B and the cutoff frequency ω _H , respectively, and ω _B > ω _H. If you set
The output phase of each circuit changes as shown in FIG. 5 depending on the frequency ω.

Therefore, the oscillation frequency of the variable frequency oscillator of FIG. 1 is set to ω ₀ where the output phase differences of both circuits are equal ±. That is, the oscillation frequency omega _{0 is, - (ω B / ω 0} -ω 0 / ω B) Q = ω H / ω 0 ∴ ω 2 0 = (ω 2 B + ω B · ω H / Q) ‥‥‥‥ _{‥‥‥‥‥‥‥ 5 ω H = 1 / (} C H × R H) and substituting equation ^{_{4, ω 0 = √ω 2 B}} + (ω B · q · i e) / (C H · kT ) .... 6. Therefore, the oscillation frequency of the variable frequency oscillator can be changed by externally controlling the current source I of FIG. 4 that constitutes the high-pass filter circuit 2.

FIG. 6 is a circuit diagram showing a specific example of the variable frequency oscillator of the present invention shown in FIG. In this figure, a portion 10 surrounded by a dotted line is a bandpass filter circuit for adding a bandpass characteristic, and resistors R ₂ and R
₃ and a resonance coil L _B for setting the pass frequency and a capacitor C _B are connected in parallel.

The output of the bandpass filter circuit 10 is supplied to a differential first amplifier 11 composed of transistors T _{1 to} T ₄ connected in a cascading manner, the output of which is buffer transistors T ₅ , T ₆ and DC cut. Capacitor C
It is supplied to the second differential amplifier 12 via ₁ and C ₂ .

Similarly, the output of the second differential amplifier 12 is output by the emitter follower via the buffer transistors T ₁₁ and T _12, and is supplied to one ends of the capacitors C _H and C _H which form the high pass filter circuit.

The other ends of the capacitors C _H and C _H have a time constant R _e.
(Emitter resistance) is connected to the emitters of the transistors T ₁₄ , T ₁₅ and the transistors T ₁₃ , T ₁₄ are connected.
_Is connected to the variable current sources I _c and I _c via. Emitter resistor R _e of the transistor T _14, T _15, as is R _e shown in Equation 4 described above, the variable current source I _c, since it is varied by the control current (i _e) flowing through the I _c, the The cutoff frequency ω _H of the high pass filter circuit 13 can be changed by changing the variable current sources I _C and I _C by an external signal. The output of the high pass filter circuit 13 is fed back to the band pass filter circuit 10 via the buffer transistors T ₁₆ and T ₁₇ to form an oscillation circuit.

I _{1 to} I ₈ represent constant current sources. Further, E _{1 to} E ₄ are bias power supplies of cascade-connected transistors, and these form a base-grounded amplifier (current amplification factor = 1) to extend the amplification band.

According to this embodiment, the variable oscillation frequency of about 400 MHz can be obtained by the IC circuit, and if the Q of the resonance circuit is properly designed, the variable frequency is about ± 100 M.
It is also possible to change the Hz.

FIG. 7 shows another embodiment of the present invention, in which the same reference numerals indicate the same functional parts. However, in this embodiment, the bandpass filter circuit is the first
The differential amplifier 1 is formed by adding a coil L _B and a capacitor C _B. Therefore, the Q of the bandpass filter circuit and the gain are linked, but the circuit configuration is simplified.

Next, an embodiment in which the variable frequency oscillator of the present invention is used as an FM demodulator will be shown. FIG. 8A shows a block diagram of the FM demodulator of the present invention, where 21 is
For example, a multiplier (mixer) to which a wideband video signal (IF) is supplied, 22 is a low-pass filter for removing high-frequency signal components, 23 is an amplifier, 24 is a linearity correction circuit for correcting the linearity of the detection output, Reference numeral 25 is a variable frequency oscillator formed by the bandpass filter circuit and the highpass filter circuit described above. The output of the variable frequency oscillator 25 is fed back as the other input of the multiplier 21, and forms a PLL circuit as a whole.

The multiplier 21 multiplies the FM-modulated wideband video IF signal fv by the center frequency f ₀ of the variable frequency oscillator 25, and outputs the signal of the phase difference from the low-pass filter 22. Then, by controlling the oscillation frequency of the variable frequency oscillator 25 so as to follow fv by the detected output, (b) in FIG.
An FM demodulation circuit having an S-shaped characteristic as shown in FIG.

