Motor de Búsqueda de Datasheet de Componentes Electrónicos |
|
AD8315ARM-REEL Datasheet(PDF) 10 Page - Analog Devices |
|
AD8315ARM-REEL Datasheet(HTML) 10 Page - Analog Devices |
10 / 20 page REV. B AD8315 –10– Control Loop Dynamics In order to understand how the AD8315 behaves in a complete control loop, an expression for the current in the integration capacitor as a function of the input VIN and the setpoint voltage VSET must be developed. Refer to Figure 3. SETPOINT INTERFACE LOGARITHMIC RF DETECTION SUBSYSTEM 3 1 VSET RFIN 4 CFLT FLTR 7 VAPC ISET = VSET/4.15k IDET IERR IDET = ISLPLOG10 (VIN/VZ) VSET VIN 1.35 Figure 3. Behavioral Model of the AD8315 First, the summed detector currents are written as a function of the input: II V V DET SLP IN Z = log ( / ) 10 (3) where IDET is the partially filtered demodulated signal, whose exact average value will be extracted through the subsequent integration step; ISLP is the current-mode slope and has a value of 115 mA per decade (that is, 5.75 mA/dB); VIN is the input in volts-rms; and VZ is the effective intercept voltage, which, as previously noted, is dependent on waveform but is 316 mV rms (–70 dBV) for a sine wave input. Now the current generated by the setpoint interface is simply: IV k SET SET =W /. 415 (4) The difference between this current and IDET is applied to the loop filter capacitor CFLT. It follows that the voltage appearing on this capacitor, VFLT, is the time integral of the difference current: Vs I I sC FLT SET DET FLT () ( – )/ = (5) = W Vk I V V sC SET SLP IN Z FLT /. – log ( / ) 415 10 (6) The control output VAPC is slightly greater than this, since the gain of the output buffer is ¥1.35. Also, an offset voltage is deliberately introduced in this stage; this is inconsequential since the integration function implicitly allows for an arbitrary constant to be added to the form of Equation 6. The polarity is such that VAPC will rise to its maximum value for any value of VSET greater than the equivalent value of VIN. In practice, the VAPC output will rail to the positive supply under this condition unless the control loop through the power amplifier is present. In other words, the AD8315 seeks to drive the RF power to its maximum value whenever it falls below the setpoint. The use of exact integration results in a final error that is theoretically zero, and the logarithmic detection law would ideally result in a constant response time following a step change of either the setpoint or the power level, if the power-amplifier control function were likewise linear-in-dB. This latter condition is rarely true, however, and it follows that in practice, the loop response time will depend on the power level, and this effect can strongly influence the design of the control loop. Equation 6 can be restated as: Vs VV V V sT APC SET SLP IN Z () log ( / ) = - 10 (7) where VSLP is the volts-per-decade slope from Equation 1, having a value of 480 mV/decade, and T is an effective time constant for the integration, being equal to 4.15 k W ¥ CFLT/1.35; the resistor value comes from the setpoint interface scaling Equation 4 and the factor 1.35 arises because of the voltage gain of the buffer. So the integration time constant can be written as: TC in s when C is in nF FLT = 307 ., m expressed (8) To simplify our understanding of the control loop dynamics, begin by assuming that the power amplifier gain function actu- ally is linear-in-dB. Also use voltages to express the signals at the power amplifier input and output, for the moment. Let the RF output voltage be VPA and its input be VCW. Further, to characterize the gain control function, this form is used: VG V PA O CW VV APC GBC = 10 (/) (9) where GO is the gain of the power amplifier when VAPC = 0 and VGBC is the gain-scaling. While few amplifiers will conform so conveniently to this law, it provides a clearer starting point for understanding the more complex situation that arises when the gain control law is less ideal. This idealized control loop is shown in Figure 4. With some manipulation, it is found that the characteristic equation of this system is: Vs VV V V kG V V sT APC SET GBC SLP GBC O CW Z O () ()/ – log / = () + 10 1 (10) where k is the coupling factor from the output of the power amplifier to the input of the AD8315 (e.g., ¥ 0.1 for a “20 dB coupler”), and TO is a modified time constant (VGBC /VSLP)T. This is quite easy to interpret. First, it shows that a system of this sort will exhibit a simple single-pole response, for any power level, with the customary exponential time domain form for either increasing or decreasing step polarities in the demand level VSET or the carrier input VCW. Second, it reveals that the final value of the control voltage VAPC will be determined by several fixed factors: Vt V V V kG V V APC SET GBC SLP O CW Z =• () = () ( ) /– log / 10 (11) Example Assume that the gain magnitude of the power amplifier runs from a minimum value of ¥0.316 (–10 dB) at VAPC = 0 to ¥100 (40 dB) at VAPC = 2.5 V. Applying Equation 9, GO = 0.316 and VGBC = 1 V. Using a coupling factor of k = 0.0316 (that is, a 30 dB directional coupler) and recalling that the nominal value of VSLP is 480 mV and VZ = 316 V for the AD8315, first calculate the range of values needed for VSET to control an output range of 33 dBm to –17 dBm. This can be found by noting that, in the steady state, the numerator of Equation 7 must be zero, that is: VV kV V SET SLP PA Z = log ( / ) 10 (12) when VIN is expanded to kVPA, the fractional voltage sample of the power amplifier output. Now, for +33 dBm, VPA = 10 V rms, this evaluates to: VmV V V SET () . log ( / ) . max == 048 316 316 1 44 10 m (13) For a delivered power of –17 dBm, VPA = 31.6 mV rms: VmV V V SET () . log ( / ) . min == 048 1 316 024 10 m (14) |
Número de pieza similar - AD8315ARM-REEL |
|
Descripción similar - AD8315ARM-REEL |
|
|
Enlace URL |
Política de Privacidad |
ALLDATASHEET.ES |
¿ALLDATASHEET es útil para Ud.? [ DONATE ] |
Todo acerca de Alldatasheet | Publicidad | Contáctenos | Política de Privacidad | Intercambio de Enlaces | Lista de Fabricantes All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |