CN201904766U - Intermediate frequency selecting circuit with function of suppressing adjacent channel interference - Google Patents

Abstract

The utility model provides an intermediate frequency selecting circuit with the function of suppressing adjacent channel interference, which comprises a frequency selecting circuit, two intermediate frequency signal input ends and two intermediate frequency signal output ends, wherein intermediate frequency signals inputted from the intermediate frequency signal input ends are selected by the frequency selecting circuit and then are outputted to the intermediate frequency signal output ends; the intermediate frequency selecting circuit further comprises a resonant circuit; and the resonant circuit is connected between the frequency selecting circuit and any one of the two intermediate frequency signal output ends. The circuit is supplied with an absorbing point outside the frequency band by arranging the resonant circuit at the output end of the frequency selecting circuit, thereby enhancing the function of absorbing a signal outside the frequency band of the intermediate frequency signal and promoting the capability of suppressing adjacent channel interference outside the frequency band.

Description

Have and face the frequency-selecting of intermediate frequency circuit that disturbs inhibit feature frequently

Technical field

The utility model relates to the electronic tuning unit technology, relate in particular to a kind of have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature.

Background technology

Along with the continuous expansion of cable TV network (CATV) and ground receiving system, Digital Television is popularized rapidly, has entered ten million average family.The Digital Television that increases except digital sensitivity index is had the higher requirement, requires it that the outer antijamming capability of very strong frequency band is arranged simultaneously fast, and promptly adjacent interference suppression ratio frequently is to satisfy corresponding national standards.This also is a very important index of digital tuner.

The intermediate frequency amplitude-frequency characteristic of electronic tuning unit is meant the amplitude-frequency characteristic of mixer stage intermediate frequency output circuit.It is the many difference frequency signals that produce after mixing, selects tv-if signal.This just requires amplitude-versus-frequency curve to the outer interference signal of frequency band goodish inhibition ability will be arranged, and it can directly influence the image quality of reception of TV signal.

In existing tuner, radio-frequency input signals enters band pass filter, and filtered signal enters wide-band amplifier, signal amplitude is amplified the decay that causes after shunt with compensating signal again.Signal after the amplification is divided into two-way output through frequency divider, and the first via forms radio frequency output (LT OUT), the second tunnel entering signal treatment circuit.Signal processing circuit comprises High Amplifier Circuit, double-tuned circuit, frequency-selecting of intermediate frequency circuit, mixing amplification etc.Promptly the second road signal enters High Amplifier Circuit, the major function of High Amplifier Circuit is, carry out automatic gain control according to the power of signal amplitude, enter double-tuned circuit then, double-tuned circuit mainly is that each radiofrequency signal that receives is carried out frequency-selecting, simultaneously out-of-band interference signal is suppressed and filtering.Enter into master chip (SN761668 chip) then and carry out mixing and amplification.Intermediate-freuqncy signal after the master chip output mixing divides two-way to enter digital Surface Acoustic Wave Filter and simulation Surface Acoustic Wave Filter.By the digital medium-frequency signal of digital Surface Acoustic Wave Filter, the amplifier by the Gain Adjustable joint amplifies again, and output two-way intermediate-freuqncy signal enters next stage complete machine mainboard and carries out decoding processing from the output of tuner terminal.And the analog if signal of simulation Surface Acoustic Wave Filter output enters analog demodulator chip (M61111FP chip), carry out the audio frequency and video demodulation process, output analog composite video signal (CVBS) and second accompanying sound intermediate frequency signal (SIF), from the terminal output of tuner, enter the next stage audio frequency and video treatment circuit of complete machine.

