JPH0249046B2 - SHUHASUFUKUCHOKAIRO - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a frequency demodulation circuit formed by a phase lock loop and suitable for integration into an IC (integrated circuit).

[Background of the invention]

Conventionally, a frequency modulation (hereinafter referred to as FM modulation) method has been widely used as a method for recording and reproducing information signals. To explain this method using the recording and playback of audio signals on a video tape recorder (VTR) as an example, generally,
In a VTR, the luminance signal is FM-modulated, and the chromaticity signal is reduced and recorded below the frequency band of the FM-modulated luminance signal.In that case, the FM-modulated audio signal is converted to FM-modulated. A method is known in which the frequency band of the luminance signal and the frequency band of the reduced converted chromaticity signal are frequency-multiplexed and recorded.

Figure 1 shows the audio FM multiplex recording system.
This is a block diagram showing a conventional example of an FM demodulation circuit, in which 1 is an AGC circuit (automatic gain control circuit), 2 is a dynamic range compression circuit, 3 is a switch circuit,
4 is an LPF (low pass filter), 5 is a VCO (voltage controlled oscillator), 6 is a phase detection circuit, 7 is a loop filter, 8 is a dynamic range expansion circuit, 9 is a recording amplifier, 10 is a reproduction amplifier, 11 12 is a magnetic tape, 12 is a magnetic head, and the part surrounded by broken lines is an IC configuration, 101 to 10.
7 is the IC pin terminal.

In the figure, during recording, the switch circuit 3 is closed to the terminal A side, and the VCO 5 operates as an FM modulator. The audio signal supplied from the IC pin terminal 101 is level-adjusted by the AGC circuit 1, and compressed by the dynamic range compression circuit 2 to a predetermined dynamic range. The audio signal from the dynamic range compression circuit 2 is supplied to the VCO 5 via a switch circuit. VCO5 changes the oscillator frequency according to the supplied audio signal, and
Output audio signal. This FM audio signal is supplied from the IC pin terminal 106 to the recording amplifier 9, where it is amplified, and then supplied to the magnetic head 12, where it is sent to the magnetic tape 1.
1 is recorded.

On the other hand, the audio signal from switch circuit 3 is
LPF connected externally between pin terminals 103 and 104
The signal is supplied to the dynamic range expansion circuit 8 via the dynamic range compression circuit 2, where it is expanded by the amount compressed by the dynamic range compression circuit 2, and outputted from the IC pin terminal 105 as a monitor signal.

Next, during playback, the switch circuit 3 is closed to the terminal B side, and the VCO 5, phase detection circuit 6 and
A frequency demodulation circuit is also formed by a phase-lock flap formed by a loop filter externally attached to the IC pin terminal 102.

Therefore, the magnetic head 12 reads the magnetic tape 1.
The FM audio signal reproduced from 1 is sent to the reproduction amplifier 1.
0 and is supplied to the phase detection circuit 6 from the IC pin terminal 107. The output signal of the VCO 5 is also supplied to the phase detection circuit 6, and the phase of the FM audio signal is detected using this output signal. The output signal of the phase detection circuit 6 is supplied to the VCO 5 via a loop filter 7 and a switch circuit.
The carrier component of the FM audio signal is band-limited by the LPF 4 and sufficiently attenuated, and is further outputted to the IC pin terminal 105 through the dynamic range expansion circuit 8. The output signal obtained at this IC pin terminal 105 is a frequency demodulated audio signal.

By the way, the lag lead filter shown in FIG. 2a is generally used as the first loop filter. Figure b shows a lag filter, consisting of a resistor R1 between the input and output terminals, and a capacitor C1 provided between the output terminal and the ground terminal. The lag lead filter has a resistor R2 connected in series to the capacitor C1 of the lag filter. Figure 3a
1 shows the amplitude characteristics of the lag-lead filter, and b shows the phase characteristics of the lag lead filter. Frequencies ₁ and ₂ in Fig. 3a are respectively expressed as ₁ = 1/2πC1・(R1+R2) …(1) ₂ = 1/2πC1・R2 …(2) Therefore, the lag lead filter is Compared to a lag filter, the phase lock loop has a larger phase margin and a wider pull-in range.

