ACE9030
giving:
division will be 495 for a 14·85 MHz crystal; the term fCRYSTAL is
used to refer to the exact crystal frequency in the following
calculations.
(fCRYSTAL x 0·03393939) –
(fCRYSTAL x 58·61818182 – 870·030 MHz).
Thus Main synthesiser (nominally 915·030 MHz) gener-
This is 870·030 MHz – fCRYSTAL x 58·58424243 and is the
difference between two large but similar numbers, one fixed
andtheotheramultipleoffCRYSTAL andwillhaveanoverallvalue
ates:
fCRYSTAL x 30501 ÷ 495 = fCRYSTAL x 61·61818182
strongly dependent on fCRYSTAL
.
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:
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 fCRYSTAL 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.
fCRYSTAL 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 ∞ fCRYSTAL
and this integer multiplication is the reason for choosing a
14·85 MHzcrystal. Thismixeroperateswithlow-sideinjection
so the actual second I.F. on the AFCIN pin (nominally
450 kHz) is:
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
avoidovershootandhunting,possiblyimplementedasalways
under-correcting the crystal or by limiting the slew rate of the
corrections.
fCRYSTAL x 61·61818182 – 870·030 MHz – 3 ∞ fCRYSTAL
= fCRYSTAL 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(fCRYSTAL ∞ 504÷14850=fCRYSTAL ∞0·03393939)tohigh-
side downconvert AFCIN to a low frequency on AFCOUT,
35
MITEL [ MITEL NETWORKS CORPORATION ]
