Motorola MagOne Handmatig - Pagina 20

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2-2

RF AMP(Q301) before being filtered again by the post-selector before being applied to the mixer.

The pre-selector and post-selector are band-shift type filters to remove unwanted signals.

2.2

First Mixer

The signal from receiver front-end is channelled into the Mixer (Q302) base to be heterodyned with

the first local signal generated from the VCO to become the first IF frequency (45.1MHz). This first

IF frequency is resonated at tank circuit C328, L308 (VHF) or L306 (UHF) and then channeled into

the pair of crystal filters.

2.3

The first IF signal is amplified to approximately 15dB by the IF amp Q303 and channeled into IF IC

(U201) to be mixed with the second local frequency (44.645MHz). The first IF frequency (45.1MHz)

and the second local frequency (44.645MHz) are mixed in U201, to produce the second IF

frequency (455KHz). The second IF frequency passes through ceramic filters (CF1, CF2) to

eliminate undesired signals and demodulated by demodulator in U201.

The 455Khz ceramic filter consist of a wide (CF1) and a narrow (CF2) filter. The filtering is changed

to wide and narrow band by the switch (SW).

2.4

U201 has a filter that filters noise and has an internal amplifier that amplifies the noise to make it a

DC voltage in order to control the squelch system. So, if the noise level is under a threshold, the

microprocessor (U101) in the radio detects the DC level and un-mutes the radio. If the noise level is

over a threshold, the microprocessor mutes the radio.

Receiver System