However, if the output signal of a loop filter consisting of such a lag lead filter is an FM demodulated audio signal, the carrier component of the FM signal is not sufficiently attenuated, so as shown in FIG.
LPF4 is required. Therefore, the number of parts increases. In particular, when converting the frequency demodulation circuit into an IC, this LPF4 becomes an external component, which not only increases the number of external components but also requires IC pin terminals, which poses a problem in terms of cost. .

[Purpose of the invention]

The purpose of the present invention is to eliminate the drawbacks of the above-mentioned prior art,
It is an object of the present invention to provide a frequency demodulation circuit that can reduce the number of IC pin terminals without deteriorating performance such as the pull-in characteristic of a phase lock loop.

[Summary of the invention]

In order to achieve this object, the loop filter of the phase lock loop has lag filter characteristics and lead filter characteristics, and the output signal of the phase detection circuit is band-limited by the lag filter characteristics to sufficiently eliminate unnecessary high-frequency components. The present invention is characterized in that it obtains a demodulated signal that is attenuated as much as possible, and limits the band of the output signal of the phase detection circuit using lag filter characteristics and lead filter characteristics to obtain a control signal for a voltage controlled oscillator.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 4 is a block diagram showing an embodiment of the frequency demodulation circuit according to the present invention, in which 7 ₁ is an input terminal of the loop filter 7, and 7 ₂ and 7 ₃ are the input terminals of the loop filter 7.
1, and the parts corresponding to those in FIG. 1 are given the same reference numerals.

In FIG. 4, during recording, switch circuit C
is closed to the A side, and the audio signal supplied to the IC pin terminal 101 is level-adjusted by the AGC circuit 1.
It is FM modulated by VCO 5 via switch circuit 3. The obtained FM audio signal is amplified by a recording amplifier 9, supplied to a magnetic head 12, and recorded on a magnetic tape 11.

During playback, the switch circuit 3 is switched to the B side, and a phase lock loop consisting of the VCO 5, the phase detection circuit 6, and the loop filter 7 forms a frequency demodulation circuit. Here, the loop filter 7 has resistors R1 and R2 between the input terminal ₇₁ and the ground terminal.
and capacitor C1 are connected in series, and resistor R
An output terminal 7 ₂ is connected to the connection point between the resistor R2 and the capacitor C1, and an output terminal 7 ₃ is connected to the connection point between the resistor R2 and the capacitor C1. Since the loop filter 7 has such a configuration, the input terminal 7 ₁ and the output terminal 7 ₂
As is clear from the comparison with FIG. 2a, a lag lead filter is formed between the input terminal ₇₁ and the output terminal ₇₃ . Thus, a lag filter is formed.

Therefore, the magnetic head 12 reproduces the
After the FM audio signal is amplified by the regenerative amplifier 10, it is supplied to the phase detector 6, and the detected output signal is supplied to the input terminal ₇₁ of the loop filter 7.
At the output terminal ₇₂ , a detection output signal is obtained by band-limiting with a lag lead filter, and this signal is supplied to the VCO 5 as a control signal. Further, a detection output signal is obtained by band-limiting with a lag filter and outputted from the IC pin terminal 105 as a demodulated audio signal to the output terminal ₇₃ .

Now, as a specific example, if the lag lead filter characteristics of the loop filter 7 are as shown in Figure 3a, and the DC loop gain of the phase lock loop is 640kHz with ₁ = 16kHz and ₂ = 80kHz, the loop If the signal obtained at the output terminal ₇₂ of the filter 7 is an FM demodulated audio signal, the frequency characteristics of this audio signal will be as shown in FIG. 5a. On the other hand, the frequency characteristics obtained at the output terminal ₇₃ of the loop filter 7 under the same conditions are shown in FIG. 5b.

