MGCM02
Data Sheet
band pass filter. This can be provided in two ways as
described in the preceding FM section.
The RSSI output from the MGCM02 will normally be
input into an a to d converter. This, together with the
baseboard 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, offset at low and high signal
levels, and threshold level of the RSSI characteristic.
For example if the RSSI output is less than Vdd/2
then the RSSI slope is positive; if greater than Vdd/2
than the RSSI slope is negative.
In the fixed gain FM mode the RSSI output is input to
a comparator. The output of this comparator then
reduces the IF amplifier gain by 32.5dB thus
enabling a larger dynamic range for the RSSI.
Hysteresis is built in to prevent oscillation when close
to the threshold level. Fig 6 shows the RSSI
characteristic. At low signal levels the RSSI output
increases with signal level, however at high signal
level when the gain is reduced in the input path, the
RSSI output is mirrored around Vdd/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 fig
5.
If the AGC mode is used for FM then this automatic
RSSI switching is disabled and the RSSI is only
operated over the lower segment of the
characteristic. The actual signal level must then be
calculated by the baseband using the measured
RSSI level and the applied AGC signal.
60 kHz
60kHz
Band Pass
Low Pass
Smoothing
Filter
I
Switched
Capacitor
Switched
Capacitor
Low Pass
Butterworth
n = 2
IF
Input
Chebychev
n = 5
Chebychev
n = 3
14dB
90∞
Rx LO
60 kHz
60 kHz
Gain
Settings
-2dB
6dB
BW
= +/- 16kHz
BW = 37.5kHz
G = 8dB
BW = 60 kHz
G = 6 dB
Q
12dB
G =14dB
21.6dB
Each gain stage is
programmable from -12dB to
+12 dB in 0.5dB increments
To FM Discriminator
and RSSI
Figure 2 - Receive Path Block Diagram showing Gain Plan and Filters
Filter Response
Passband Detail
0
0
5
20
40
10
60
15
20
25
80
100
120
30
40
50
60
70
80
90
0
30
60
90
120
150
Frequency (kHz)
Frequency (kHz)
Figure 3 - Band Pass Filter Response
10
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