參數(shù)資料
型號(hào)： MGCM02
廠商： Zarlink Semiconductor Inc.
英文描述： TDMA/AMPS IF and Baseband Interface
中文描述： 時(shí)分多址/ AMPS二元IF和基帶接口
文件頁(yè)數(shù)： 9/21頁(yè)
文件大小： 529K
代理商： MGCM02
Data Sheet
MGCM02
9
Operating Description
Receive
Basic Architecture
The MGCM02 provides a highly integrated receive
solution
for
dual
mode
telephones. The input to MGCM02 is normally from
the output of the
fi
rst IF
fi
lter. The signal is ampli
fi
ed
by
a
variable
gain
downconverted in a quadrature mixer to a low IF of
60kHz as I (In phase) and Q (Quadrature) signals.
The local oscillator for this mixer is generated from
an on chip VCO using external tank components.
This oscillator operates at twice the LO frequency to
allow the generation of accurate quadrature LO
signals for the mixer. High side or low side LO
injection is programmable. Further stages of voltage
controlled gain are then provided at 60kHz with
matched ampli
fi
ers in the I and Q channels.
IS136/AMPS
mobile
ampli
fi
er
before
being
Gain control is provided from an external analogue
control signal. This signal is digitised by an on chip 8
bit analog to digital converter. The digital outputs are
then used to control the ampli
fi
ers. Figure 2 shows
the ampli
fi
er con
fi
guration in more detail. The
preampli
fi
er has 4 gain settings and there are four
ampli
fi
er stages at 60 kHz each providing -12 to
+12dB gain in 0.5 dB steps. The gain control range
will be more than adequate for most TDMA
applications. The digitally controlled ampli
fi
ers
provide a highly accurate and linear control of gain.
The conversion rate of the analog to digital converter
is 45 kHz.
The I and Q signals are then combined and passed
through a switched capacitor polyphase bandpass
fi
lter. This
fi
lter is a
fi
fth order Chebyshev. The
advantage of using a switched capacitor
fi
lter is that
it gives very stable performance and no calibration is
required. The circuit also provides rejection of the
image frequency following the down conversion to
60kHz.
TDMA IS136 Mode
Following the bandpass
fi
lter the signal is mixed
down to baseband I and Q signals which are output
from differential outputs. There is additional
baseboard
fi
ltering to remove spurii from the
downconverters and clock breakthrough from the
switched capacitor
fi
lters. Further detail of the
MGCM02 receive path is shown in Fig 2. The
baseboard outputs can be fed directly into analog to
digital converters in a baseband circuit.
AMPS FM Mode
FM demodulation can be done using the I and Q
baseband signals if supported by the baseband
however the MGCM02 contains a FM demodulator.
In FM mode the baseband I and Q output stages are
disabled and the 60 kHz IF signal from the bandpass
fi
lter is input to a limiting ampli
fi
er and FM
discriminator. The FM discriminator consists of a
shift register acting as a delay line. The output of the
discriminator is a digital signal which must be
fi
ltered
to recover the audio signal. The discriminator output
is routed through the cascaded baseband I and Q
low pass smoothing
fi
lters and
fi
nally through an
output buffer stage. The cut-off frequency of the low
pass switched capacitor
fi
lters can be set at 25kHz
for optimum
fi
ltering. External components can be
used to optimise the gain and frequency response of
the output ampli
fi
er.
There are two methods of controlling the ampli
fi
er
gain in FM Mode.
1)
Fixed gain. The ampli
fi
er gain is set to a preset
level. This gain level (Pre-ampli
fi
er and VGA gain
= 26dB) allows the minimum sensitivity
requirements to be met, but at high signal levels
the gain is automatically reduced by 32.5 dB.
This optimises the signal levels through the
bandpass
fi
lter preventing overload and
excessive phase distortion. Further details on
this are given in the following RSSI section.
2)
AGC Control. In this mode the gain is controlled
by the VGA control input. The RSSI level is
monitored by the baseband controller and the
gain level set appropriately. This mode gives
improved performance in strong fading
environments.
RSSI
The MGCM02 also contains RSSI circuitry. This
would normally be used when using the FM
discriminator to provide the received signal strength
to the phone microcontroller. This RSSI circuit has
over 70dB dynamic range.
A block diagram of the RSSI circuit is shown in
fi
gure
5. The switched capacitor
fi
lter has a limited dynamic
range of approximately 50dB due to aliased noise
from the sampling process used. In order to enable
the RSSI to operate over a larger dynamic range
gain control is required in the ampli
fi
ers before the
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