That is, the DC loop gain of the phase lock loop is K, and the transfer function of the loop filter 7 is F.
(S), the transfer function H(S) of the phase lock loop is generally expressed as H(S)=KF(S)/S/1+KF(S)/S (3). By the way, the transfer function F(S) between the input terminal 7 ₁ and the output terminal 7 ₂ of the loop filter 7 is F(S)=R241/SC1/R1+R2+1/SC1 (4), so from the output terminal 7 ₂ The frequency characteristic of the obtained audio signal is 20kHz, as shown in Figure 5a.
The amplitude is raised in the frequency band from 150kHz to
Moreover, the attenuation at 1MHz is only about -20dB.

On the other hand, the transfer function F(S) between the input terminal 7 ₁ and the output terminal 7 ₃ is expressed as F(S)=1/SC1/R1+R2+1/SC1 (5). Therefore, the frequency characteristics of the audio signal obtained at this output terminal ₇₃ are flat up to 40 kHz, as shown in FIG. 5b, and the attenuation amount at 1 MHz is -40 dB.

Therefore, the output terminal 7 ₃ of the loop filter 7
The carrier component of the audio signal obtained is sufficiently attenuated, and furthermore, a control signal for the VCO 5 sufficient to obtain good pull-in characteristics of the phase lock loop can be obtained from the output terminal ₇₂ .

In this way, by providing the lag lead filter that constitutes the loop filter 7 with lag filter characteristics, it is possible to obtain an audio signal in which the carrier component is sufficiently attenuated without providing an extra LPF, and the audio of the reproduced audio can be without deteriorating
It is possible to reduce the number of IC pin terminals by reducing the number of parts.

FIG. 6 is a block diagram showing another embodiment of the frequency demodulation circuit according to the present invention, in which 13 is a voltage-current conversion circuit, 14 is an input terminal, and 108 and 109 are
This is an IC pin terminal, and parts corresponding to those in FIG. 1 are given the same reference numerals.

In Figure 6, when recording, IC pin terminal 1
The audio signal input from 01 is supplied to the dynamic range compression circuit 2 via the AGC circuit 1, and is compressed so that the dynamic range is reduced to 1/2. Thereafter, the audio signal is supplied to a voltage-current circuit 13 (hereinafter referred to as a VI conversion circuit) via the switch circuit 3, and is converted into a current signal. This current signal is supplied to VCO5 and FM modulated,
The obtained FM audio signal is supplied from the IC pin terminal 106 to the magnetic head 12 via the recording amplifier 9.
It is supplied to the magnetic tape 11.

The V-I conversion circuit 13 has an IC pin terminal 108.
A resistor R5 is connected through the IC pin terminal 109, and a variable resistor R4 and a capacitor C3 are connected through the IC pin terminal 109.
The other end of the variable resistor R4 is connected to the input terminal 14 to which a predetermined voltage is applied. Resistor R5 determines the center frequency of VCO5.
The variable resistor R4 determines the DC current of the current signal of the I conversion circuit 13, and the variable resistor R4
By changing the conversion gain of VCO5
This is for adjusting the deviation of FM modulation. Further, the capacitor C3 prevents oscillation of the V-I conversion circuit 13 and constitutes a HPF (high pass filter).

During reproduction, the FM audio signal reproduced from the magnetic tape 11 by the magnetic head 12 is amplified by the reproduction amplifier 10 and then supplied from the IC pin terminal 107 to the phase detection circuit 6. The phase detection circuit 6 is
Compare the phase between the FM audio signal and the output terminal of VCO5. A resistor R ₃ connected to the output stage of the phase detection circuit 6 and a capacitor C 2 connected via the IC pin terminal 102 act as a load for the phase detection circuit 6 . Therefore, the high frequency of the output signal of the phase detection circuit 6 is attenuated by a lag filter whose transfer function Fa(s) is Fa(s) = 1/SC ₂ /R ₃ +1/SC ₂ (6), and the signal is demodulated. The resulting audio signal is supplied to the dynamic expansion circuit 8 and the switch circuit 3. The dynamic expansion circuit 8 increases the dynamic range of this audio signal by 2.
By doubling the original dynamic range, the noise generated during the recording and playback process is reduced and the IC pin terminal 1
Output from 05.

