crystal mc1206[ @ ? ~ . . . motorola ?semiconductor products gnc.
,, ..- ,. ,:? crystal switching methods for .,, .: mci 2060/mc12061 oscillators introduction ? is dso an economical dual diode in the configuration crystal switching can be achieved electronically for the needed for tfis circuit. MC12060 and mc 12061 crystal oscillator integrated cir- the mpn3401 is used with the mc 12061 circuit be- cuits by utilizing diodes as rf switches. the switching is cause it offers a large off-to-on impedance ratio for low dc controlled by applying a forward bias to the diode associ- bias currents at frequencies within the range of the ated with the desired crystal and applying a reverse bias mc12061. ,*!. to the remaining diodes, related to the unselected crystals. ?*{,3, $};. , \ ,,\?,$, in addition to functioning with the mc 12060/ ,,:2:,>, ~~ * 1~.?~~ dc bias requirements . . . ...! $ ,$, .tbk+ l.. .j\, .ij,, mc 12061 ic?s, the switching circuit designs described ., ~orward bias for ,the. ,desired cryst~=~$~~yti:g diode here can dso be used? in other applications where it i: ?, ~~, (dl ,? d2; .d3y d4, or .d5) is applie~,~b~~~~ing the five desired to electronically switch series-resonant crystals ~ ? ?position switch. the? bias current is#~~,#~y set. by r17 with a minimum of frequency pulling. !<<,~r,. ?\:*\t\.mw , . and r2 (r4, r6, r&,? and r1.o h$~e, ,%- 4 )} *...,.,. .* ~?+.,. *<-$. general iq?-??*>(, v-vd - (~) (for mc1206 ?: ??$~lr~+{r~711 (r3+ ?~r13)} the mc 12060 and mc 12061 crystal oscillators are :j&~$i$~ specified for operating frequency ranges of 100 khz to $y~~~~~$ 2.o mhz, and 2.0 mhz to 20 mhz respectively. their? ? ?ti?:$. while one diode (or one diode pair in the case of fig- ~~> outputs consist of a sin~e-ended ttl signal, plus ,,$pm- .? ure 1) is always forward biased, the remaining diodes are : elementary sine wave tid ??ecl si~ds. the sim~~~~$~~ ., reverse biased to minimize their capacitance. this is outputs are capable of driving an ac load of ~@#* at accomplished with a single polarity supply by using pullup 500 mvp-p (typical) when an external re~~~~y~@ used ~ resistors (r12, r13, r14, ri 5, and r16) from the posi- to increase the current in the emitter f~~~f? output. tive potential to each switch terminal. therefore, the the ecl and ttl outputs are capabl~:$~~~ting five and ~ cathodes of the diodes corresponding to the unselected ten gate loads respectively. .*\ \,, . . .t!c, :11 crystals are pulled up to approximately the supply volt- series resonant crystals conne#ed~&#$ween pins 5 and age. since one diode (or diode pair) is always selected, 6 are required for use with~#~~$,#scfilatorsl the tot~ ?? current is flowing through r 17 continuously, causing a effective ac series resistan~ ?{,~~~ ..*,., voltage drop. therefore, the anodes of the unselected sistance plus any addi@na& resistance contributed by diodes will be negative with respect to their cathodes. ~si> ,:$. switching compone,~&)$ be~een these pins must ?be less when using a 5.0 volt supply, this reverse bias will be 1.6 than 4 k ohs for$~~3~~# 12060, and less than 155 ohms volts for the mc 12060 and 1.2 volts for the mc 12061 for the mc 120@:~~.,#;i?$ c~stal switching array. for additi~>@formation on these ic?s, see the device data she$ki;&~aefigineering bulletins eb-58 and eb-59. additional considerations sch~at$+ ??diagrams for the mc 12060 and mc 12061 a sufficient amount of forward current through the cryg{~$~~ltching circuits are given in figures 1 and 2 diode selectirig the desired crystal is required to insure ~~~~~tively. the same basic technique is employed a low value for diode resistance kd (see figure 3). this is f~~each ic except that an addifiond diode-resistor pair important for two reasons: (d6, r18 through d1o, r22) is incorporated for the 1. to minimize the effects of diode capacity on the mc 12060 to offset its greater sensitivity to ac loading. crystal?s natural series-resonant frequency. the mpn3401 pin diode and the msd7000 pn junc- 2. to minimize the totrd effective external resistance tion diode are used to switch the crystals. the msd7000 between pins 5 and 6 of the integrated circuit. was selected for use with the ?mc 12060 oscillator because from figure 3 it is apparent that as rd is made smal- of its low capacitance (1.5 pf max. for vr = o volts). it ler, xs is decreased and cs is increased. a large value for circuit diagrams external to motorola products are included as a means of illustrating typical semiconductor applications; consequently, completa information sufficient for construction purposes is not necessarily givan. the information in this application note has been care- fully checked and is believed to be antirely reliabla. however, no responsibility is assumed for inaccuracies. furthermore, such information doss not convey so the purchaser of:the semiconductor devices describad any. iicanse. undar tha patent, righ= of motorola inc. b;.others. ?1) .
