ACE9030MSLASHIWSLASHFP2N datasheet(35/39 Pages) MITEL

35 / 39 page

ACE9030

division will be 495 for a 14·85 MHz crystal; the term f

CRYSTAL is

used to refer to the exact crystal frequency in the following

calculations.

Thus Main synthesiser (nominally 915·030 MHz) gener-

ates:

CRYSTAL x 30501 ÷ 495 = fCRYSTAL x 61·61818182

This result is independant of the chosen comparison fre-

quency which is given above as an illustration only.

With this drive to the first mixer the actual first I.F.

(nominally 45 MHz) is:

CRYSTAL x 61·61818182 – 870·030 MHz

The second mixer is required to downconvert 45 MHz to

450 kHz so needs an LO2 at 44·550 MHz which is 3

∞ f

CRYSTAL

and this integer multiplication is the reason for choosing a

14·85 MHz crystal. This mixer operates with low-side injection

so the actual second I.F. on the AFCIN pin (nominally

450 kHz) is:

CRYSTAL x 61·61818182 – 870·030 MHz – 3 ∞ fCRYSTAL

CRYSTAL x 58·61818182 – 870·030 MHz

This signal feeds the F.M. discriminator to extract the

modulation and is also used to derive the AFC information.

The AFC mixer uses a 504 kHz clock derived from the

crystal (f

CRYSTAL ∞ 504 ÷ 14850 = fCRYSTAL ∞ 0·03393939) to high-

side downconvert AFCIN to a low frequency on AFCOUT,

giving:

CRYSTAL x 0·03393939) –

CRYSTAL x 58·61818182 – 870·030 MHz).

This is 870·030 MHz – f

CRYSTAL x 58·58424243 and is the

difference between two large but similar numbers, one fixed

and the other a multiple of f

CRYSTAL and will have an overall value

strongly dependent on f

CRYSTAL .

To evaluate the sensitivity of the system consider a

+ 1 ppm change in crystal frequency, giving AFCOUT at

53·130 kHz instead of its nominal at 54 kHz, a shift of 870 Hz

or 1·61 %. This frequency can be counted in the

microcontroller using a timebase derived from f

CRYSTAL or any

other crystal reference as the error in the timebase due to the

crystal is swamped by the changes in AFCOUT.

The counting of AFCOUT should be over a period long

enough to resolve changes of around 1 % which means at

least 2 ms but is normally a little longer to filter off some noise

and modulation on the signal; around 10 ms is a good starting

value for system development.

Once the crystal error has been estimated the DAC’s

controlling the crystal frequency can be adjusted to bring the

whole cellular terminal into frequency alignment with the

basestation which is normally assumed to be very accurately

held at the correct frequency; effects like Doppler shift will be

significantly less than 1 ppm for all intended users. Some

damping in the control loop for the crystal will be needed to

avoid overshoot and hunting, possibly implemented as always

under-correcting the crystal or by limiting the slew rate of the

corrections.

ACE9030M/IW/FP2N Folha de dados(PDF) 35 Page - Mitel Networks Corporation

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