On the other hand, the audio signal supplied to the switch circuit 3 is transferred to the V-I conversion circuit 13 through a resistor R4 and a capacitor.
The high frequency range is lifted by the lead filter consisting of C ₃ and the transfer function Fb (s) is Fb (s) = R ₄ + 1/SC ₃ /1/SC ₃ (7), which becomes a current signal and is sent to VCO5. Supplied. The characteristics of this V-I conversion circuit 13 and lead filter will be described in detail later in FIG. 7. In this way, the detection output signal from the phase detection circuit 6 supplied to the VCO 5 passes through the above-mentioned lag filter and lead filter, and therefore equalizes the transfer function Fc
(s) is Fc(s)=Fa(s)×Fb(s)=1+SC ₃ R ₄ /1+SC ₂ R ₃
...(8) This is the signal that has passed through the lag lead filter.

The relationship between the time constant τ _a =C ₂ R ₃ of the lag filter and the time constant τ _b =C ₃ R ₄ of the lead filter is set to τ _a >τ _b .

FIG. 7 is a circuit diagram showing a specific example of the switch circuit and V-I conversion circuit of FIG. 6, in which 15 is a negative feedback amplifier, 16 is a switch, 17 is a constant current source,
18 is an input terminal, 19 is an output terminal, and parts corresponding to those in FIG. 6 are given the same reference numerals.

In FIG. 7, during recording, the switch 16 is
Closed to the REC terminal side, transistor Q7, resistor R
11 and voltage source E1 form a constant current source. At this time, since transistor Q8 is off,
No emitter current flows through transistors Q5 and Q6, and transistors Q5 and Q6 are off.

Input terminal A (this is contact A of switch circuit 3)
The audio signal output from the dynamic compression circuit 2 (Fig. 6) is supplied from (Fig. 6) to the base of the transistor Q3. Since a negative total feedback amplifier is configured, a substantially equal signal of this audio signal is supplied to the base of transistor Q4.

Here, the base-emitter voltage of the transistor is added to the input terminal 14 by adding the DC voltage of the input terminal A or the input terminal B during playback (this is the contact B of the switch circuit 3 (Fig. 6)). When a voltage is applied and no audio signal is supplied to the input terminal A, the transistor Q forming the differential pair
The collector currents of Q3 and Q4 are made equal. Only the signal current corresponding to the input signal flows through the load resistor R4, and when the resistance value of the load resistor R4 is increased, the difference between the collector currents of the transistors Q3 and Q4 becomes smaller;
When the load resistance of transistor Q3,
The difference in collector current of Q4 becomes large. This difference in collector current corresponds to the audio signal supplied to input terminal A. Therefore, by changing the load resistance R4, the VI conversion gain of the VI conversion circuit can be adjusted.

By the way, transistors Q1 and Q12 and resistor R
13, R17 form a current mirror circuit, transistors Q2, Q10 and resistors R14. R15 also forms a current mirror circuit, and transistor Q1
3. Q15 and resistors R18 and R20 also form a current mirror circuit, and resistors R13 and R1
4, R15, R17, R18, and R20 have the same resistance value. Therefore, transistors Q1, Q3, Q1
Since the collector currents of transistors Q2, Q4, Q10, Q13, and Q15 are equal to each other, the current _I1 is the difference between the collector currents of transistors Q12 and Q15, and therefore, the collector current of transistors Q3, This is the difference between the collector currents of Q4, and is a signal current corresponding to the audio signal supplied from input terminal A.

