JPH0691371B2 - Signal processor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal processing device for amplitude-modulating a signal and passing it through a bandpass filter.

2. Description of the Related Art In recent years, in video signal processing devices and the like, higher performance of devices has been demanded, and bandpass filters and the like also require very high performance bandpass filters.

In general, a bandpass filter after amplitude modulation requires a wideband bandpass filter when the band of the modulated wave is wide. A wideband bandpass filter is designed by combining a lowpass filter and a highpass filter. The group delay characteristic of such a wideband bandpass filter exhibits a downwardly convex characteristic near the center frequency of the bandpass filter. Conventionally, a bandpass filter after amplitude modulation has been configured by using several stages of phase compensation circuits in order to correct the above-described downward convex group delay characteristic.

FIG. 5 shows a block diagram of a conventional bandpass filter. 51
Is a modulated wave input terminal after amplitude modulation, 52 is a modulated wave output terminal after passing through a band pass filter, 53 is a high pass filter (abbreviated as HPF), 54, 55 and 56 are group delays of the configuration shown in FIG. Compensation circuit, 57 is a low-pass filter (abbreviated as LPF)
Is.

As described above, when the bandpass filter is composed of HPF and LPF, the amplitude characteristic becomes flat and the group delay characteristic shows a downward convex characteristic (see FIG. 2). A group delay compensating circuit as shown in FIG. 6 is required to compensate for the downward convex characteristic, and in the case of a wide band pass filter, about three stages are required as shown in FIG. In FIG. 6, L61 is a coil having a variable peak frequency of the group delay amount, and the peak frequency is Wc.
The characteristic diagram of the group delay compensating circuit in such a case is as shown in FIG.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, a bandpass filter of this type has a group delay compensating circuit 54 to 56, as shown in the characteristic diagram of the bandpass filter of FIG. Ripple occurs in both characteristics. Further, in the group delay compensation circuit shown in FIG. 6, it is difficult to adjust the number of turns of the coil L61. As described above, there is a problem in that adjustment is required to correct ripples in the band due to the phase compensation circuit and variations in constants of the phase compensation circuit, which is expensive.
Furthermore, since the delay time of the bandpass filter itself becomes longer in proportion to the number of stages that use the phase compensation circuit, use a delay circuit with the same delay time for other systems when adjusting the time with the signals of other systems. There was a problem that it became necessary to do.

The present invention solves such conventional problems and provides an inexpensive and high-performance band limiting system after amplitude modulation.

Means for Solving the Problems The present invention relates to a system in which a signal is amplitude-modulated and a modulated wave is passed through a band-pass filter, a group delay correction circuit for correcting a phase of a modulated wave and the group delay correction circuit. An amplitude modulator that amplitude-modulates the output of as a modulated wave,
A modulation output of the amplitude modulator is provided, and a bandpass filter having a line-symmetrical characteristic about a carrier frequency as an axis,
The group delay correction circuit is characterized in that the phase of the modulated wave is changed so as to cancel the group delay characteristic of the bandpass filter.

Action By setting the group delay characteristic of the modulated wave to the direction that cancels the group delay characteristic of the bandpass filter (negative direction with respect to the frequency) by the group delay variable circuit, the group delay characteristic of the modulated wave is the carrier frequency. Since it has a convex shape, the convex group delay characteristic can be corrected with the carrier frequency of the bandpass filter as an axis, and a satisfactory band-limited system after amplitude modulation can be obtained.

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

FIG. 1 is a block diagram of a quadrature two-phase amplitude modulation system according to an embodiment of the present invention. In Figure 1, 1,
Reference numeral 2 is an input terminal for the first and second modulated waves, and 3 and 4 are input terminals for the first and second carrier waves which are orthogonal in phase. 5,6
Are group delay correction circuits for respectively correcting the phases of the first and second modulated waves, 7 is a multiplier, 8 is a multiplier, 9 is an adder for adding the outputs of the multipliers 7 and 8, and 10 is a band pass filter. The filter 11 is an output terminal.