loading and therefore frequency pulling will be greater selecting the nominrd 1.0 mhz crystal is approximately for higher frequency crystals and will increase as the total -0.0040 +? 0.0031 = -0.0009 percent. similarly, absolute number of crystals. to be switched is increased. however, pulling for the 8.0 mhz crystal becomes -0.004 +0.0001 by using the switching techniques shown in figures 1 and = -0.0039 percent. pulling effects of the switching circuits 2, any frequency pulling in addition to that for a single when selecting the 0.2 mhz crystal offset pulling caused crystal connected directly to pins s and 6 (i.e. pulling by the ic to give approximately zero absolute crystal pull. caused by the ics alone) ?is negligible below approxi- when desirable, a trim capacitor can be added in series mately 1 mhz for the mc 12060 and 15 mhz for the with the crystals and adjusted to pull the oscillq(lor up in mc 12061. measurements of this additiond pulling are frequency. , ,, ~~.iv, ~},:,: ,, ~.,, \\$,,, summarized in table 1. typical frequency pulling values several options are possible to reduce #~~~ming for ,, :: ~ . . \ ,?1., \ device i mci 2060 ,,,,*&#@?l :.., y 0.5 1.0 2.0 2.5 f:, %?s 13.4 20.0 r .f i ref. -0.005 n n.. in this case the devices are operating with a single cr~sta~$ ? connected directly to pins 5 and 6 with no crystal @~~ch- ing circuits. the table ii values have been take~,~$~j,re?*r- ence jn establishing the pulling (noted in t&ti~~~~d~used by the switching networks. when using t~~~&~sti switch- k~), .@?* ing circuits, complete pulling from *?*hystd?s series . . ., ! >, resonant frequency is obtained by ~~~%~]~~ly adding the respective values in tables i and~$k~ @~,.&xample, absolute crystal pulling for the five cr~s?~,:,@tching system when ..>>: ~t.$~< . . .,,.!,,,. both the mc 12060 and mc 12061 crystal switching cir- cuits. using a higher voltage supply for the bias networks will allow larger values of bias resistors to be used at the same diode current, resulting in reduced loading. also, rf decoupling chokes may be added between resistors r2, r4, r6, r8, and rio and capacitors c6 through c 10. where frequency pulling is not as critical, l1 in figure 1 may be eliminated. these options are left to the dis- cretion of the user.
-. ??. is~eqqire jl - ?-??? -c--??--: ? . . !the:,i~,,pvu~,,~gu, l1lg lvidu/uuu uiuug wjuiu.%j iiia ?[?cd .,t~,:.. .~:i,, of bias current has a typical value of 11,5~-qo = 11 ~ ~ :~ s+. ~ ~!. o~s ?at ?100 khz and l15@= 113~[ 2 .mhz; resulting cs values are respectively 0.005 pf. since typical series resonant clys lti. ,11 ,lll. 160, the frequency range exhibit equivalent seri~q ~znn~i~nn~~ vlaues of rd = xd:= 7~:s~ ? ohms give rs = xs = 39.7 k values, - cx, ranging from 0.024 pf to o.c. - ~. , . . . . . . . is isatisfled. mso, since the equiv~ent- seriec r~c:~t~fi~a fif unselected cr~~h~%ill be virtually isolated from the the diode is much less. than the. maximum ~---,.-. . - ----- ~ri~~? .naoi g.. tinn (a b fih-.l-ffi. +ha nfirl 9nan ;+o- 9 : crease ir fre~ency. ? 1l7. u- u ~6, otis it 20 mhz. to maximize the s~!~~~q~valent resistor (rs), the parallel resistor r~ is&4,@,<8qual to the reactance xd r 0.265 pf and .-. ,?+?1. :- +l:. at the highest operatin&~~~~#&flcy. for the mc 120i ,--, --r-?. .-.-? }17 n~ itpm 1 ohms. since rs i$~&&& much greater than 4 k ohms, the . . .w..o.gl. ?? ?l effective resist- - oscfll,ator. ~@~&+~&~ation will become greater with a de- . ?. . ..? . ..