JPH0618292B2 - FM demodulator - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM demodulator suitable for use in an FM demodulation circuit of an SHF receiver for satellite broadcasting, for example.

Background Art and its Problems As is well known, in an SHF receiver, the BS tuner is
It functions to select a desired channel from the first intermediate frequency signal from the BS converter, demodulate and reproduce a video / audio signal. This BS tuner is constructed, for example, as shown in FIG.

In the figure, the input terminal (1) is supplied with a first intermediate frequency signal of, for example, 1035 to 1335 MHz from a BS converter, and this intermediate frequency signal is passed through an intermediate frequency amplifier (2) to a mixer (3).
Is supplied to. An oscillation signal is supplied from the local oscillation circuit (4) to the mixer (3), and a second intermediate frequency signal in the 400 MHz band is obtained from the output side thereof. Channel tuning is performed by changing the frequency of the oscillation signal.

Further, the second intermediate frequency signal (hereinafter referred to as FM signal S _FM ) obtained from the mixer (3) is obtained by using a series circuit of a band pass filter (5) having a pass band of 27 MHz, an amplifier (6) and a limiter (7). It is supplied to the FM demodulation circuit (8) via the. The demodulation output S obtained from the FM demodulation circuit (8) is supplied to the video signal processing section (9) and the audio signal demodulation section (10).

The video signal processing unit (9) is composed of a de-emphasis circuit that restores the pre-emphasized video signal to the original waveform, an energy diffusion signal removal circuit, and a video amplification circuit, and the video signal S _V is obtained from this.

On the other hand, the audio signal demodulation unit (10) extracts the audio subcarrier from the demodulation output S and demodulates it to obtain a PCM audio signal.
It is composed of a K demodulation circuit and a PCM demodulation circuit that demodulates a PCM audio signal and extracts an audio signal from a digital signal. From this, a left audio signal S _L and a right audio signal S _R are obtained.

Further, in FIG. 1, (11) is a stabilized power supply circuit, which applies a predetermined voltage, for example +15 V, to the terminal (1) as a power supply of the BS converter.

The BS tuner FM demodulation circuit as shown in FIG.
In (8), as mentioned above, the band is 27 MHz in the high frequency band of 400 MHz.
And a wide FM signal is supplied. Therefore, it is desired that the FM demodulator used in the FM demodulation circuit (8) is capable of wideband demodulation in a high frequency range and has high sensitivity.

Conventionally, as an FM demodulator, for example, 2 tuning type, phase shifting type, P
The LL type is used.

FIG. 2 shows a two-tuning type, (13) and (14) are tuning circuits having the same characteristics, and each tuning circuit (13) and (14) is an FM signal supplied to the input terminal (12). It is designed to be tuned to a frequency which is vertically symmetrical with respect to the center frequency of S _FM . And the output (A of these tuning circuits (13) and (14)
The M detection output) is supplied to the subtractor (17) via the diodes (15) and (16), respectively, so that it has an S-shaped characteristic as a whole. Therefore, the demodulation output S is obtained at the output terminal (18) derived from the subtractor (17).

Also, FIG. 3 shows a phase shift type, (20) is a phase shift circuit for providing a phase difference corresponding to the frequency of the FM signal S _FM supplied to the input terminal (19), and (21) is a phase comparator. . FM signal S _FM
Is supplied to the phase comparator (21) directly and via the phase shift circuit (20). Therefore, a comparison error output, that is, a demodulation output S is obtained at the output terminal (22) derived from the phase comparator (21).

Further, FIG. 4 shows a PLL type, which is composed of a phase comparator (24), a low-pass filter (25) and a voltage controlled oscillator (26).
The FM signal S _FM supplied to the input terminal (23) is compared with the oscillation frequency of the voltage controlled oscillator (26) in the phase comparator (24), and the comparison error signal from this is passed through the low pass filter (25) to the voltage controlled oscillator ( Returned to 26). Therefore, the demodulation output S is obtained at the output terminal (27) derived from the low-pass filter (25).

