Gift to

and 5. The purpose of the
divide-by-ten prescaler is to
lower the vco fr equency a
bit so that the programmabl
e cou nter can f unction
reliabl y. The output of the
prescaler is 3.5 to 3.799
MHz, and this feeds the
programm able or divide-byn
c o u nte r. I u se d a n
MC12013P IC fo r th e divideby-t
e n circ uit; however ,
almost any prescaler IC wi ll
work.
Looki ng at the circ uit,
transistor Q1 buffers and
am pl if ies the vco ou t put to
a sufficie nt level to ensure
proper cou nti ng by the prescaler.
Tra nsistor Q1 has a
f ixed gain of ten, and C15
co uple s the desired amount
of rf into the MC12 013 P.
About 0.8-vo lts p-p of rf is
needed at pi n 15 of the
12013P to ensure proper
counting.
T h e p rog r amm a bl e
co unter has the responsibility
o f di viding t he vco output
in to 10-kH z increments
over th e 299-channel range
w hen the lo op is locked. To
do t his, th e co unter has to
be designed to di vi de-by­
3500-to-3799 in 299 steps.
T he followi ng f o rm u l a
shows ho w t he desir ed
cou nter range was der ived:
n (num ber counter d ivides
b y ) i s equal t o : (3 .5
MH z - 3.799 MHz )/1000
Hz.
Dividing eac h f requ ency
by the 100D-Hz reference
yields the desired coverage
of the counter. The 100D-Hz
f requen cy is th e ph ase
detector sampling fre quency
or ref er ence fr equency.
A 1000-Hz ref erence is
neede d at the phase det ecto
r to provide the 1D-kHz
step at th e vco. This can be
prove n, as fo llows. For the
moment, assume th at the
loop is locked and that the
cou nte r is set at divide-by­
3500. The vco f requ ency
now equals: Fvco is eq ua l
to (presca le r)(divid e-by -n
numb er)(ph ase detecto r
ref erence). Thi s yield s (10)
(3500)(1 000), or 35 MHz.
If t he pr og ramm ab le
c
., ,,, .oo
- 1211 ( 13
or
.-----t---l~
cte
H
or
f--+-- +l
3 .~ TO
3.7 9 9 MH!
1KHz
P
'" '"
, 2: :5 4
7' "
.. ,J,
3.3K
~ 5V ~5V
l1 :" ue
f--­
f-----
. ..
I Kll ( 15
3 - 15
" 2NJ 9 47
'" roo
4
8 20 .01
"" 1CI
" 10 3 n I.
".
'"
FIl O,", S WITCH DECOOER
5 6 1 8
"" 7
f--­
f--
"
p
,
l--1>--r1'O
."
'". ,
Cl6 _511
ur
MC12 0 13 P ""IO
Fig. 4. Phase detector, loop filter, reference fo r phase dete ctor, and veo prescaler.
1 :. e
_
CLR
C 0 C/ L
e U 4 6
eL K 7 4 19 6 (:2
IO MH,
a A aD
ra
COUNT LOAO
3 112 SN741 4 of1--- - - - - + - -"'- --- = = = "'--- - - ---- - --'
> ell z
I.
r I'
·'511
"
" "
ce
.. ,
"'
'"
2491( " .% ,~
"
ua ,
IOOpF " 2491< 1% , L /Iol 308 ce
•
-" ." "
cr
,
2 261< I'Jl. • 2. 2.."-
ut
/101(4044 cz
IOO pF
.. • cr
• a , ~
3. 3 1< 2201(
.-
." ••
11:4 4 7
2: 21' F· 1511
." "
9 10 I ' 12
7\7 \
rl :'. c
_
~
ce cs
, ." .r ." .r '" ." ." '" .or ."
p p p p
counte r is advanced to
3501, the vco fr equency
now becomes Fvco is equa l
to (10)(3501X1000) or 3501
" 10 3 II II
'Q:f
e U!> 6
CL ~ 7 419 6 C2
IMHz
CLR QA a D
13 7 ~ 12
f---
Fig. 5. 3500 to 3799 programmab le d ivider.
M Hz . The vco now has incre
ased in frequency the
desired 10 kH z, and it wi ll
incr em en t 10 kH z each
""
IK," z TO
P,"A SE
OETECTOR
F IG. 2
time t he counte r is advanced
o ne step.
It should be appa rent
that th e vc o reall y is give n
73 Magazine . October, 1980 213