Masurarea Lung. de Unda - Frecv

Badea Cristian George

Alecu Alexandru Cristian 431A

Masurarea lungimii de unda in ghid si a frecventei

Pentru masurarea lungimii de unda si a frecventei se foloseste o linie de masura dispozitiv carea se utilizeaza in general atat la masurarea semnalelor cat si la masurarea retelelor .

Din punct de vedere constructiv linia de masura este o portiune de ghid uniform, prevazuta cu o fanta longitudinala in care se introdoce sonda.

Schema bloc a liniei de masura este data in figura de mai jos :

Linia de masura de mai sus este compusa din generator cu undametru cu transmisie, atenuator, sonda, sistem de cuplaj direct, detector, gol sau scurt fix sau scurt mobil in functie de ce se doreste sa se masoare .

Masurarea lungimiide unda in ghid se bazeaza pe propietatea undei stationare ca distanta dintre doua minime sau maxime consecutive este (g/2. Pentru o perecizie de masurare buna trbuie ca ondulatia distributie sa fie cat mai pronuntata. In practica se masoara distanta dintre doua minime si pentru o citire cat mai buna se foloseste o citire mediata.Se folosesc doua metode pentru masurarea lungimii de unda prin determinarea distantei dintre doua minime :

Metoda deplasarii sondeiSe monteaza la capatul liniei de masura un scurtcitcuit. Se deplaseaza sonda in lungul liniei si se noteaza minimele. Se calculeaza distanta dintre doua minime din care se obtine (g.

Metoda deplasarii scurtcitcuitului

Se monteaza la capatul liniei de masura un scurtcitcuit mobil . Se deplaseaza scurtcitcuitul si e noteaza pozitiile pentru care apar minime ale semnalului cu sonda fixata intr-o pozitie oarecare. Se calculeaza distanta dintre doua minime si se determina (g.

Cele doua metode din punctul de vedere al rezultatelor difera foarte putin deoarece in cazul deplasarii sondei se masoara de fapt lungimea de unda in ghidul cu fanta care variaza foarte putin fata delungimea de unda in ghidul inchis.

Pentru ca rezultatele masuratorilor sa fie cat mai corectela inceputul lucrarii se regleaza un nivel constant laintrare pentru ca dioda sa nu iasa din zona de detectie.Se tine cont de faptul ca maximul cel mai mare se obtine cu linia terminata in gol si se regleaza nivelul pentru ca acesta sa coboare sub 20mV.

Dupa cele trei masuratori s-au obtinut valorile din urmatorul tabel si pe baza acestor date s-au ridicat urmatoarele grafice:

Cele trei distributii sunt realizate cu ghid avand la capat scurtcitcuit,sarcina adaptata,gol.Valorile variabilelor oI , ah ,ak (distante) se dau in mm, iar V1, V2, V3 (tensiuni) in volti.

Se masoara minimele prin metoda sondei mobile si prin medierea valorilor simetrice si se obtine d = 68.91mm si prin metoda scurtcitcuitului mobil si prin mediere si se obtine d =68.4 mm Se obtin (g = 137.82 mm si (g = 136.8 mm. Eroarea relativa este de (r =0.75%.

Frecenta in ghid se calculeaza cu formula :

unde c viteza luminii

(g lungimea de unda in ghid

(c lungimea de unda critica

Se alege din cele doua lungimi de unda de mai sus (g = 136.8 mm pentru ca determinarea este mai precisa .

Se da a = 72mm si (c =2a =144mm

Se obtine f = 3002.4 106 Hz.

Din reglajele efectuate la inceputul lucrarii s-a obtinut f=3004.8 106 Hz.Deci masuratorile s-au efectuat cu o eroare relativa de (r =0.07%.

Raspunsuri la intrebari

1. Curbele obtinute au acest aspect deoarece detectorul liniei de masura are o caracteristica de detectie patratica si aceasta face ca valorile mici ale amplitudinii sa fie facute si mai mici si deaceea curbele au o aliura sinusoidala cu toate ca prin minime curba ar fi trebuit sa fie cat mai abrupta.

2. Prezenta zgomotului se observa prin faptul ca minimele sunt asimetrice.

3. In cazul terminatiei in gol minimele nu sunt nule datorita micilor perturbatii introduse de fanta de la sonda.

4. Nivelul semnalului afecteaza lungimea de unda prin caracteristica detectorului care poate fi liniara sau patratica in functie de nvelul semnalului.Sarcina nu afecteaza lungimea de unda pentru ca daca ar afecta nu s-ar mai putea masura ( prin cele doua metode.

Nivelul semnalului trebuie sa fie mare pentru ca minimele sa fie cat mai pronuntate

5. Atenuatorul are rolul de a micsora nivelul semnalului unuei inverse(reflectate) care se propaga pe linia de masura pentru ca aceasta sa nu afecteze regimul de functionare al generatorului modificarea frecventei sau amplitudinii oscilatiilor.

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Masurarea Lung. de Unda - Frecv

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