MGCM01
OPERATING DESCRIPTION
Receive
TDMA IS136 mode
The receive path filtering is shown in more detail in Figure 4.
The inputs to the MGCM01 are I and Q signals at an IF
of 60 kHz. These can be generated by a quadrature
demodulator circuit such as the Zarlink Semiconductor
MGCR01 (Saturn) AGC amplifier and demodulator. This
device is normally used for mixing direct to baseband but
can also mix down to low IF quadrature signals. The I and
Q signals are passed through anti-alias filters to prevent
spurious responses in the subsequent switched capacitor
filter. The anti-alias filter is a third order Butterworth with a
230kHz cutoff. The I and Q signals are then combined
and passed through a switched capacitor bandpass filter.
This filter is a tenth order Chebychev. 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.
Following the bandpass filter the signal is mixed down to
baseband I and Q and is output from differential outputs.
There is additional baseband filtering to remove spurious
signals from the down converters and clock breakthrough
from the switched capacitor filters. Further detail of the
filtering in the the MGCM01 receive path is shown in
Figure 4. The baseband outputs can be fed directly into
A to D converters in a baseband circuit.
AMPS FM Mode
Demodulation can be performed using the I and Q
baseband signals. However, the MGCM01 also includes
a limiting amplifier and an 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. External components can
be used to optimise the gain and frequency response of
the output amplifier.
Further information on using MGCM01 in FM mode is
provided in application note "MGCM01 in AMPS
environment".
RSSI
The MGCM01 also contains RSSI circuitry. This would
normally be used when using the FM discriminator to
provide the received signal strength to the phone’s
microcontroller. The RSSI circuit has over 70dB dynamic
range. In the presence of strong signals the RSSI circuit
switches in a 32dB attenuator in the IF I and Q input stages
to optimise dynamic range. The RSSI circuit will not normally
be used in I/Q mode except if required to monitor base station
signal strength.
A block diagram of the RSSI circuit is shown in Figure 7.
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 the RSSI output is
input to a comparator. The output of the comparator then
switches a 32·5dB attenuator in the 60kHz I and Q input
stages and enables a larger dynamic range for the RSSI.
Hysteresis is built in to prevent oscillation when close to
the threshold level. Figure 8 shows the RSSI characteristic.
At low signal levels the RSSI output increases with signal level;
however, at high signal level when the attenuator is switched
in the input path, the RSSI output is mirrored around V
DD
/2
and decreases with increasing signal level. The slope is the
same at high level as at low level but is, of course, negative.
The actual slope (or gain) and settling time for the RSSI are
set by external components as shown in Figure 7.
The RSSI output from the MGCM01 will normally be input
into an A to D converter. This, together with the baseband
controller can convert the RSSI signal to a monotonic digital
output as required by the IS136 specifications.
Calibration will be required to determine the slope and offset
at low and high signal levels, and the threshold level of the
RSSI characteristic. For example if the RSSI output is less
than V
DD
/2 then the RSSI slope is positive; if greater than
V
DD
/2 then the RSSI slope is negative.
I
ANTI-ALIAS
LOWPASS
BANDPASS
SWITCHED
CAPACITOR
60kHz
CHEBYCHEV
n=3
BW = 230kHz
n = 10
BW =
660kHz
CHEBYCHEV
n=3
BW = 37·5kHz
BUTTERWORTH
n=3
BW = 60kHz
LOWPASS
SWITCHED
CAPACITOR
SMOOTHING
FILTER
LOWPASS
I
60kHz
BUTTERWORTH
Q
Q
TO FM DISCRIMINATOR AND RSSI
Figure 4 - Receive path filters
6
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