In these FM demodulators, the two-tuning type and the phase-shifting type can be realized up to a relatively high band and wideband demodulation is possible, but the threshold level V _th cannot be expanded and the sensitivity is not good. That is, Fig. 5 shows the C / N in the two tuning type and the phase shifting type.
The relationship between the (FM signal / noise ratio) and the S / N (demodulation output / noise ratio) is shown, but the threshold level V _th exists relatively high.

On the other hand, the PLL type can extend the _threshold level V _th . Moreover, although broadband demodulation is possible in the high frequency band (UHF band), since the circuit includes the voltage controlled oscillator (26), extra phase jitter increases and the residual S / N deteriorates. Further, in the case of this PLL type, its detection characteristic is as shown in FIG. 6A, and when an overmodulated FM signal S _FM as shown in FIG. 6B is input, demodulation as shown in FIG. The output S is obtained, the lock is displaced in the overmodulation portion, and the distortion characteristic is rapidly deteriorated. Incidentally, in the case of satellite broadcasting, since the frequency deviation is larger than the transmission bandwidth, the distortion characteristic of video and audio is deteriorated as compared with other systems.

By the way, as the FM demodulator, a phase shift tracking type as shown in FIG. 7 has been proposed. In the figure, the FM signal S _FM supplied to the input terminal (28) is supplied to the phase comparator (31) through the 90 ° phase shifter (29) and the limiter (30) and the FM signal S _FM is supplied. Is supplied to the phase comparator (31) via the tracking filter (32) and the limiter (33). The filter (32) is composed of a single tuning circuit having a single-peak characteristic, and its tuning frequency can be changed.

The comparison error signal from the phase comparator (31) is supplied to the output terminal (36) via the low-pass filter (34) and the amplifier (35), and the output of the amplifier (35) is output from the filter (3) described above.
It is supplied to 2) as a control voltage. And filter (32)
The tuning frequency of is controlled to match the frequency of the input FM signal S _FM . When the tuning frequency of the filter (32) completely matches the frequency of the FM signal S _FM , this filter (32)
There is no phase difference in the passing output of. However, they do not completely match due to the control delay, and a phase difference according to the frequency of the FM signal S _FM is added to the passing output of this filter (32). Therefore, the comparison error signal from the phase comparator (31) corresponds to the FM signal S _FM, and accordingly, the demodulation output S is obtained at the output terminal (36).

In the case of the example of FIG. 7, wide band demodulation is possible by the tracking operation of the tracking filter (32), and noise and interfering waves are limited by the band limitation by the tracking filter (32). High sensitivity with extended V _th can be obtained. Further, since the voltage controlled oscillator is not used as compared with the PLL type (see FIG. 4), there is no possibility that the residual S / N is deteriorated due to the phase jitter of this type. Further, in the case of the phase tracking type as shown in FIG. 7, the detection characteristic is as shown in FIG. 8A, and when an overmodulated FM signal S _FM as shown in FIG. The demodulation output S as shown in FIG. 7C is obtained, and the output waveform is clipped in the overmodulation portion, and there is no sharp deterioration in the distortion characteristic even in the overmodulation signal as compared with the PLL type.

As described above, the phase-tracking type shown in FIG. 7 is superior to the above-mentioned PLL type in terms of distortion characteristics. However, this phase tracking type has a tracking filter (32) with high selectivity and good selectivity (high Q).
Is difficult to realize, and is conventionally used only in a band of 10 MHz or less, for example, and is not used in the above-mentioned BS tuner FM demodulation circuit (8) which requires a band of 400 MHz.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, the present invention provides a phase-tracking type in which good identification can be obtained even in a high frequency range.

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a phase comparator having a low input impedance through a series circuit of a coil in which an FM signal is composed of a transistor and a parallel resonance circuit and a 90 ° phase shifter. The FM signal is supplied to the input side and the other input side of the phase comparator, and a demodulation output is obtained from the output side of the phase comparator, and the demodulation output causes the resonance frequency of the parallel resonance circuit. Is controlled, and in particular, the coil forming the 90 ° phase shifter is directly connected to the collector of the transistor.

