ACE9040 datasheet(21/33 Pages) MITEL

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ACE9040

TRANSMIT INPUT SIGNAL PRECONDITIONING

Microphone Amplifier (MIG)

Microphone signals input at MI via switch MLI are

amplified by a gain selectable amplifier of either 22 dB or

32 dB, controlled by the MIG bit in the “Handsfree mode”

control message. The microphone amplifier’s input can also

be connected to the line input LI via the MLI switch. The

microphone amplifier’s output drives the soft limiter via switch

MIS which also allows higher level signals from the line input

LI to bypass the microphone amplifier to drive the soft limiter

directly. These two switches are controlled by the MIS and MIL

bits in the “Operation mode” control message.

Soft Limiter

Signal amplitude is restricted without clipping to the

correct level for maximum F.M. deviation by the soft limiter.

This operates as an AGC system, controlled by the signal

amplitude at the compressor input and the hard limiter output

later in the signal processing path. If the signal is too large at

either of these points the soft limiter forward gain is reduced.

The nominal soft limiter gain range of 0 to - 30 dB in 64 steps

of 0·5 dB covers all normal volume changes occuring during

a conversation. Soft limiter attack and decay times are set by

internal clocks derived from SCLK and ramp the gain steps at

nominal rates of one step down every 40

µs when the signal

is too large and one step up every 1·68 ms when the signal is

too small.

The soft limiter output at pin SLO is externally coupled

with a capacitor to the input pin IPS of the following gain adjust

stage, INPSENSE. When the speech channel is used to send

DTMF signalling tones a switch, controlled by bit DTMFM in

the “Operation mode” control message, selects the internal

DTMF signal rather than the speech signal at input IPS.

For test purposes TEST[1:0] bits in the “Initializing Mode

1” control message can configure switches to give access to

the softlimiter comparator inputs. When TEST[1:0] bits are

high the 3 & 97 % window comparator is switched from the

hard limiter output to PREIN input and the 92 % comparator is

switched to IPS input.

INPSENSE AMPLIFIER and TXBPF FILTER

Both microphone and transmit voice path tolerances are

trimmed in the INPSENSE gain adjustment block following the

soft limiter and DTMF switch. INPSENSE has a nominal gain

of 8·0 dB with a trim range of - 12·0 to + 12·4 dB relative to the

nominal gain set by bits INPS[4:0] in the “Initializing mode”

control message.

Transmit signal preconditioning is completed by a band-

pass filter TXBPF to limit the audio signal to a speech

bandwidth of 300 Hz to 3·4 kHz. This filter uses switched

capacitor techniques and is preceded by an anti-alias filter and

followed by a smoothing filter to remove the clock from its

output. The typical frequency response is shown in figure 4

which also shows the mask defined by type approval limits.

HANDSFREE FUNCTIONAL BLOCKS

Between the transmit bandpass filter and the compressor

two extra functions are included for use with handsfree

operation. Firstly an attenuator HFGAIN is provided to give

progressive signal reduction in handsfree mode with a range

of 0 to - 49 dB in 7 dB steps, set by bits THF[2:0] in the

“Handsfree mode” control message. The HFGAIN

attenuator’s output drives pin TBPO which is externally

capacitively coupled to the compressor input at pin CIN and

internally drives both the soft limiter and sidetone circuits.

The second function provided for handsfree operation is

a signal rectifier whose output, filtered by an external capacitor

at pin TXC, drives output pin HF via switch HFS with a d.c.

transmit level. Switch HFS is controlled by bit HFS in the

“Handsfree mode” control message. Under control of bit HFS

both receive and transmit levels are available at HF output pin

for external comparison to implement the handsfree function.

The handsfree system is further described in the section

HANDSFREE OPERATION.

COMPRESSOR

ACE9040 provides a 2:1 compressor to halve the transmit

dynamic range as required by analog cellular systems. Within

the operating signal range each 2 dB change in input level

gives a 1 dB change in output level. A transmit signal is input

through pin CIN and output on pin COUT, the signal is

referenced to a mid-supply voltage. CRCIN and CRCOUT are

connections for the external attack and delay time constant

setting components.

The compressor’s nominal unity gain level is 707 mV.

Above this level the signal at CIN is attenuated and below this

level the signal is amplified to achieve the 2:1 dB compression.

Table 20 gives the nominal, Vmax and Vmin levels at CIN and

nominal levels at COUT corresponding with the TACS and

AMPS systems for 0 dB, maximum and minimum deviation.

The gain of the INPSENSE amplifier should be set so that

the compressor operates within these signal levels for a given

input signal at the MIC or LI input.

For the usual attack time of 3·0 ms and decay time of

13·5 ms a 180 k

Ω resistor is connected between CRCIN and

CRCOUT pins and a 100 nF capacitor between CRCIN and

GND pins. An 82 nF capacitor should be connected between

the VMIDTX and GND pins. Attack and decay time is

measured with a 12 dB step, - 8 dB to - 20 dB relative to the

unaffected level. Attack and decay times are respectively

defined at points on the output envelope where it reaches

x 1·5 and x 0·75 of the final steady state level.

External compressor connections allow the use of

external coupling capacitors to remove d.c. offsets and

optionally an external compander. The compressor can be

internally bypassed allowing use of ACE9040 without

companding in non-cellular applications, or for test purposes.

Bits COMP[2:1] in the “Initializing mode 0” control message

control the operation of the internal compander and are used

to switch both the transmit compressor and receive expander

into or out of the signal path. When not in use the internal

compressor and expander are both powered down.

Table 20 Compressor CIN and COUT signal levels for TACS and AMPS

System

Vmax at CIN

Vmin at CIN

Nominal Input at CIN

Nominal Output at COUT

Vrms

mVrms

(0dB) levels mVrms

levels mVrms

TACS

1.0

+ 25

1.77

- 30

200

AMPS

1.0

+ 23

2.25

- 30

225

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