Exista numeroase cazuri cand se doreste transformare unui radioreceptor dotat cu gama undelor scurte (tip MA) in vederea receptionarii semnalelor cu banda laterala unica (BLU sau SSB) sau a telegrafiei nemodulate (CW). In asemenea situatii se prefera reintroducerea purtatoarei unui oscillator local numit BFO. Semnalul se amplifica pe anodul sau catodul diodei detectoare de amplitudine care in acest caz indeplineste rolul de mixer.
In cele ce urmeaza este descris un BFO ce poate fi atasat unui receptor de frecventa intermediara de 455KHz. Pentru a putea receptiona semnale emise in banda laterala inferioara sau cea superioara s-a prevazut posibilitatea reglarii frecventei in domeniul 453 … 457 KHz, prin intermediul a doua diode varicap tip BB139.
Oscilatorul de tip Colpits utilizeaza un transistor T de tip BC 107-108 sau BF214-215. Alimentarea montajului se realizeaza de la o sursa stabilizata si filtrate de 12Vcc sau chiar din aparatul de receptie.
Inductanta L contine 170 spire CuEm cu diametrul de 0.1mm, bobinata pe un miez oala de F.I. – 455MHz utilizat curent in receptoarele industriale.
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