The operation of the amplitude modulation system configured as described above will be described below.

The first and second modulated waves supplied to the input terminals 1 and 2 are respectively input to the group delay correction circuit 5 and 6 having the characteristics shown in FIG. 3, the group delay is varied, the multiplier 7 and the multiplication are performed. Input terminal on device 8
The carrier waves given by 3 and 4 are respectively multiplied and added by the adder 9. The added signals are input to the bandpass filter 10 having the characteristics shown in FIG.
Get 10. Here, the group delay variable amount in the group delay correction circuits 5 and 6 is changed according to the characteristics of the bandpass filter 10. For example, if the delay amount of the (Wc + Wo) frequency is delayed with respect to the Wc frequency in the bandpass filter, the delay amount at the Wo frequency is set to be faster than the delay amount at the DC (direct current component) by the group delay correction circuit. The group delay correction circuits 5 and 6 may be designed (see FIGS. 2 and 3).

FIG. 4 shows an embodiment of the group delay correction circuit. 40 is a modulated wave input terminal, 41 is a modulated wave output terminal, Q42 is a group delay variable transistor, C44 and R45 are group delay variable capacitors and resistors, C46 is an amplitude characteristic compensation capacitor, and Q43 is an emitter follower. is there. In this circuit, transistor Q42 has C
Since the impedance of 44 changes according to the frequency, a modulated wave having the frequency characteristic shown in FIG. 3 can be obtained from the output end 41. In this way, the group delay correction circuit can be configured with a relatively simple circuit, and the amount of variable group delay can be determined by C44 and R45. Ripple seen in the amplitude characteristic and the frequency characteristic as in the conventional example can be reduced, and problems such as the number of turns of the coil do not occur.

As described above, in this embodiment, it is possible to configure a band limiting system after amplitude modulation that is easier and has higher performance than the conventional one.

EFFECTS OF THE INVENTION As described above, the present invention limits the band of the group delay characteristic of the modulated wave according to the group delay characteristic of the bandpass filter, the amplitude modulator and the output of the amplitude modulator. , By providing a symmetrical band-pass filter around the carrier frequency, it is a relatively simple and inexpensive band-pass filter that does not require a phase compensation circuit, and has a high-performance amplitude with little ripple in both amplitude and group delay characteristics. It is possible to realize a band limiting system after modulation.

[Brief description of drawings]

FIG. 1 is a block diagram of an amplitude modulation system according to an embodiment of the present invention, FIG. 2 is a frequency characteristic diagram of a bandpass filter required in the present invention, and FIG. 3 is a frequency characteristic of a group delay correction circuit used in the present invention. FIG. 4 is a block diagram showing an embodiment of a group delay correction circuit, FIG. 5 is a block diagram of a conventional bandpass filter, and FIG. 6 is an example of a group delay compensation circuit used in a conventional bandpass filter. FIG. 7 is a frequency characteristic diagram of the group delay compensation circuit of FIG. 6, and FIG. 8 is a frequency characteristic diagram of a conventional bandpass filter. 1 ... Modulated wave input terminal, 2 ... Modulated wave input terminal, 3 ... Carrier wave input terminal, 4 ... Carrier wave input terminal, 5 ... Group delay correction circuit, 6 ... Group delay correction circuit, 7 ... Multiplier, 8 ... Multiplier Bowl, 9
… Adder, 10… Band pass filter, 11… Output terminal.

Claims (1)

[Claims] 1. In a system for amplitude-modulating a signal and passing the modulated wave through a bandpass filter, a group delay correction circuit for giving a modulated wave and correcting the phase thereof, and an output of the group delay correction circuit are modulated. An amplitude modulator that performs amplitude modulation as a wave, and a bandpass filter that is provided with a modulation output of the amplitude modulator and has line symmetry about a carrier frequency as an axis, and the group delay correction circuit has a phase of a modulated wave. Is changed so as to cancel the group delay characteristic of the bandpass filter.