By the way, generally, the center frequency f ₀ of the variable frequency oscillator 25 is adjusted from the outside so that the detection output and the frequency change of the variable frequency oscillator 25 are completely proportional to each other.
When the oscillation frequency ω ₀ of the variable frequency oscillator 25 deviates from the center frequency of the FM-modulated video signal fv, the linearity deteriorates as shown in FIG. 8B. Therefore, in the case of the present invention, the detection output is configured to form the control voltage of the variable frequency oscillator 25 via the linearity correction circuit 24.

The linearity correction circuit, for example, y = ax
+ Bx ² + cx ³ + ...... nx ⁿ is a high-order function to be added, and external terminals t ₁ t ₂ t ₃ ............ for setting the coefficients a, b, c. Provide t _n . Then, by giving an appropriate coefficient, the detection output is corrected so as to change into a straight line as shown by the alternate long and short dash line.

[0030]

As described above, the variable frequency oscillator of the present invention is configured so that the output of the bandpass filter circuit is positively fed back through the highpass filter circuit, and the cutoff frequency of the highpass filter circuit is variable. Since it is configured to change the oscillation frequency, it is possible to control the oscillation frequency without using the varicap that forms the resonance circuit as seen in the conventional variable frequency oscillator, and to adjust the coil without adjustment. Cost reduction can be achieved by adopting this method. In addition, there is an advantage that the number of external elements can be reduced and the circuit can be made compact by using the IC. Further, by applying the variable frequency oscillator of the present invention to a demodulation circuit for a video signal (FM signal), it is possible to obtain a wideband detection signal with little distortion.

[Brief description of drawings]

FIG. 1 is a block diagram showing a conceptual diagram of a variable frequency oscillator of the present invention.

FIG. 2 is a circuit diagram showing a configuration example of a bandpass filter circuit.

FIG. 3 is a circuit diagram showing another configuration example of a bandpass filter circuit.

FIG. 4 is a circuit diagram for explaining the configuration of a high pass filter circuit.

FIG. 5 is a graph for explaining an oscillation frequency of a variable frequency oscillator formed by a bandpass filter circuit and a highpass filter circuit.

FIG. 6 is a circuit diagram showing a specific example of a variable frequency oscillator according to the present invention.

FIG. 7 is a specific circuit diagram showing another embodiment of the variable frequency oscillator of the present invention.

FIG. 8 is a block diagram when the variable frequency oscillator of the present invention is applied to an FM demodulator and its detection characteristics.

FIG. 9 is a diagram showing a circuit example of a conventional variable frequency oscillator.

[Explanation of symbols]

 1 band pass filter circuit 2 high pass filter circuit 21 multiplier 22 low pass filter 24 linearity correction circuit 25 variable frequency oscillator

Claims (5)

[Claims] 1. An output signal of a bandpass filter having a center frequency of ω _B is supplied to a highpass filter having a cutoff frequency of ω _H, and the output signal of the highpass filter is positively fed back to the input side of the bandpass filter. Center frequency ω _B and cutoff frequency ω _H
A variable frequency oscillator characterized in that an oscillation frequency ω _O is set in a range where ω _B <ω _H. 2. The variable according to claim 1, wherein a parallel resonance circuit is connected between the differential input terminals or between the differential output terminals of the differential amplifier to form a bandpass filter. Frequency oscillator. 3. A high-pass filter having a cut-off frequency set by the emitter resistance of a grounded base transistor and the capacitance value of a capacitor connected to the emitter of the transistor. Variable frequency oscillator according to. 4. A bandpass filter in which a parallel resonance circuit is connected between the differential input terminals of a differential amplifier, and a differential output of the differential amplifier via a DC blocking differential amplifier, respectively.
The capacitor is supplied to one end and the other end of the capacitor is supplied to a high-pass filter circuit connected to the emitter of a transistor having an emitter resistance, and the respective outputs of the high-pass filter circuit are connected to both ends of the parallel resonance circuit via a resistor. And a variable frequency oscillator configured to change the oscillation frequency by changing the emitter current of a transistor forming the high pass filter. 5. A variable frequency oscillator in which the output of a bandpass filter circuit is input to a highpass filter circuit, and the output of the highpass filter circuit is positively fed back to the bandpass filter circuit, and the phase of the variable frequency oscillator and the FM modulation signal. And a linearity correction circuit that corrects the linearity of the output signal of the multiplier, wherein the oscillation frequency of the variable frequency oscillator is controlled by the output of the linearity correction circuit. FM demodulator.