(MC12060 / MC12061) Crystal Switching Methods

www.DataSheet4U.com CRYSTAL MC1206[ @ “ ‘Semiconductor products .. gnc. ~ MOTOROLA ,, ..- ,. www.DataSheet4U.com ,:” cRYsTAL SWITCHINGMETHODs FOR .,,: . MCI 2060/MC12061OSCILLATORS : is dso an economical dual diode in the configuration needed for tfis circuit. The MPN3401 is used with the MC 12061 circuit because it offers a large off-to-on impedance ratio for low dc bias currents at frequencies within the range of the ,*!. MC12061. ‘*{,3, $};. ,,,?,$, ,,:2:,>, *1~.’~~ ......!.tbk+ $ ~~ ,$, DC BIAS REQUIREMENTS .ij,, l...J, ~orward bias for ,the. ,desired cryst~=~$~~yti:g diode (Dl ,’ D2; .D3Y D4, or .D5) is applie~,~b~~~~ing the five ‘position switch. The’ bias current is#~~,#~y set. by R17 desired to electronically switch series-resonant crystals ~ ‘ !<<,~r,. , ‘:*t.mw . and R2 (R4, R6, R&,’and R1.Oh$~e, ,%)} (5.0 volts). .* *...,.,. ~?+.,. *<-$. V-VD IQ’-’’*>(, - (~) (For MC1206 ‘1) . GENERAL “: “’$~lR~+{R~711 (R3+ ‘~R13)} The MC 12060 and MC 12061 crystal oscillators are :J&~$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~~~~$~~ ., reverse biased to minimize their capacitance. This is outputs are capable of driving an ac load of ~@#* at accomplished with a single polarity supply by using pullup 500 mVp-p (typical) when an external re~~~~Y~@used ~ resistors (R12, R13, R14, RI 5, and R16) from the posito increase the current in the emitter f~~~f” output. tive potential to each switch terminal. Therefore, the The ECL and TTL outputs are capabl~:$~~~ting five and ~ cathodes of the diodes corresponding to the unselected .* ,, . ..T!c,:11 ten gate loads respectively. crystals are pulled up to approximately the supply voltSeries 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~ ~.,\> ‘{,~~~ ,:$. switching compone,~&)$ 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$ki;&~AEfigineering Bulletins EB-58 and EB-59. ADDITIONAL CONSIDERATIONS A sufficient amount of forward current through the Sch~at$+ ‘“diagrams for the MC 12060 and MC 12061 diode selectirig the desired crystal is required to insure cryg{~$~~ltching circuits are given in Figures 1 and 2 a low value for diode resistance KD (see Figure 3). This is ~~~~~tively. The same basic technique is employed f~~each IC except that an addifiond diode-resistor pair important for two reasons: 1. To minimize the effects of diode capacity on the (D6, R18 through D1O, R22) is incorporated for 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 junc2. 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 ‘MC12060 oscillator because From Figure 3 it is apparent that as RD is made smalof its low capacitance (1.5 pF max. for VR = O volts). It ler, Xs is decreased and Cs is increased. A large value for INTRODUCTION ‘ Crystal switching can be achieved electronically for the MC12060 and MC 12061 crystal oscillator integrated circuits by utilizing diodes as RF switches. The switching is controlled by applying a forward bias to the diode associated with the desired crystal and applying a reverse bias to the remaining diodes, related to the unselected crystals. in addition to functioning with the MC 12060/ MC 12061 IC’S, the switching circuit designs described ., here can dso be used’ in other applications where it i: ‘, ~~, Circuit completa fully diagrams checked and external to Motorola for products are included purposes However, devices as a means of illustrating givan. The typical for semiconductor inaccuracies. righ= applications; Note Furthermore, of Motorola consequently, has been careinformation information sufficient construction reliabla. is not necessarily information tha patent, in this Application is believed to be antirely of:the no responsibility describad is assumed such doss not convey so the purchaser semiconductor any. Iicanse. undar Inc.b;.others. www.DataSheet4U.com Loading and therefore frequency pulling will