Therefore, since the coil forming the 90 ° phase shifter is connected in series to the output side of the filter, the impedance seen from the filter to the phase comparator side becomes high when the FM signal is in the high frequency band, and the impedance of the filter becomes high. Q can be set only by the damping resistor that constitutes the resonance circuit, and a filter having a good selectivity characteristic even in a high range can be realized. Therefore, it is suitable for use in the FM demodulation circuit of the BS tuner. Also, the filter and 90 ° phase shifter are connected in series, and the filter and 90 °
There is no need to interpose a coupling circuit with the phase shifter, and a simple structure can be achieved.

Embodiment An embodiment of the present invention will be described below with reference to FIG.

In the figure, for example 400 MHz supplied to the input terminal (37)
The band FM signal S _FM is supplied to the tracking filter (39) through the limiter (38). This filter (39) is composed of a single tuning amplifier circuit having a steep single peak characteristic, and its tuning frequency can be changed. Further, the phase characteristic changes linearly near the tuning frequency and becomes constant at ± 90 ° outside the band, and this relationship is maintained even if the tuning frequency changes. Further, as the tracking characteristic, the tuning frequency vs. control voltage characteristic changes linearly, which makes the control sensitivity large.

The passing output S _FM ′ of the filter (39) is supplied to the 90 ° phase shifter (40), and the signal S _FM ″ 90 ° phase shifted from this is supplied to one input side of the phase comparator (41). In this case, the phase comparator (41) has a low input impedance in order to drive it sufficiently in the high range.

In addition, the FM signal S _FM passed through the limiter (38) is an attenuator.
It is supplied to the other input side of the phase comparator (41) via (42). In this case, when the signal S _FM ″ supplied to one input side of the phase comparator (41) has a level of 0 dB, the level of the FM signal S _FM supplied to the other input side is, for example, −20 dB.
It is said that

The comparison error signal from the phase comparator (41) is supplied to the output terminal (46) via the low pass filter (43), the DC amplifier (44) and the buffer amplifier (45).

The output of the DC amplifier (44) is supplied as a control voltage V _C to the above-mentioned filter (39) via a buffer amplifier (47). The tuning frequency of the filter (39) is the FM signal S.
Controlled to match the _FM frequency. In this case, when the tuning frequency is exactly the same as the frequency of the FM signal S _FM , there is no phase difference in the passing output S _FM ′ of this filter (39). However, they do not completely match due to the control delay, and a phase difference corresponding to the frequency of the FM signal S _FM is added to the passing output S _FM ′ of this filter (39). Therefore, the phase comparator
The comparison error signal from (41) corresponds to the FM signal S _FM, and accordingly, the demodulation output S is obtained at the output terminal (46).

In this example, the tracking filter (39), 90 ° phaser
The (40) and the phase comparator (41) are configured as shown in FIG. In FIG. 10, parts corresponding to those in FIG. 9 are designated by the same reference numerals.

As shown in the figure, the filter (39) is composed of a single tuning amplifier circuit composed of an npn-type transistor (48) and a parallel resonance circuit (49), and a terminal (50) connects a limiter () to the base of the transistor (48). The FM signal S _FM via 38) is supplied. Also,
The parallel resonance circuit (49) is composed of a coil (51), a resistor (52) and a capacitor (53) {capacitors (53a), (53d), variable capacitance diodes (53b), (53c)}. Then, the control voltage V _C is supplied from the terminal (54) to the anodes of the variable capacitance diodes (53b) and (53c) via the buffer amplifier (47), and the capacitances of the variable capacitance diodes (53b) and (53c) are changed. As a result, the resonance frequency of the resonance circuit (49) is changed, and the tuning frequency of the filter (39) is changed so as to match the frequency of the FM signal S _FM as described above.

Further, (55) is a coil which constitutes the 90 ° phase shifter (40),
The passing output S _FM ′ of the filter (39) passes through this coil (55) and is supplied to one input side of the phase comparator (41). On the other hand, the other input side of the phase shift comparator (41) is supplied with the FM signal from the terminal (56) via the attenuator (42). Then, a comparison error signal is obtained between the terminals (57a) and (57b).

