20

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.

AFC Amplifier and Discriminator

Fig. 18 Audio Discriminator And AFCOUT Circuit

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

Fig. 17 Typical OSC8 Loop Filter

The input signal to the pin AFCIN is approximately a

squarewave from the second (final) I.F. amplifier-limiter at 450

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.

One path samples the input signal at 504 kHz, with a clock

generated by a divide-by-two from the CL clock. This sampling

gives a mixed-down output AFCOUT at 54 kHz when the input

is at exactly 450 kHz and otherwise tracks the offset to allow

estimation of the local oscillator error and the crystal error so

that an AFC loop can be built to adjust the crystal to exactly the

C8B

10nF

12k

VDDA

AFCOUT

TO ACE9050

(FOR AFC

SOFTWARE)

AUDIO

TO ACE9040

PRECISION

BANDPASS

FILTER

C2

R1

R2

R3

C1

EXTERNAL FEEDBACK

(BANDPASS FILTER)

XO

AFCIN

CL

FROM CRYSTAL

OSCILATOR

D = 2 OR 3,

FROM BUS

1 2 3 . . . . M

M = 37 OR 40,

FROM BUS

=1

DRIVER

BP

6 dB

ATT'N

L.PASS

FILTER

: D

DQ

DQ

: 2