MC12061 Motorola, MC12061 Datasheet - Page 4

• Manufacturer Part Number: MC12061
• Description: (MC12060 / MC12061) Crystal Switching Methods
• Manufacturer: Motorola

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place a high imp@@@k/in series with them. The MSD7000
large $~%e
resistance will not be exceptionally
rent is 1.15 mA. This current is sufficie]
series “impedance ‘of the MPN3401
2 MHz the impedance
suiting Cs values in this case are ,0.008 pF
Typical series resonant
exhibit
0.003 pF and the maximum series resistant
both items 1 and 2 above are met.
DECOUPLING UNSELECTED C~~S~LS
typically haq~~.7~~F
Figure 3J$~$ ~N?= 1.6 volts, and the MPN3401 typically
0.75 ~~~~~~!~volts
proximately
300 times greater than typical values of equivalent crystal
series resonant
is~eqqire JL - ‘-’”’-c--’’--: ‘
2 .MHz; resulting CS values are respectively
0.005 pF. Since typical series resonant
frequency
values, -Cx, ranging from 0.024 pF to O.C. - ~. , . .. ... .
is isatisfled. Mso, since the equiv~ent- seriec r~c:~t~fi~a fif
is satisfied.
ofims and at 20 MHz 3.3/-37°
for”the MC12061 is 155 ohms. Again th~”r
standpoint
crystal, and insuring tha${~~,~~scfllator will lock on a new
crystal frequency w~~,:~~ttified
low value for Rs, maintains
of
o~s
the diode is much less. than the. maximum ~---,.-. . - -----
~ri~~’
U,.
series equivalent capacitance (CT =
ody
!The:,i~,,PvU~,,~GU,
: Isolating unselected crysta~~*\@~important
w”
The objective f~~,~e~dupling
.
Fnr
bias
“.
.nAoi
g~ww...
3.270 to 0.33% respectively.
‘at ’100 kHz and l15@=
th.
. ..”
current has a typical value of 11,5~-Qo = 11~ ~ :~
equivalent
G..
MP
. ..=
of minimizing,~~req~~y
range
””,,”,,
reverse bias condition, the resulting diode RS
equal CD. This series capacitance
tinn
17nA
.
-“”
capacitance
exhibit
(A
(T
-.
CX capacity
1 circ,lit
.
of reverse bias. Since RD is extremely
b
J.
“JLWW’.,
L1lG
of shunt capacitance CD (refer to
:/a,
is nominally
fih-.l-ffi.
“..11.0,
crystals in’ this fK
LvIDu/uuu
equivalent
(CA.
tho
.,.”
‘–.
considerable
L“,
. .
=, 2.6-jl .98
from a previous one.
unselected
values 01
AinAa
Ul”uw
-----
. .... -.w:&,~#~r*ments of
1., ~a/~?&
,,Gap%,>%>~2x;e++
This, combined with a
Cst Cx
+ha
.,,U
large and CS will ap-
Therefore,
Cs Cx ) decreases by
UIUUG WJUIU.%J
PTN
LL@
pull of the selected
~ ,,
]*:. +;.,
113~[
,J**~> .’>.\\?:p+~
seri~q ~znn~i~nn~~
nfirl
..AQ
fn[ward
.“,
‘.$.\.,,
clys
,,
~6,
r 0.265 pF and
.-. ,”+”1. :- +L:.
,--,
}17
. .
It. to keep the
diode low. At
; ohms. The ra~c.
,equencp:%;g~p
F
‘e q%q$
effective resist-
.
and O.OO&~%l,$k”
o.Q*$:~pto
9nAn
*“””,
crystals is to
coupling be-
.W..o.gl.
--r-”.
Lti.
n~
= 19.4-j10
—
“A”?wation
the total
bias
from the
OtiS
is 30 to
,,
1L7.
;+o-
,11
itpm
J.U,,,
.-.-”
””
U-
cur.
IIIA
,lll.
it
“L
u
9
&
1
.:A*
-
,,.!;,*
“X$~$= XS = 3.97 k ohms. This vrdue of RS is much greater
:
vlaues of RD = XD:= 7~:S~? ohms give Rs = Xs = 39.7 k
unselected
$g~&:$#in RD = XD = 7.95 k ohms, giving a new value of
bias diodes D1 through
amount
20 MHz. To maximize the s~!~~~q~valent
the parallel resistor R~ is&4,@,<8qual to the reactance XD
at the highest operatin&~~~~#&flcy. For the MC 120i
ohms. Since Rs i$~&&&much greater than 4 k ohms, the
oscfll,ator. ~@~&+~&~ationwill become greater with a de-
crease iR fre~ency.
an~f~$c~fculate
?kan 155 ohms, the maximum effective resistance speci-
fication for the MC12061. Therefore,
now have sufficient isolation from the unselected crystals
to prevent erratic performance.
(Dl -D5). Ml bias elements contribute
load, regardless of which crystal position is selected. This
with a 5-volt supply. One diode (D I-D5) in the MSD7000
dud
ing. R18 throu~
corresponding
ohms, and 10 k ohms for Figures 1 and 2 respectively.
OSCILLATOR AC LOADING
quency pulling and sine wave distortion.
shown in Figures 1 and 2 the ac loading is primardy
attributable
occurs because
parallel
MC 12060, additional
networks
ated to minimize the loading effects of the common bias
resistor, R17. In addition, a modified approach is used to
through
and, at the same time, supply sufficient forward current
the second diode (D6-DI O) is used for reducing ac load-
Usi#&,,the-;me
The vrdues used for R1, R3, R5, R7 and R9 are 82 k
Oscillator ac loading must be minimized to reduce fre-
Due to a greater
“[’CD
diode package is used to switch the crystal and
of current
capacitance
(D1O, R22)
for this device. An RF choke, L1, is incorpor-
cr~~h~%ill
to the biasing networks
to the unselected crystals.
RS and XS at 20 MHz for the MC 12061
the RF signal is coupled
formulas to determine the required RD
R22 are essential to supply a small
for reverse bias of
(Co)
elements are used in the switching
are added to minimize ac loading
sensitivity
be virtually
D5. The networks
of the unselected
s+. ~ ~!.
.~:i,,
to ac loading
for the five diodes
.,t~,:..
isolated
the oscillator will
to an effective ac
For the circuits
diodes D1 -D5
resistor (Rs),
. . .
through
(D6, R18)
from the
crystals.
160, the
of the
the