FIG. 11 shows the tracking filter (3
9), an equivalent circuit of the phase shifter (40) and the phase comparator (41), where R is the resistance value of the resistor (52) and L ₁ is the coil
Inductance of (51), C is the capacity of capacitor (53), L
₂ is the inductance of the coil (55), and R ₀ is the impedance as seen from the coil (55) on the phase comparator (41) side.

From this equivalent circuit, the transfer function is Given in. From this, the tuning frequency ω ₀ is given by the following equation.

From equations (1) and (2), the transfer function at the frequency where the filter (39) is tuned is Next, it can be seen that occurs 90 ° phase difference in the output current i ₂ with respect to the input current i _{s (S FM)} at the tuning frequency (S _FM ").

In this example, although the input impedance of the phase comparator (41) is low (for example, R ₀ = 50Ω) as described above,
In the high frequency range of 00 MHz, the impedance of the coil (55) is high (L ₂ is, for example, 300 nH), and the impedance seen from the filter (39) to the phase comparator (41) side is high. Therefore, the Q of the filter 39, that is, the selectivity characteristic of the filter 39 can be set only by the damping resistor 52 of the resonance circuit 49.

Thus, according to this example, the selectivity characteristic of the tracking filter (39) can be set only by the damping resistor (52) of the resonance circuit (49), and the filter having a good selectivity characteristic even in a high frequency range. Is realized.

Since this example is basically the same as the phase shift tracking type shown in FIG. 7, it is of course possible to obtain the same effects as the example of FIG.

Therefore, the FM demodulator of this example is suitable for use in the FM demodulation circuit of the BS tuner. Further, according to this example, the filter (39) and the 90 ° phase shifter (40) are connected in series, so that they can be integrally configured and a simple configuration can be achieved. it can.

Incidentally, FIG. 12 shows the relationship between the received C / N and the voice bit error rate. In FIG. 12, the curve a is a double tuning type,
The curve b is for the PLL type, and the curve c is for this example, and this example has a smaller voice bit error rate than the PLL type.

According to the present invention described above, since the passing output of the tracking filter is supplied to the phase comparator through the coil constituting the 90 ° phase shifter, the filter is output when the FM modulation signal is in the high frequency band. The impedance seen from the phase comparator side is high, and the input impedance of the phase comparator is low in order to drive sufficiently in the high frequency range. However, this low input impedance does not affect the resonance circuit of the filter, and the filter Q Can be set only by the damping resistance, and a filter with good selectivity characteristics can be realized even in a high frequency range. Therefore, the F according to the present invention
The M demodulator is suitable for use in the above-mentioned BS tuner FM demodulation circuit. Further, since the filter and the 90 ° phase shifter are connected in series, there is no need to interpose a coupling circuit between the filter and the 90 ° phase shifter, and the structure can be simplified.

[Brief description of drawings]

FIG. 1 is a block diagram of a BS tuner, FIGS. 2 to 8 are diagrams for explaining a conventional FM demodulator, FIG. 9 is a block diagram showing an embodiment of the present invention, and FIG. FIG. 11 is a specific circuit diagram of the main part thereof, FIG. 11 is an equivalent circuit thereof, and FIG. 12 is a diagram for explaining one embodiment. (37) is an input terminal, (39) is a tracking filter, (40) is
90 ° phase shifter, (41) phase comparator, (46) output terminal, (49)
Is a parallel resonance circuit, and (55) is a coil.

Claims (1)

[Claims] 1. A filter comprising an FM signal and a parallel resonance circuit having a coil and a variable capacitance diode connected to a transistor and a collector of the transistor, and a 90 ° phase shifter connected to the collector of the transistor. The FM is supplied to one input side of a phase comparator with low input impedance through a series circuit with a coil.
The signal is supplied to the other input side of the phase comparator, the demodulation output is obtained from the output side of the phase comparison, the capacitance of the variable capacitance diode is controlled by this demodulation output, and the resonance frequency of the parallel resonant circuit is obtained. An FM demodulator, characterized in that