be greater for higher frequency crystals and will increase as the total number of crystals. to be switched is increased. However, by using the switching techniques shown in Figures 1 and 2, any frequency pulling in addition to that for a single crystal connected directly to pins S and 6 (i.e. pulling caused by the ICS alone) ‘is negligible below approximately 1 MHz for the MC 12060 and 15 MHz for the MC 12061. Measurements of this additiond pulling are summarized in Table 1. Typical frequency pulling values selecting the nominrd 1.0 MHz crystal is approximately -0.0040 +‘ 0.0031 = -0.0009 percent. Similarly, absolute pulling for the 8.0 MHz crystal becomes -0.004 +0.0001 = -0.0039 percent. Pulling effects of the switching circuits when selecting the 0.2 MHz crystal offset pulling caused by the IC to give approximately zero absolute crystal pull. When desirable, a trim capacitor can be added in series with the crystals and adjusted to pull the oscillq(lor up in , ,,,,~~.iv, ~.,, frequency. ~},:,: \$,,, Several options are possible to reduce #~~~Ming for ,,: ~ : .. ,?1., Device I MCI 2060 0.5 1.0 2.0 2.5 F:, ,,,,*&#@’l :.., y %?S 13.4 R .f I 20.0 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 @~~ching circuits. The Table II values have been take~,~$~j,re?*rence jn establishing the pulling (noted in T&ti~~~~d~used by the switching networks. When using t~~~&~sti switchk~), .@’* ing circuits, complete pulling from *’*hystd’s series .. .,! >, resonant frequency is obtained by ~~~%~]~~ly adding the respective values in Tables I and~$k~@~,.&xample, absolute crystal pulling for the five cr~s?~,:,@tching system when ..>>: ...,,.!,,,. ~t.$~< “ both the MC 12060 and MC 12061 crystal switching circuits. Using a higher voltage supply for the bias networks will allow larger values of bias resistors to be used at the same diode current, resulting in reduced loading. Also, RF decoupling chokes may be added between resistors R2, R4, R6, R8, and RIO and capacitors C6 through C 10. Where frequency pulling is not as critical, L1 in Figure 1 may be eliminated. These options are left to the discretion of the user. www.DataSheet4U.com s+. ~!. ~ 20 MHz. To maximize the s~!~~~q~valent resistor (Rs), the parallel resistor R~ is&4,@,<8qual to the reactance XD at the highest operatin&~~~~#&flcy. For the MC 120i 160, the vlaues of RD = XD:= 7~:S~? ohms give Rs = Xs = 39.7 k ohms. Since Rs i$~&&&much greater than 4 k ohms, the unselected cr~~h~%ill be virtually isolated from the oscfll,ator. ~@~&+~&~ationwill become greater with a de: crease iR fre~ency. Usi#&,,the-;me formulas to determine the required RD is satisfied. an~f~$c~fculate RS and XS at 20 MHz for the MC 12061 Fnr th. MP 17nA 1 circ,lit tho AinAa fn[ward bias cur. . $g~&:$#in RD = XD = 7.95 k ohms, giving a new value of rent is 1.15 mA. This current is sufficie] It. to keep the “X$~$= XS = 3.97 k ohms. This vrdue of RS is much greater series “impedance ‘of the MPN3401 PTN diode low. At ----?kan 155 ohms, the maximum effective resistance speci1., LL@ 19.4-j10 2 MHz the impedance is nominally ~a/~?& = ,, .:A* ,,.!;,* fication for the MC12061. Therefore, the oscillator will ofims and at 20 MHz 3.3/-37° =, 2.6-jl .98; ohms. The ra~c. now have sufficient isolation from the unselected crystals suiting Cs values in this case are ,0.008 pF and O.OO&~%l,$k” ,equencp:%;g~p to prevent erratic performance. Typical series resonant crystals in’ this fK The vrdues used for R1, R3, R5, R7 and R9 are 82 k F exhibit equivalent CX capacity values 01 o.Q*$:~pto ohms, and 10 k ohms for Figures 1 and 2 respectively. 0.003 pF and the maximum series resistant ‘e q%q$ “A”?wation for”the MC12061 is 155 ohms. Again th~”r . .... -.w:&,~#~r*ments of ‘.$..,, ,J**~> .’>.\?:p+~ both items 1 and 2 above are met. OSCILLATOR AC LOADING ~ ,, ,, ]*:. +;., Oscillator ac loading must be minimized to reduce fre,,Gap%,>%>~2x;e+