With reference to figure 1, in the frequency-selecting of intermediate frequency circuit of above-mentioned tuner, radiofrequency signal and local oscillation signal carry out mixing in master chip, mixed intermediate-freuqncy signal is by the two output pin MIXOUT1 and MIXOUT2 (the 37th and 38 pins of the SN761668 chip) output of described master chip, carry out after the filtering again carrying out tuning frequency-selecting (frequency-selecting of intermediate frequency) through capacitor C 13 and C14 respectively by the tuning circuit that capacitor C 12 and inductance L 11 and L12 form, at last from input pin IFIN1 and IFIN2 (the 35th and 36 pins of SN761668 chip) input master chip, carry out next step intermediate frequency again and amplify in master chip, the power supply circuits that+5V power supply is formed by resistance R 55 and capacitor C 11 are the intermediate frequency amplifier circuit power supply of master chip inside.Along with the development of digital television techniques, above-mentioned frequency-selecting of intermediate frequency and the outer filter circuit of band can not satisfy the requirement of present digital tuner to adjacent channel interference rejection capability index.

Summary of the invention

The purpose of this utility model be to provide a kind of have face the frequency-selecting of intermediate frequency circuit that disturbs inhibit feature frequently, by outside frequency band, increasing absorption point when the intermediate-freuqncy signal frequency-selecting, strengthen the interference rejection capability frequently that faces of intermediate-freuqncy signal.

Technical solutions of the utility model are:

The utility model provide a kind of have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, comprise frequency selection circuit, two signal input end of intermediate frequency and two intermediate-freuqncy signal outputs, frequency-selecting outputs to the intermediate-freuqncy signal output to the intermediate-freuqncy signal of described signal input end of intermediate frequency input through frequency selection circuit, wherein, described frequency-selecting of intermediate frequency circuit also comprises a resonant circuit, and described resonant circuit is connected in described frequency selection circuit and the two intermediate-freuqncy signal outputs and carries out resonance absorbing between arbitrary.

Described have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, it also comprises another resonant circuit, it is connected between described frequency selection circuit and another intermediate-freuqncy signal output and carries out resonance absorbing.

Described have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, wherein, described frequency selection circuit comprises two inductance and an electric capacity, and described two inductance series connection back is in parallel with described electric capacity, the two ends of described electric capacity respectively with described two signal input end of intermediate frequency with two the intermediate-freuqncy signal output is corresponding is connected.

Described have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, wherein, described resonant circuit is a LC resonant circuit in parallel.

Described have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, wherein, described two signal input end of intermediate frequency and two intermediate-freuqncy signal outputs provide by the master chip of electronic tuning unit.

Described have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, wherein, described master chip is the SN761668 chip, and described two signal input end of intermediate frequency are respectively 37 and 38 pins of SN761668 chip, and described two intermediate-freuqncy signal outputs are respectively 35 and 36 pins of SN761668 chip.

Described have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, it also comprises power supply circuits, is described master chip power supply.

Described have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, wherein, described power supply circuits comprise resistance and the electric capacity that is series at successively between power supply and the ground, the series connection node of described resistance and electric capacity links to each other with the series connection node of two inductance in the described frequency selection circuit.

Described have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, it also comprises a resistance, and is in parallel with the electric capacity of described frequency selection circuit.

Described have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, wherein, the resistance of described resistance is 3K Ω.

The utility model provide a kind of have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, provide frequency band outer absorption point by increasing resonant circuit, increase frequency band external signal absorption function, improves out-of-band adjacent frequency interference rejection capability.

Description of drawings

Fig. 1 is the circuit diagram of existing frequency-selecting of intermediate frequency circuit;

Fig. 2 the utlity model has to face the circuit diagram that disturbs the frequency-selecting of intermediate frequency circuit better embodiment of inhibit feature frequently;

Fig. 3 is the intermediate frequency curve comparison diagram of the frequency-selecting of intermediate frequency circuit that provides of existing frequency-selecting of intermediate frequency circuit and the utility model.

Embodiment

In order to make the purpose of this utility model, technical scheme and advantage clearer,, the utility model is further elaborated below in conjunction with drawings and Examples.Should be appreciated that specific embodiment described herein only in order to explanation the utility model, and be not used in qualification the utility model.

