ACE9040 datasheet(22/33 Pages) MITEL

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ACE9040

FINAL MODULATION PREPARATION

Pre-emphasis

A pre-emphasis filter follows the compressor to boost

the amplitude of higher audio frequencies by tilting the fre-

quency response by 6 dB per octave across the whole speech

band as shown in figures 5 and 8. To prevent overload in the

pre-emphasis filter the signal first passes through an

attenuator set to suit the system in use. If an external

compander is used or the companding function is bypassed

the gain is set to 0 dB. When using the internal compander the

gain is

set to – 7 dB for TACS or – 11·0 dB for AMPS.

Compander bypass is determined by control bits COMP[2:1].

The pre-emphasis filter and attenuator input is pin PREIN and

the output is an internal connection to the hard limiter. The pre-

emphasis filter, but not the attenuator, can be bypassed if the

PREEMPH bit in the “Operation mode” control message is set

to “1”.

Hard Limiter

To ensure compliance with the peak deviation specifi-

cation for cellular telephone systems, a hard limiter follows the

pre-emphasis filter to remove any transient level changes that

have passed through the soft limiter. This limiter will handle

large signals and has symmetrical clipping levels close to the

supply rails V

DD & VSS (GND). To ensure clipping at the same

hard limiter input signal level with both the nominal power

supply voltages, hard limiter gain is adjusted by an external pin

“V485”. For the nominal supply voltages of 4.85 V and 3.75 V

gain is respectively 16 dB (V485 pin at “1”) and 13.5 dB (V485

pin at “0”). An 8 dB attenuator follows the limiter to prevent any

further clipping of the signal in the following transmit lowpass

filter.

TX Lowpass Filter TXLPF

A TXLPF lowpass filter with an optimised stop band

response limits the signal bandwidth to a cut-off frequency of

3·0 kHz, the frequency response is shown in figure 6. The

combined frequency response of the pre-emphasis and

lowpass filter stages is shown in figure 8. It is possible for test

purposes to bypass this lowpass filter by setting bit TEST[0] in

the “Initializing mode 1” control message to a “1”.

Speech Deviation Level Setting

A controlled gain stage AUDIODEV sets the output level

to give the required FM deviation for speech. The gain is set

by bits AUDEV[3:0] in the “Initializing mode 1” control mes-

sage. AUDIODEV is followed by a transmit audio mute switch

enabled by bit TXM in the “Operation mode” control message.

A buffer drives output pin TLPO with the transmit speech

signal (and DTMF when in use) which is added with DATA/ST

and SAT tones in the modulation combiner.

Fig. 12 Transmit Data and DTMF Paths

TRANSMIT DATA AND DTMF PATHS

Transmit Data

Data communication from mobile terminals to base

stations in the AMPS and TACS cellular phone systems takes

place over the Reverse Control Channel (RECC) during call

set-up and in short bursts over the Reverse Voice Channel

(RVC) during a call.

RECC or RVC data is transmitted for AMPS or TACS as

a 10 kHz or 8 kHz Manchester Coded FSK signal

respectively. The data signal is generated by the ACE9050

“System Controller and Data Modem” or similar digital circuit

to drive ACE9040’s DATI input pin. The DATI input data signal

is filtered using a 4th order Butterworth lowpass filter with

nominal - 3 dB points of 16 kHz for TACS, or 20 kHz for

AMPS. This filter is implemented using switched capacitor

techniques and is preceded by a continuous time anti-alias

DATI

DTMF[3:0]

DTWIST DTMFMODE[2:1]

DTMFM

IPS

TONEM

RXI

RECEIVE

VOICE PATH

TRANSMIT

VOICE PATH

DATO

DTMF

VMID

DATM

16/20kHz

DATAF[1:0]

-2.8 to + 3.2dB

DATD[3:0]

DATADEV

LOW-PASS

FILTER

DTMF

PRE-

EMPHASIS

MODE

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