Diode Charge Pump AM-FM Demodulators

Transcription

1 Diode hage Pum AM-FM Demodulatos Fequency-to-voltage convetes fom at of a wide vaiety of instumentation cicuits. hey also find use in adio as FM demodulatos. One inteesting configuation fo this alication is the Diode hage Pum cicuit (DP), which also doubles as an AM detecto. he DP is basically a ulse-diven half-wave voltage double. Its use as a demodulato deives fom the analysis of chage tansfe taking lace between cicuit comonents. In this aticle we will attemt to exlain why the demodulation ocess takes lace in the DP. Following, the cicuit will be studied unde A sine-wave excitation. Let s begin then analysing a voltage double diven by a eiodic tain of single-olaity ulses having a duty cycle of 5% (Fig..a). We shall model this situation by a switch that toggles between a battey deliveing volts and a esisto R connected to gound (Fig..b). he switch stays in each osition equal eiods of time. In Fig..a, is esonsible fo uming chage towads the outut caacito, which acts as a esevoi. Oeation of the cicuit is as follows. When the switch is in osition a a ulse of height is alied to. he chage eceived by this caacito is distibuted between and esisto R. At the end of the ulse, dischages though R and D (switch in osition b ). Diode D does not conduct (is an oen cicuit) on this inteval. As a consequence, dischages though R. When the switch etuns to the a osition, the oeation cycle is eeated. If the ulse ate is sufficiently high, s dischage will be incomlete on each cycle and a continous cuent will flow though R. In the steady state, chage consevation dictates that: q q q () Hee, q is the chage eceived by e ulse; q is the chage tansfeed to, also e ulse (it estoes the chage lost by this caacito in the eceding cycle) and q is the chage that divets though R (faction of q that doesn t each ).

2 he voltage acoss inceases in an amount due to q. We may wite then: q () Assuming ideal diodes (zeo voltage do when conducting): whee o is the instantaneous value of the outut voltage. Substituting the above elationshi into eq.() yields: ( ) q (3) he voltage acoss inceases by an amount o due to q. Accodingly, we may wite: q (4) If we assume o << o, the outut voltage may be consideed to be aoximately constant. s dischage cuent in each oeating cycle can then be aoximated by the constant cuent i o / R. hage q may be found integating this cuent ove one half cycle of the inut signal. hus: q i dt R dt Woking out the value fo o yields: R (5) and: q (6) R is the eetition eiod of the inut ulses. Fo o to be much smalle than o, the following estiction must hold: R

3 o: R. (7) 5 Being the outut voltage o aoximately constant, we may wite: Substituting eqs. (3), (6) and (8) into eq.(): q dt (8) R R ( ) R R R Solving fo o we obtain, with /f: fr (9) fr We must bea in mind that f is the ulse ate o numbe of ulses e second. Fo thee to exist lineaity between o and f, it must be satisfied that: o that: Unde these conditions: fr R () R f () lealy, a linea elationshi exists between o and the ulse ate f, and also between the outut and the height of the inut ulses. he outut linealy follows any fequency o amlitude inut changes. Hence, the DP may act as an AM/FM demodulato. Sine-wave excitation analysis he DP was subjected to tests with AM and FM modulated A sine-wave inuts. In each case, successful ecovey of the modulating signal could be achieved. It has then

4 been found advisable to analyse the demodulation ocess with these new inut conditions. Sine excitation suggests that it is best to look at the cicuit as being a half-wave voltage double. With this in mind, if the souce voltage g has an amlitude and fequency f, then fo the unloaded case the steady-state outut voltage will be v o. Note that caacito will be chaged to volts (Fig..a). Uon connection of a esistive load R (Fig..b), the outut voltage v o will no longe be a ue D value. It will consist of a D comonent o and some ile sueimosed on it. D will conduct fo a bief time in the neighbohood of the ositive eaks of g, tansfeing chage fom to. his last caacito dischages acoss R when D is off, which occus duing most of the time inteval between ositive eaks of the inut voltage. elenishes its chage when D biefly conducts in the neighbohood of the negative eaks of g. As a esult of these actions v o will have some ile added, as stated befoe, and also will the voltage acoss. Let o be the eak-to-eak value of the ile sueimosed on o, and that of the ile comonent acoss. If o << o, then: () R hage consevation thoughout one cycle of the inut signal dictates now that: hus:

5 o will be given by the exession: R 5. Solving fo o yields: fr.5 (3) If we let that:.5 fr (4) then:.5 fr (5) which is the linea elationshi we ae looking fo. hen, fo an FM signal: f R fm.5 (6) whee ofm eesents the outut voltage vaiations following fequency changes f of the inut.

6 Fo an AM signal: fr am (7).5 Hee, oam eesents the outut voltage vaiations following amlitude changes of the inut signal. hus, the outut linealy follows the modulating signal. It is desiable that the ile at the unmodulated inut fequency be much smalle than the D outut. hen, the following must also be satisfied: R o: fr (8) Summaizing: fr. 5 alculations have assumed ideal diodes, so coections ae needed to comensate fo eal wold-diode voltage dos. Fo the no-modulation and FM cases, - D may be substituted fo, whee D is the eak-voltage do in diodes D and D (assumed equal). Highe caie amlitudes should hel ovecoming these voltage dos. Howeve, ovisions should be made to otect diodes fom excessive eak cuents. onducted measuements he cicuit deicted in Fig.3 was used fo the FM demodulation tests. With the selected values fo, and R, the following figues wee obtained: fr (egetfully, not much geate than ) and: fr 4.37 A Hewlett-Packad 86A Sweee Geneato was used as the signal souce fo a - olt amlitude.7mhz caie, and accodingly, the following values wee obtained at the inut of the two-tansisto amlifie stage:

7 -No modulation: o m D -FM modulated caie with f /-75kHz (calibation not checked) at a khz ate: ofm.4m eak -AM modulated caie at 3% with khz,.3 eak: oam 4m eak Amlification was used in the FM case fo easy viewing of the ecoveed modulation. Fig.3 icuit used fo testing FM demodulation with the DP Ramon agas Paton vagas@inictel.gob.e Lima-Peu, South Ameica Januay 3 st, 5

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