CMX148 datasheet(23/75 Pages) CMLMICRO

23 / 75 page

PMR Audio and Data Processor

CMX148

D/148/6

AF Response - TX Mode

Pre-Emphasis TIA/EIA-603 Compliance

Hard Limiter

-90

-80

-70

-60

-50

-40

-30

-20

-10

100

1000

10000

100000

Frequency (Hz)

12.5kHz & HPF Enabled (C2H=3400H)

25kHz & HPF Enabled (C2H=2C00H)

Template ((TIA/EIA)

Ref. 0dBV

RBW 10Hz

VBW 30Hz

Sweep Time: 160 secs

C1H 4002H

+6dB/octave

Figure 13 Audio Frequency Pre-emphasis

Modulator Output Routing

The sub-audio component can be combined with the audio band signal and this composite signal routed

to both MOD1 and MOD2 outputs, or the sub-audio and audio band signal can be output separately (sub-

audio to MOD2 and audio band to MOD1), in accordance with the settings of:

AuxADC and Tx MOD Mode - $A7 write

Input Gain and Output Signal Routing - $B1 write

Alternatively, the combined sub-audio and audio band composite signal can be output on MOD1 and

MOD2 in a phase/quadrature (I/Q) format suitable for direct upconversion to the final RF signal.

Due to the nature of the I/Q modulation, this mode is only feasible in RF channels/systems which have a

maximum frequency deviation of 3kHz or less. Additional test modes are provided for calibrating external

circuits. Tx I/Q mode is particularly suitable for data transmission.

InputAGC

An Automatic Gain Control system can be enabled by setting the relevant bits of the Program Block P4.9.

The setting of the Input1 Gain stage is recorded when the device enters Tx mode and if the signal

exceeds the pre-set threshold, the Input1 Gain is automatically reduced in 3.2dB steps until it falls within

the operational levels or the range of the gain stage is exhausted. When the signal level drops, the gain

will be automatically increased in 3.2dB steps at the rate set in P4.9 until the initial values has been

reached. For maximum effect the system should be designed such that the +22.4dB setting of the Input1

Gain stage achieves the nominal levels. To ensure consistent operation, it is recommended that the

Input1 Gain stage value be re-initialised before entering Tx mode. The signal that is used as an input to

this process can be selected to be either the:

•

Output of Input1 Gain Stage

•

Output of the Pre-emphasis Filter

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