= . -?? . ?jlww?., .,.? ul?uw .?, rent is 1.15 ma. this current is sufficie] series ?impedance ?of the mpn3401 ptn 2 mhz the impedance is nominally ll@ ofims and at 20 mhz 3.3/-37 =, 2.6-jl .98 suiting cs values in this case are ,0.008 pf typical series resonant crystals in? this fk exhibit equivalent cx capacity values 01 0.003 pf and the maximum series resistant for?the mc12061 is 155 ohms. again th~?r both items 1 and 2 above are met. u,. w? g~ww... ??,,?,, (t j. ?..11.0, l?, .,,u ..aq . *???, j.u,,, & is satisfied. usi#&,,the-;me formulas to determine the required rd fnr th. mp 17na 1 circ,lit tho ainaa fn[ward bias cur. an~f~$c~fculate rs and xs at 20 mhz for the mc 12061 it. to keep the $g~&:$#in rd = xd = 7.95 k ohms, giving a new value of diode low. at ?x$~$= xs = 3.97 k ohms. this vrdue of rs is much greater ----- 1., ~a/~?& = 19.4-j10 ?kan 155 ohms, the maximum effective resistance speci- ,,.!;,* ,, .:a* fication for the mc12061. therefore, the oscillator will ; ohms. the ra~c. and o.oo&~%l,$k? now have sufficient isolation from the unselected crystals ,equencp:%;g~p to prevent erratic performance. f o.q*$:~pto the vrdues used for r1, r3, r5, r7 and r9 are 82 k ?a??wation ?e q%q$ ohms, and 10 k ohms for figures 1 and 2 respectively. . . . . . -.w:&,~#~r*ments of ? .$.\.,, ,j**~> .?>.\\?:p+~ oscillator ac loading ~ ,, ,, ]*:. +;., ,,gap%,>%>~2x;e++ oscillator ac loading must be minimized to reduce fre- decoupling unselected c~~s~ls quency pulling and sine wave distortion. for the circuits : isolating unselected crysta~~*\@~important from the shown in figures 1 and 2 the ac loading is primardy standpoint of minimizing,~~req~~y pull of the selected attributable to the biasing networks for the five diodes crystal, and insuring tha${~~,~~scfllator will lock on a new (dl -d5). ml bias elements contribute to an effective ac :/a, crystal frequency w~~,:~~ttified from a previous one. load, regardless of which crystal position is selected. this the objective f~~,~e~dupling unselected crystals is to occurs because the rf signal is coupled through the place a high imp@@@k/in series with them. the msd7000 parallel capacitance (co) of the unselected crystals. typically haq~~.7~~f of shunt capacitance cd (refer to due to a greater sensitivity to ac loading of the figure 3j$~$ ~n?= 1.6 volts, and the mpn3401 typically mc 12060, additional elements are used in the switching 0.75 ~~~~~~!~volts of reverse bias. since rd is extremely networks for this device. an rf choke, l1, is incorpor- large $~%e reverse bias condition, the resulting diode rs ated to minimize the loading effects of the common bias resistance will not be exceptionally large and cs will ap- resistor, r17. in addition, a modified approach is used to proximately equal cd. this series capacitance is 30 to bias diodes d1 through d5. the networks (d6, r18) 300 times greater than typical values of equivalent crystal through (d1o, r22) are added to minimize ac loading series resonant capacitance (ca. therefore, the total and, at the same time, supply sufficient forward current with a 5-volt supply. one diode (d i-d5) in the msd7000 cs cx ) decreases by dud diode package is used to switch the crystal and series equivalent capacitance (ct = cs t cx the second diode (d6-di o) is used for reducing ac load- ing. r18 throu~ r22 are essential to supply a small diodes d1 -d5 ody 3.270 to 0.33% respectively. this, combined with a . . . amount of current for reverse bias of low value for rs, maintains considerable coupling be- corresponding to the unselected crystals.
figure 1 ? schematic diagra,p o~crystal switcnng for the MC12060 r1, r3, i5, r7, r9= 10k~ c6 ?cio= 0.01 pf r2, r4, r6, r8, rio, ri? = 2.2 k~ d1 ? d5 = mpn3401 r12, r13, r14, r15, r16= l.okfi xl ?x5=2.0mhz?20 mhz cl = 0.001 pf c2 ?c5 =0.1 pf figure 2 ? schematic diagram of crystal switching for the mc12061 ,.. .: ?..... . . . . . . . . . . . . ,. .:
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