On the other hand, a predetermined reference voltage is applied to the input terminal 18, and the collector current _I2 of the transistor Q19 is determined by the resistor R5 connected to the IC pin terminal 108. Also, transistor Q1
6, Q17, Q18 and resistors R21, R22 form a current mirror circuit, and the transistor Q
The collector current of transistor Q18 is equal to the collector current _I2 of transistor Q19. Therefore, the collector current of transistor Q20 becomes I ₁ +I ₂ . here,
Transistors Q20, Q21 and resistors R23, R2
4 forms a current mirror circuit, and as a result, the collector current I ₃ of transistor Q21
becomes I ₃ = I ₁ + I ₂ . The output terminal 19 is connected to the VCO 5 (FIG. 6), and _the sum current I ₃ of the current signal I 1 corresponding to the audio signal and the DC current I ₂ becomes the input current of the VCO 5. In this case, the direct current I ₂ determines the center frequency of the VCO 5, and the current signal I ₁ determines the deviation of the VCO 5.

For this reason, even if there are variations in the modulation sensitivity of the VCO 5, the signal current _I1 supplied to the VCO 5 can be changed by adjusting the resistor R4 in the V-I converter circuit. The deviation of the VCO 5 can be adjusted without changing the amplitude level of the audio signal supplied from the input terminal A.

In FIG. 7, during playback, the switch 16 is
Closed to the PB terminal side, transistor Q8, resistor R1
2 and the voltage source E1 form a constant current source. In this case, transistor Q7 is turned off, so transistors Q3 and Q4 are turned off.

Input terminal B is supplied with a reproduced audio signal from a lag filter consisting of resistor R3 and capacitor C2. In this case, as in the case of recording, the difference current between the collector currents of transistors Q5 and Q6 corresponds to the audio signal supplied from input terminal B.
This difference current is applied to the transistor Q2 as a signal current _I1 .
0 collector. And output terminal 1
A sum current I ₃ of the signal current I ₁ and the DC current I ₂ appears at 9, and is supplied to the VCO 5 (FIG. 6).

Capacitor C connected to IC pin terminal 109
3 is a negative feedback amplifier 1 in combination with variable resistor R4
A feedback rate of 5 is determined, and the high frequency range of the feedback signal is suppressed. This feedback factor β(s) can be expressed as β(s) = 1/SC3/R ₄ +1/SC3 (9), so the transfer function G of the negative feedback amplifier 15 is
(s) is G(s)=1/β(s)=R ₄ +1/SC3/1/SC3...(10), and the lead whose low frequency gain is 1 as shown in equation (7) above. This is a characteristic of the filter. Therefore,
The control signal input to the VCO 5 converts the detection output signal of the phase detection circuit 6 into the capacitor C2 and the resistor R3.
This can be obtained by passing it through a lag filter consisting of the above and the above-mentioned lead filter.

Furthermore, as described above, this V-I converter circuit sets the deviation at the VCO 5 to a predetermined value without changing the amplitude level of the audio signal supplied to the input terminal A during recording. Since the amplitude level of the signal current is set, during reproduction, an audio signal having the same amplitude level as the audio signal supplied to input terminal A during recording is supplied to input terminal B.

FIG. 8 is a circuit diagram showing a specific example of the VCO5 in FIG. 6, 20 to 23 are constant current sources, and parts corresponding to those in FIGS. 6 and 7 are given the same reference numerals. .

This VCO5 is an emitter-coupled unstable multivibrator. The oscillation period T of this multivibrator is such that the charging voltage of the capacitor C10 becomes 2・R100/R100+R101・V _cc due to the current _I3 supplied from the VI conversion circuit shown in FIG. 7 through the terminal 19. Since this is twice the charging time required, T=4・C10・k・V _cc /I ₃ [However, k=R100/R100+R101]. Therefore, the oscillation frequency is =I ₃ /4・C10・k・V _cc , and the amount of change in the oscillation frequency with respect to the change in current I ₃ (that is, the modulation sensitivity of VCO 5) is ∂/∂I ₃ =1 _/4.C10.k.Vcc , the oscillation frequency is controlled in accordance with the audio signal converted into current by the VI conversion circuit shown in FIG. At this time, load resistance R4 in FIG.
By changing the resistance value, the V-I conversion gain of the V-I conversion circuit can be changed, so even if there are variations in the modulation sensitivity of VCO5,
Deviation of VCO5 can be kept constant.

