CN102811101A - Image frequency detection method - Google Patents

Abstract

The invention relates to an image frequency detection method, which is characterized by comprising the following steps of: mixing microwave input frequency fIN and variable local frequency fO; filtering a signal in a mixing process in the step 1 by using a 35M band-pass filter, and when intermediate frequency fIF is detected in the filtered signal, stopping the variation of the variable local frequency fO with a local frequency value fOI, varying the variable local frequency fO into fOL=fOI-2fIF, and continuing mixing the fOL serving as a variable local frequency value and the microwave input frequency fIN; filtering the signal in the mixing process by using the 35M band-pass filter, and if the intermediate frequency fIF cannot be detected, determining that the microwave input frequency fIN free from the interference of image frequency is equal to the summation of fO and fIF; and if the intermediate frequency fIF can be detected, determining that the microwave input frequency fIN free from the interference of the image frequency is equal to a difference between fO and fIF. The method has the characteristics of simple structure, low cost and no use of an image frequency suppression mixer.

Description

A kind of image frequency detection method

Technical field

The present invention relates to a kind of image frequency detection method, relate to a kind of detection method that does not receive the microwave incoming frequency of image frequency interference, is a kind of image frequency detection method in the search type superhet frequency measuring receiver.

Background technology

Search type superhet frequency measuring receiver is realized frequency search through changing local frequency, when local frequency equals the receiver IF-FRE with the beat that receives signal, carries out frequency measurement.The frequency measurement result is:

f _IN＝f _O+f _IF

Because input signal f _INUnpredictable search working with local oscillator of frequency and arrival time, make incoming frequency f _INImage frequency f _IN-2f _IFAlso possibly drop in the receiver medium-frequency band, bring image frequency to disturb, real incoming frequency f _INMaybe for:

f _IN＝f _O-f _IF

For final frequency measurement brings error, the broadband image frequency rejection mixer approximately the image frequency inhibition degree of 20dB can be provided and volume is big, cost is high.

Summary of the invention

The technical problem that solves

For fear of the weak point of prior art, the present invention proposes a kind of image frequency detection method, overcomes in the above-mentioned frequency measuring method because of existing image frequency that the frequency measurement result is brought interference.

Technical scheme

A kind of image frequency detection method is characterized in that step is following:

Step 1: with microwave incoming frequency f _INWith variable local oscillator f _OCarry out mixing; Said microwave incoming frequency f _INLower band f in system _NWith system bandwidth upper limit f _MBetween change; Said variable local oscillator f _OAt f _N-f _IFTo f _M-f _IFPress stepping f in the scope from low to high _ΔChange; Said f _ΔSpan is 0.2 megahertz～20 megahertzes;

Step 2: with 35,000,000 band pass filters the signal in step 1 optical mixing process is carried out filtering, when in filtered signal, obtaining intermediate frequency f _IFThe time, variable local oscillator f _OStop to change and the local oscillator value of this moment is f _OI, change variable local oscillator f then _OBe f _OL=f _OI-2f _IF, and as variable local oscillator value with microwave incoming frequency f _INProceed mixing; Said variable local oscillator is still by stepping f _ΔChange;

Step 3: with 35,000,000 band pass filters the signal in step 2 optical mixing process is carried out filtering, if can not detect intermediate frequency f _IF, the microwave incoming frequency f that not disturbed by image frequency _IN=f _O+ f _IFIf still can detect intermediate frequency f _IF, the microwave incoming frequency f that not disturbed by image frequency _IN=f _O-f _IF

Beneficial effect

A kind of detection method that does not receive the microwave incoming frequency of image frequency interference that the present invention proposes compared with prior art, has following characteristics: simple in structure, at the bottom of the cost, do not use image frequency rejection mixer.

Description of drawings

Fig. 1: search type superheterodyne receiver schematic diagram.

Embodiment

Combine embodiment, accompanying drawing that the present invention is further described at present:

Referring to Fig. 1, the present invention realizes through following hardware: microwave mixer, intermediate frequency amplifier, local oscillator, filter, frequency measurement module constitute.The microwave incoming frequency f that needs detection _INBetween 6.200MHz and 6.600MHz, change variable local oscillator f _O(f in changing by stepping 0.2MHz from low to high _OBetween 6165MHz and 6565MHz, change), microwave incoming frequency f _INWith variable local oscillator f _OMixing is carried out filtering with 35MHz band pass filter (bandwidth 6MHz) to the signal after the mixing and is obtained intermediate frequency f _IFAt variable local oscillator f _OIn the change procedure, as intermediate frequency f _IFLocal oscillator f during for detectable signal _OStop to change, and in the frequency measurement module, test the f of this moment _IFFrequency values; The local oscillator value of supposing this moment is 6365MHz, uses 6365-2*f _IFMHz is as system's local oscillator value; Detect f _INWith 6365-2*f _IFThe intermediate frequency output that the MHz local oscillator is produced.If detect less than intermediate frequency f as stated _IF, then frequency measurement result uses f _IN=6365+f _IFExpression; If still can detect intermediate frequency f as stated _IF, then frequency measurement result uses f _IN=6365-f _IFExpression; The microwave incoming frequency f that not disturbed by image frequency _INValue is accomplished test.

Claims (1)

1. image frequency detection method is characterized in that step is following:

Step 1: with microwave incoming frequency f _INWith variable local oscillator f _OCarry out mixing; Said microwave incoming frequency f _INLower band f in system _NWith system bandwidth upper limit f _MBetween change; Said variable local oscillator f _OAt f _N-f _IFTo f _M-f _IFPress stepping f in the scope from low to high _ΔChange; Said f _ΔSpan is 0.2 megahertz～20 megahertzes;

Step 2: with 35,000,000 band pass filters the signal in step 1 optical mixing process is carried out filtering, when in filtered signal, obtaining intermediate frequency f _IFThe time, variable local oscillator f _OStop to change and the local oscillator value of this moment is f _OI, change variable local oscillator f then _OBe f _OL=f _OI-2f _IF, and as variable local oscillator value with microwave incoming frequency f _INProceed mixing; Said variable local oscillator is still by stepping f _ΔChange;

Step 3: with 35,000,000 band pass filters the signal in step 2 optical mixing process is carried out filtering, if can not detect intermediate frequency f _IF, the microwave incoming frequency f that not disturbed by image frequency _IN=f _O+ f _IFIf still can detect intermediate frequency f _IF, the microwave incoming frequency f that not disturbed by image frequency _IN=f _O-f _IF

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