Motorola Internal Use
13
V2288 – Circuit Description
1)
The antenna signal
FMin
is routed from the headset ground line through the antenna
matching circuit into the tuned front end on
Pin 47
of the
U1001
. The front end and
the FM Oscillator are tuned by the
TUNE
signal from the internal PLL circuit of the
IC.
2)
The output signal from the tuned front-end and the FM Oscillator are routed into the
mixer stage. The output of the mixer is a 10.7 MHz IF signal; this is due to the FM
Oscillator running 10.7 MHz above the antenna signal.
3)
The 10,7 MHz IF signal than passes the first IF filter and the first amplifier stage.
Behind the amplifier the signal path is split in two-signal path.
•
The first supports the AM FM Indicator Stage that converts the IF signal to an
analogue voltage. This voltage is routed to
Pin 22
of the
U1001
as
FM_RSSI
. The
FM_RSSI
level is
corresponding to the receiver field strength.
•
The second passes the signal to the 2
nd
IF filter and IF amplifier to feed into the
Demodulator stage.
4)
The FM demodulator stage is using the
10.7 MHz crystal Y1003
to demodulate the IF
signal. The output supports three different parallel filter stages.
•
The 1st is filtering the frequency-band from 0 to 15kHz that contains the
L+R
information.
•
The 2nd is filtering the frequency-band from 23 to 53kHz that contains the
HELP
Signal
.
•
The 3rd is filtering the
19KHz
pilot tone and is working to detect the pilot to support
the 38 kHz PLL/VCO stage.
