Ramsey Electronics AM25 Manual de instrucciones descargar pdf (Pagina 8)

AR1 • 8

CIRCUIT DESCRIPTION

The antenna is coupled through C1 to a three-section tuned LC filter input

network. 118-135 Mhz signals are amplified by VHF transistor Q1 and fed to

the input of U1, the SA602 mixer-oscillator. L6 and its associated capacitor

network establish the LO (local oscillator) frequency at 10.7 Mhz higher than

the incoming 118-135 Mhz signals. The LO frequency may be tuned across

about 15 Mhz by the varactor tuning network formed by D1 and R1. The 10.7

Mhz difference between the LO and the received signal is fed through the

10.7 Mhz ceramic filter FL1, amplified by Q2 and applied to U2 the MC1350 IF

amplifier IC with AGC input. The 10.7 Mhz IF is peaked by L7, and the AM

audio is demodulated by D2 and fed through the four op-amps of U3, the

LM324, where volume control, AGC output, audio filtering and squelch

functions are managed. The LM386 (U4) audio output is quite low in

background noise and is capable of driving simple communications speakers

to excellent volume levels.

PARTS LAYOUT DIAGRAM

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Ramsey Electronics AM25 Manual de instrucciones Pagina 8