JPS62139418A - Frequency converting circuit - Google Patents

Abstract

PURPOSE:To improve disturbance characteristics of local disturbance, adjacent disturbance, etc., without influence to desired signal band by inserting a trap for attenuating a video frequency band after a frequency converting element. CONSTITUTION:When a required input signal, a local signal and a disturbance signal adjacent to the local signal are simultaneously inputted to a frequency conversion element 11, a signals having video frequency band and several frequency components higher than an intermediate frequency band are outputted simultaneously with the required intermediate frequency signal. In this case, frequency components higher than the intermediate frequency band are attenuated by low pass characteristics formed by capacitors 12, 13 and a coil 14 and an unnecessary component is removed from a video band signal by a trap consisting of a coil 16 and a capacitor 17.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a frequency conversion circuit for a tuner, and is particularly effective in suppressing interference caused by unnecessary spurious components in the intermediate frequency band, such as local interference and adjacent interference. .

[Background of the invention]

Conventionally, in order to deal with this type of interference, a method has been adopted in which signals other than the desired signal are blocked before the first frequency conversion circuit, and third-order distortion is suppressed as much as possible in the frequency conversion circuit.

However, as the degree of attenuation of signals other than the desired signal increases,
Attenuation also occurs in the desired band, and it is extremely difficult to attenuate only the interfering signal, especially when the desired frequency is high or when the interfering signal is close to the desired signal.

Regarding high frequency interference from local stations, see Japanese Patent Application Laid-Open No. 58-31622.
Although it is shown in the issue, there is no discussion of spurious components generated by interference signals near the desired frequency or near the local frequency that are transmitted to the frequency conversion circuit. Alternatively, a common method is to increase the degree of attenuation outside the desired signal band using an interstage circuit.

[Purpose of the invention]

An object of the present invention is to provide a tuner that improves interference characteristics such as local interference and adjacent interference without affecting the desired band.

[Summary of the invention]

In the present invention, when the interfering signal injected into the frequency conversion element is close to the desired signal or the local signal, the difference component between the interfering signal frequency and the desired signal frequency or the interfering signal frequency and the local signal frequency is generated by second-order distortion. A spurious component of the difference component of In order to prevent such components from being generated, in the above process, signals in the video frequency band are blocked at a stage subsequent to the frequency converting element in order to reduce components harmful to the screen.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in more detail with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention.

In FIG. 1, 1 is a high frequency circuit consisting of a high frequency amplification circuit, a high frequency tuning circuit, etc., 2 is a local oscillation circuit, 3 is a frequency conversion circuit, and 4 is an amplifier circuit connected after the frequency conversion circuit 3.

11 is a frequency conversion element; 12, 13, and 14 are capacitors and coils forming a low-pass type intermediate frequency tuning circuit; and 15 is an amplification element for performing amplification in the amplifier circuit. The series circuit consisting of the coil 16 and capacitor 17 is
It is a trap with resonance in the video frequency band.

FIG. 2 shows the degree of attenuation of the intermediate frequency tuning circuit used in the frequency conversion circuit in FIG. 1 without this trap. Here, 21 indicates a video frequency band, and 22 indicates an intermediate frequency band.

Desired input signal (fo, +:p) to frequency conversion element 11
When a local signal (fe) and a disturbance input signal (f+ro, i) close to the local signal are input simultaneously, the frequency conversion element converts the desired intermediate frequency signal (f+
3. rp) and at the same time in the video frequency band (fe-f
uD-1), and the sum and difference of the other signals injected into the frequency conversion element.

Signals with frequency components higher than several intermediate frequency bands are output.

The signals are connected to capacitors 12, 13 . Selection is performed by an intermediate frequency tuning circuit formed by the coil 14, but in the case of this circuit, since it is a low-pass type, the attenuation characteristic shown in FIG. 2 is used.

Frequency components higher than the intermediate frequency band can be easily attenuated to a level that causes no problem.

However, as shown in the attenuation characteristics shown in Fig. 2, the video band signal is transmitted as it is to the rear stage 4, and due to the distortion of the rear stage amplification element, the desired intermediate frequency signal (fo, IF) and the video frequency band (fe-fuD) are , +) from fD, IF
±(fe-fuD-+) is generated and becomes a harmful intermediate frequency interference signal on one screen.

Also, the interference input signal (fuD) input to the frequency conversion element
2) is close to the desired input signal (fD, Rp), similarly fD・IF±(fo-np-fuo, 2)
An intermediate frequency interference signal is generated.

A trap consisting of a coil 16 and a capacitor 17 removes unnecessary components occurring in the video frequency band. By inserting this trap, the signal in the video frequency band is attenuated before being transmitted to the subsequent stage, so that the above process no longer holds true, and the interference characteristics can be greatly improved. This trap becomes inductive in the intermediate frequency band and causes parallel resonance with the capacitor 13, but by placing one resonance point between the intermediate frequency band and the video frequency band, the characteristics can be reduced.

Traps can be inserted without any effect. In addition, in this embodiment, the coil 16 and the capacitor 17 can be configured with a coil for blocking intermediate frequency components during power supply and a bypass capacitor, respectively, and this circuit can be used without increasing costs. can be configured.

The trap can be formed by a parallel resonant circuit, as shown in another embodiment of FIG. 3, or even before a tuned circuit, as shown in another embodiment of FIG. As shown in another embodiment of FIG. 5, the present invention is effective not only in a low-pass type intermediate frequency tuning circuit but also in a ground type tuning circuit.

〔Effect of the invention〕

According to the present invention, it is possible to easily suppress interference components caused by the secondary type in local interference characteristics, adjacent interference characteristics, etc., without changing the characteristics of the high frequency band and the characteristics of the intermediate frequency band. The interference characteristics can be greatly improved, and the design of the circuit connected to the subsequent stage becomes extremely easy.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIG. 2 is a characteristic diagram showing the pass attenuation characteristics of an intermediate frequency tuning circuit of a frequency conversion circuit in a conventional tuner, and FIGS. 3 to 5 are FIG. 3 is a circuit diagram showing another embodiment of the present invention. ■...High frequency circuit, 2...Local oscillation circuit, 3...
Frequency conversion circuit, 4...-amplification circuit, 11... Frequency conversion element, 12.13... Intermediate frequency tuning capacitor,
14... Intermediate frequency tuning coil, 15... Amplifying element. 16... Coil for trap formation, 17... Capacitor for trap formation, 21... Video frequency band, 22.
...Intermediate frequency band. Part II! 1

Claims (1)

[Claims] A frequency conversion circuit comprising a frequency conversion element and an intermediate frequency tuning circuit, characterized in that a trap for the purpose of attenuating a video frequency band is inserted after the frequency conversion element.