MGCM02
Data Sheet
14
The programming will be described in more detail in
the following sections. Serial bus timing is shown in
fi
gure 5.
Receive Programming
MGCM02 has two basic receive modes:
1.
I/Q mode. The 60 kHz IF signal is mixed down to
baseband I and Q signals. This mode is used for
IS136 TDMA and may also be used for AMPS.
2.
FM mode. The baseband I/Q path is powered
down and the MGCM02 discriminator is used for
demodulation. This mode can be used for
AMPS.
These modes are selected by the RX mode bit -
Word 7, Bit 11
Additional control is provided by the receive control
bits - Word 7 Bits 9:6. These program MGCM02 gain
in I and Q mode and allow the input attenuator to be
manually controlled.
Normally the standard gain setting of CONT<1:0> =
00 should be used. The extra gain is provided in the
ampli
fi
er after the bandpass
fi
lter and the low pass
baseband
fi
lter.
CONT<2> sets the bandwidth of the 60 kHz
bandpass
fi
lter. The low bandwidth mode (CONT<2>
= 1) should be used for FM mode however the higher
bandwidth mode (+/-20 kHz) may be used in TDMA
operation.
The FM gain control CONT<3> is inactive unless
RSS <0> is set to 1 as described in the next section.
CONT<2> sets the bandwidth of the 60 kHz
bandpass
fi
lter. The low bandwidth mode (CONT<2>
= 1) should be used for FM mode however the higher
bandwidth mode (+/-20 kHz) may be used in TDMA
operation.
RSS allows manual control of the input attenuator in
conjunction with CONT<3>.
LOI - Word 7 bit 3 - is used to select high side or low
side LO injection for the downconverter to 60kHz.
The internal 60 kHz I and Q signals are also
reversed to take account of this and enable the
image rejection to operate correctly.
Extra post discriminator
fi
ltering can be programmed
using LPC<1:0> and PDF<1:0> - Word 8 Bits 15-12.
The PDF bits connect the switched capacitor
baseband
fi
lters after the FM discriminator and are
used
to
improve
the
performance. The LPF bits allow the cut-off
frequency to be reduced to 25 kHz. The LPC and
PDF bits should only be used in FM mode. In TDMA
mode they should be set to zero.
discriminator
output
RX
Mode
0
I / Q
1
FM
CONT <3:0>
Mode
X
X
0
0
Gain +0dB
X
X
0
1
Gain +12dB
X
X
1
0
Gain +15dB
X
X
1
1
Gain +18dB
X
0
X
X
+/- 20kHz Bandwidth
X
1
X
X
+/- 16kHz Bandwidth
0
X
X
X
FM Low Gain
1
X
X
X
FM High Gain
RSS
Operation
0
Normal FM Mode
1
Manual 32 dB Gain Enable
LOI
Operation
0
High side LO
1
Low side LO