ACE9030
C8B to close the loop. An integration is needed to set the VCO onto the correct frequency and other components can then
ensure loop stability. Enough filtering must be provided to give a clock output suitable for all of its uses - microprocessor clock
and audio filtering. A typical loop filter is shown in figure 17.
VDDA
12k
10nF
C8B
Fig. 17 Typical OSC8 Loop Filter
AFC Amplifier and Discriminator
AFCOUT
DQ
CL
TO ACE9050
(FOR AFC
: 2
SOFTWARE)
=1
DRIVER
AFCIN
DQ
1
2 3 . . . . M
6 dB
ATT'N
M = 37 OR 40,
FROM BUS
XO
L.PASS
FILTER
: D
FROM CRYSTAL
OSCILATOR
D = 2 OR 3,
FROM BUS
AUDIO
TO ACE9040
PRECISION
BANDPASS
FILTER
BP
R1
R3
R2
C1
C2
EXTERNAL FEEDBACK
(BANDPASS FILTER)
Fig. 18 Audio Discriminator And AFCOUT Circuit
The input signal to the pin AFCIN is approximately a
squarewavefromthesecond(final)I.F. amplifier-limiterat450
or 455 kHz and is A.C. coupled to the pin through a capacitor
of typically 1 nF to drive a Schmitt trigger and become a logic
signal. This signal carries the received modulation - speech,
SAT and signalling data. AFCIN is first amplified and limited
and then processed in two separate paths as in figure 18.
Onepathsamplestheinputsignalat504 kHz,withaclock
generatedbyadivide-by-twofromtheCLclock.Thissampling
givesamixed-downoutputAFCOUTat54 kHzwhentheinput
is at exactly 450 kHz and otherwise tracks the offset to allow
estimation of the local oscillator error and the crystal error so
thatanAFCloopcanbebuilttoadjustthecrystaltoexactlythe
correct frequency. Further details of this feature are given in
the APPLICATIONS HINTS section. AFCOUT is also used by
the modem in the ACE9050 to receive signalling DATA.
The other path is the audio discriminator which is a purely
digital implementation of the quadrature technique where the
exclusive-OR gate acts as a digital multiplier and compares
two samples of the AFCIN signal separated by a programma-
ble delay to extract the audio and drive the speech and SAT
paths in the ACE9040. The delay length, M, and the crystal
divider, D, are both programmable for the various crystal and
intermediate frequency choices.
After demodulation the audio is low-pass filtered on-chip
to remove the doubled sampling clock and then bandpass
20
MITEL [ MITEL NETWORKS CORPORATION ]
