Data Sheet
MGCM02
Operating Description
Receive
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.
Basic Architecture
In FM mode the baseband I and Q output stages are
disabled and the 60 kHz IF signal from the bandpass
filter is input to a limiting amplifier 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 filtered
to recover the audio signal. The discriminator output
is routed through the cascaded baseband I and Q
low pass smoothing filters and finally through an
output buffer stage. The cut-off frequency of the low
pass switched capacitor filters can be set at 25kHz
for optimum filtering. External components can be
used to optimise the gain and frequency response of
the output amplifier.
The MGCM02 provides a highly integrated receive
solution for dual mode IS136/AMPS mobile
telephones. The input to MGCM02 is normally from
the output of the first IF filter. The signal is amplified
by
a
variable gain amplifier before being
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 amplifiers in the I and Q channels.
There are two methods of controlling the amplifier
gain in FM Mode.
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 amplifiers. Figure 2 shows
the amplifier configuration in more detail. The
preamplifier has 4 gain settings and there are four
amplifier 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 amplifiers
provide a highly accurate and linear control of gain.
The conversion rate of the analog to digital converter
is 45 kHz.
1) Fixed gain. The amplifier gain is set to a preset
level.This gain level (Pre-amplifier 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 filter 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.
The I and Q signals are then combined and passed
through a switched capacitor polyphase bandpass
filter. This filter is a fifth order Chebyshev. The
advantage of using a switched capacitor filter 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.
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.
TDMA IS136 Mode
Following the bandpass filter the signal is mixed
down to baseband I and Q signals which are output
from differential outputs. There is additional
baseboard filtering to remove spurii from the
downconverters and clock breakthrough from the
switched capacitor filters. 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.
A block diagram of the RSSI circuit is shown in figure
5. The switched capacitor filter 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 amplifiers before the
9
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