With reference to figure 2, the utility model provides has to face and frequently disturbs the frequency-selecting of intermediate frequency circuit better embodiment of inhibit feature to comprise: by being series at+power supply circuits that resistance R 21 between 5V power supply and the ground and capacitor C 21 are formed, the frequency selection circuit of forming by inductance L 21, inductance L 22 and capacitor C 22, two resonant circuits forming by inductance L 23 and capacitor C 23 and inductance L 24 and capacitor C 24, and the resistance R 22 that is used for impedance matching, balance gain respectively.As shown in the figure, described circuit also comprises master chip (SN761668 chip), described master chip comprises two intermediate-freuqncy signal output pin MIXOUT1 and MIXOUT2 (the 37th and 38 pins of SN761668 chip) and two intermediate frequency input pin IFIN1 and IFIN2 (the 35th and 36 pins of SN761668 chip), the two ends of described capacitor C 22 connect two intermediate-freuqncy signal output pin MIXOUT1 of master chip and MIXOUT2 respectively to obtain the intermediate-freuqncy signal input, inductance L 21 and inductance L 22 series connection backs are in parallel with capacitor C 22, the link of described inductance L 21 and L22 also is connected between described resistance R 21 and the capacitor C 21 to insert power supply circuits, described resistance R 22 is in parallel with capacitor C 22, the resonant circuit that described inductance L 23 and capacitor C 23 compose in parallel is connected between described intermediate-freuqncy signal output pin MIXOUT1 and the intermediate-freuqncy signal input pin IFIN1, and the resonant circuit that described inductance L 24 and capacitor C 24 compose in parallel is connected between described intermediate-freuqncy signal output pin MIXOUT2 and the intermediate-freuqncy signal input pin IFIN2.

In foregoing circuit, radiofrequency signal and local oscillation signal carry out mixing in master chip SN761668, mixed intermediate-freuqncy signal is from master chip pin the 37th and the output of 38 pin, after the tuning frequency-selecting of frequency selection circuit, undertaken absorbing outside the frequency band by resonant circuit again, import from the pin the 35th and 36 pin of master chip at last, in master chip, carry out next step intermediate frequency again and amplify, and give the intermediate frequency amplifier circuit power supply of master chip inside by power supply circuits.

Example 1, for the digital signal of DVB-T standard (terrestrial wireless digital video broadcasting), the centre frequency of intermediate-freuqncy signal is 36MHz ± 4MHz, at this moment, power taking sense L23 and L24 are 270nH respectively, and capacitor C 23 and C24 are respectively 51pF and 100pF, and then the frequency of two resonant circuits is respectively:

Inductance L 23 (270nH) and C23 (51pF) parallel resonance:

$f=\frac{1}{2\mathrm{\π}\sqrt{\mathrm{LC}}}=\frac{1}{2\mathrm{\π}\sqrt{270\×51}\×{10}^{-6}}=42\mathrm{MHz}$

Inductance L 24 (270nH) and C24 (100pF) parallel resonance:

$f=\frac{1}{2\mathrm{\π}\sqrt{\mathrm{LC}}}=\frac{1}{2\mathrm{\π}\sqrt{270\×100}\×{10}^{-6}}=30\mathrm{MHz}$

Be that the intermediate frequency curve has two absorption points outside near the frequency band 42MHz and 30MHz respectively.

Example 2, for ATSC system (United States of america digital television standard), the frequency of digital intermediate frequency is 44MHz

± 3MHz, at this moment, power taking sense L23 and L24 are 560nH respectively, capacitor C 23 and C24 are respectively

19pF and 30pF, then the frequency of two resonant circuits is respectively:

Inductance L 23 (560nH) and C23 (19pF) parallel resonance:

$f=\frac{1}{2\mathrm{\π}\sqrt{\mathrm{LC}}}=\frac{1}{2\mathrm{\π}\sqrt{560\×19}\×{10}^{-6}}=49\mathrm{MHz}$

Inductance L 24 (560nH) and C24 (30pF) parallel resonance:

$f=\frac{1}{2\mathrm{\π}\sqrt{\mathrm{LC}}}=\frac{1}{2\mathrm{\π}\sqrt{560\×30}\×{10}^{-6}}=39\mathrm{MHz}$

Equally, the intermediate frequency curve has two absorption points outside near the frequency band 49MHz and 39MHz respectively.

