ACE9030 datasheet(21/39 Pages) MITEL

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ACE9030

7, D6

Crystal Freq.

Sampling Rate

I.F.

Delay as I.F. cycles

0, 0

12·80 MHz

6·400 MHz

450 kHz

2·742

0, 0

14·85 MHz

7·425 MHz

450 kHz

2·363

1, 0

12·80 MHz

4·267 MHz

455 kHz

4·266

1, 0

14·85 MHz

4·950 MHz

455 kHz

3·677

0, 1

15·36 MHz

5·120 MHz

450 kHz

3·252

1, 1

15·36 MHz

7·680 MHz

455 kHz

2·251

DATA1 bit D

Set D

Set M

Intended I.F.

Intended Crystal

450 kHz

12·8 or 14·85 MHz

455 kHz

12·8 or 14·85 MHz

450 kHz

15·36 MHz

455 kHz

15·36 MHz

Table 2

Table 3

rapidly increase relative to the I.F. signal and so avoid low

frequency beats, when the system could sit in the state where

a steady part of one cycle is compared with a steady part of

another for a long period of time and so give no output.

The accuracy of the delay is not important as a small error

will only give a D.C. offset in the output but the delay must be

consistant to avoid adding modulation to the output so in the

ACE9030 it is derived from the crystal frequency.

The sampling rate must not be a harmonic of the I.F., or

very close to one, to prevent the sampling phase becoming

synchronised to the signal and so missing all edges, leading

to the modulation being lost for long periods of time at the beat

frequency (a 14·85 MHz crystal cannot be used in D = 3 mode

with an I.F. at 450 kHz as 14·85 MHz

÷ 3 is 4·95 MHz which is

11 x 450 kHz). It cannot be assumed that a sampling rate

greater than 4 MHz always meets the Nyquist criterion for the

I.F. signal at nominally 450 or 455 kHz because the input

signal is often a square wave from a limiting amplifier and if not

is converted to a switching logic signal in the Schmitt trigger

input buffer giving many significant harmonics. The modula-

tion deviation is up to 14·5 kHz and is multiplied by the

harmonic number to give increasingly wide deviation such that

the spectrum eventually becomes continuous, but at a low

level, for the very high (e.g.17th or above) harmonics. A

sampling rate of a few MHz will then retain all required

information and allow distortion free demodulation but is

undersampling in Nyquist terms so aliasing effects must be

avoided by choosing a frequency separated from the nearest

harmonic of the I.F. by at least twice the modulation frequency.

All combinations given in tables 2 and 3 can safely be used but

care is needed if a different crystal or I.F. is required. For

example, a 14·4 MHz crystal cannot be used in

÷ 2 mode with

a 450 kHz I.F. but

÷ 3 can be used and with an M of 40 will give

a delay of 3·75 I.F. cycles and alias-free demodulation.

Sampling the I.F. signal at a rate of only 9·3 to 16·9 times

the I.F. will remove the fine detail of the modulation from each

individual cycle of the I.F. but the modulation bandwidth is very

low (both speech and tones) compared to this sampling rate

so the information will be preserved as infrequent whole

sample steps, which when averaged over many samples will

show the correct modulation.

To explain the operation of the discriminator an example

diagram of the sampling points and the comparison delay is

given in figure 19, with the effect of modulation on the input

shown by fine lines. The increasing separation of these dotted

filtered to nearer telephone bandwidth by an on-chip amplifier

with off-chip feedback components.

The I.F. signal is digitised at a rate set by the crystal in use

and by the divider D in figure 18 and so will be in the range

4·267 to 7·680 MHz. These rates are all greater than the

maximum audio frequency of 3·4 kHz by a factor of at least

1254 ( which is over 210 ) and so the quantisation allows better

than 63 dB signal to noise ratio in the final audio, even though

only single bit quantising is used. The I.F. is oversampled by

a much smaller ratio and so will have a smaller signal to noise

ratio if measured in its total bandwidth, but this bandwidth is

reduced in the demodulation process to give a good audio

signal to noise ratio in the system.

To power down the discriminator a Normal command can

be used:

The two control bits D

7, D6 set the values for D and M as

in table 2. From this table of frequencies and division ratios it

is possible to calculate the length of the delay M in terms of

cycles of the input I.F. to understand the discrimination proc-

ess shown in table 3.

It can be seen that a 12·8 or 15·36 MHz crystal will give a

delay of a few whole cycles plus or minus one quarter cycle to

a very good accuracy and that a 14·85 MHz crystal similarly

gives some whole cycles plus or minus an odd third of a cycle.

These non-integer delays are needed because the delays

are not locked to the I.F. input on AFCIN, and to get a

demodulated output the comparisons must include an edge

time, at least for some samples. The odd quarter or third of a

cycle ensures that the phase of the start of the delay time will

DATA1

DATA2

DATA3

7 D6 xxxxxx

10xxxxxx

xx1xxx00

DATA1

DATA2

DATA3

xxxxxxxx

01 D

5 xxxxx

xxxxxxxx

where the discriminator is powered down if DATA2:D

5 is LOW

or is set active if DATA2:D

5 is HIGH.

The values of the programmable constants D and M are

set by a Set-up command, which can also use DATA1 bit D

for the lock logic filter period and DATA1 bits D

2, D1, D0 for the

OSC8 mode programming:

ACE9030 Datasheet(PDF) 21 Page - Mitel Networks Corporation

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