As described above, in this embodiment, by providing the V-I conversion circuit 13, the deviation of the VCO 5 can be adjusted without changing the amplitude level of the audio signal to be recorded.
Adjustment of the oscillation center frequency of the VCO 5 and deviation adjustment can be performed independently. For this reason, optimal phase lock loop characteristics can be obtained and a high quality FM demodulated audio signal can be obtained.

Further, a lag lead filter is constituted by a lead filter (HPF) provided in the V-I conversion circuit 13 and a lag filter including a resistor R3 and a capacitor C2 provided at the next stage of the phase detection circuit 6, and this is a phase filter. Since it forms the loop filter of the lock loop, from this lagged filter
A control signal for VCO5 was obtained, and unnecessary high-frequency components were sufficiently attenuated from the lag filter mentioned above.
An FM demodulated audio signal is obtained. Therefore, a separate LPF for obtaining such an FM demodulated audio signal is not required, and the number of parts can be reduced.

〔Effect of the invention〕

As explained above, according to the present invention, the loop filter forming the phase lock loop is
It can also be used as an LPF to sufficiently attenuate unnecessary high-frequency components of the FM demodulated audio signal, reducing the number of parts used, and especially reducing the need for external parts when converting to an IC.
It is possible to reduce the number of IC pin terminals,
A frequency demodulation circuit with excellent functionality can be provided at low cost despite the drawbacks of the prior art described above.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an example of a conventional frequency demodulation circuit, Fig. 2a is a circuit diagram showing a lag-lead filter, Fig. 2b is a circuit diagram showing a lag-lead filter, and Figs. Characteristic diagram, 4th
5 is a block diagram showing an embodiment of the frequency demodulation circuit according to the present invention; FIGS. 5a and 5b are characteristic diagrams showing the lag-lead filter characteristics of the loop filter in FIG. 4 and the frequency characteristics of the audio signal due to the lag filter characteristics, respectively; FIG. 6 is a block diagram showing another embodiment of the frequency demodulation circuit according to the present invention, FIG. 7 is a circuit diagram showing a specific example of the switch circuit and voltage-current conversion circuit of FIG. 6, and FIG. FIG. 7 is a circuit diagram showing a specific example of the voltage controlled oscillator shown in FIG. 6; 3...Switch circuit, 5...Voltage controlled oscillator, 6...Phase detection circuit, 7...Loop filter, ₇₁ ...Input terminal, ₇₂ , ₇₃ ...Output terminal,
13... Voltage-current conversion circuit, 101... IC pin terminal to which an audio signal is input, 105... IC pin terminal to which a demodulated audio signal is output.

Claims (1)

[Claims] 1. Formed by a phase lock loop consisting of a phase detection circuit, a loop filter, and a voltage controlled oscillator, a phase detection output signal of a frequency-modulated information signal passes through the loop filter to control the voltage. In the frequency demodulation circuit configured to be supplied to an oscillator, the loop filter has a lag lead filter characteristic between an input terminal to which the phase detection output signal is supplied and a first output terminal; and a second output terminal, the control voltage of the voltage controlled oscillator is output to the first output terminal, and the demodulated information signal is output to the second output terminal. A frequency demodulation circuit characterized in that it is configured to output. 2. A first resistor, a second resistor, and a capacitor are connected in series between the input terminal and the ground terminal, and the first output terminal is connected to the connection point of the first and second resistors, and the second resistor is connected in series between the input terminal and the ground terminal. 2. The frequency demodulation circuit according to claim 1, wherein output terminals of the first resistor and the capacitor are respectively connected to connection points of the first resistor and the capacitor. 3. In the loop filter, a lag filter and a reed filter are connected in series between the input terminal and the first output terminal, the second output terminal is connected to the output terminal of the lag filter, and the lag filter and the reed filter are connected in series. They have transfer functions of Fa(s)=1/1+τ _a s and Fb(s)=1+τ _b s , and the time constant τ _a of the above lag filter
2. The frequency demodulation circuit according to claim 1, wherein the time constant τ _b of the lead filter satisfies τ _a >τ _b .