Continuation is with reference to figure 3, wherein curve A is the intermediate-freuqncy signal curve of circuit shown in Figure 1, curve B and C are respectively the intermediate-freuqncy signal curve behind resonant circuit of interpolation, from the amplitude-frequency characteristic of the intermediate frequency curve of accompanying drawing 3 as can be seen, the intermediate frequency amplitude-versus-frequency curve midband broad of curve A, selecting frequency characteristic are relatively poor relatively, also be that out-of-band adjacent frequency interference rejection capability is relatively poor relatively, be difficult to satisfy digital tuner neighbour's requirement of index frequently.And curve B and C have squareness factor preferably, and selecting frequency characteristic are preferably arranged owing to the existence of the outer absorption point of frequency band.The test result that can suppress index according to the neighbour is frequently improved design, promptly suitably adjusts the inductance in the formula and the parameter value of electric capacity, makes resonance frequency be absorbed in different frequencies.Simultaneously, according to test result and the different requirement of client, can select two resonant circuit individualisms or existence simultaneously, to provide the adjacent absorption point frequently in left side, the adjacent absorption point frequently in right side or both sides that absorption point is all arranged, with the out-of-band adjacent interference rejection capability frequently of effective raising, fully satisfy the requirement of technical indicator.

Resistance R 55 in the circuit is chosen as about 3K Ω, plays the effect of impedance matching, balance gain, can make the intermediate frequency curve that phase-frequency characteristic is preferably arranged.

Except above-mentioned requirements, require the inductance in the circuit to be fine-tuning element usually so that the peak dot frequency of intermediate frequency amplitude-versus-frequency curve can be in ± 1.5MHz-± 2MHz scope fine setting to meet design requirement.

Should be understood that; the above only is a better embodiment of the present utility model; not in order to limit protection range of the present utility model; for those of ordinary skills; any modification of being done according to the above description, be equal to and replace and improvement etc., all should be included within the protection range of the present utility model.

Claims (10)

1. one kind has and faces the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, comprise frequency selection circuit, two signal input end of intermediate frequency and two intermediate-freuqncy signal outputs, frequency-selecting outputs to the intermediate-freuqncy signal output to the intermediate-freuqncy signal of described signal input end of intermediate frequency input through frequency selection circuit, it is characterized in that, described frequency-selecting of intermediate frequency circuit also comprises a resonant circuit, and described resonant circuit is connected in described frequency selection circuit and the two intermediate-freuqncy signal outputs and carries out resonance absorbing between arbitrary.

2. according to claim 1 have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, it also comprises another resonant circuit, it is connected between described frequency selection circuit and another intermediate-freuqncy signal output and carries out resonance absorbing.

3. according to claim 1 and 2 have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, it is characterized in that, described frequency selection circuit comprises two inductance and an electric capacity, described two inductance series connection back is in parallel with described electric capacity, the two ends of described electric capacity respectively with described two signal input end of intermediate frequency with two the intermediate-freuqncy signal output is corresponding is connected.

4. according to claim 3 have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, it is characterized in that described resonant circuit is a LC resonant circuit in parallel.

5. according to claim 3 have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, it is characterized in that described two signal input end of intermediate frequency and two intermediate-freuqncy signal outputs provide by the master chip of electronic tuning unit.

6. according to claim 5 have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, it is characterized in that, described master chip is the SN761668 chip, described two signal input end of intermediate frequency are respectively 37 and 38 pins of SN761668 chip, and described two intermediate-freuqncy signal outputs are respectively 35 and 36 pins of SN761668 chip.

7. according to claim 5 have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, it also comprises power supply circuits, is described master chip power supply.

8. according to claim 7 have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, it is characterized in that, described power supply circuits comprise resistance and the electric capacity that is series at successively between power supply and the ground, and the series connection node of described resistance and electric capacity links to each other with the series connection node of two inductance in the described frequency selection circuit.

9. according to claim 3 have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, it also comprises a resistance, and is in parallel with the electric capacity of described frequency selection circuit.

10. according to claim 9 have face the frequency-selecting of intermediate frequency circuit that frequently disturbs inhibit feature, it is characterized in that the resistance of described resistance is 3K Ω.

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