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AD8315-EVAL Folha de dados(PDF) 11 Page - Analog Devices |
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AD8315-EVAL Folha de dados(HTML) 11 Page - Analog Devices |
11 / 20 page REV. B AD8315 –11– Check: The power range is 50 dB, which should correspond to a voltage change in VSET of 50 dB ¥ 24 mV/dB = 1.2 V, which agrees. Now, the value of VAPC is of interest, although it is a dependent parameter, inside the loop. It depends on the characteristics of the power amplifier, and the value of the carrier amplitude VCW. Using the control values derived above, that is, GO = 0.316 and VGBC = 1 V, and assuming the applied power is fixed at –7 dBm (so VCW = 100 mV rms), the following is true using Equation 11: VV V V kG V V V V APC SET GBC SLP O CW Z () ( )/ – log / (. )/ . – log( . . . / ) .–. . max = =¥ ¥ ¥ == 10 10 1441 0480 0316 0 316 0 1 316 30 05 25 m (15) VV V V kG V V V zero APC SET GBC SLP O CW Z () ( )/ – log / (. )/ . – log (. . . / ) .– . min = =¥ ¥ ¥ == 10 10 0241 0480 0316 0 316 0 1 316 05 05 m (16) both of which results are consistent with the assumptions made about the amplifier control function. Note that the second term is independent of the delivered power and a fixed function of the drive power. RF PA DIRECTIONAL COUPLER VRF VCW RF DRIVE: UP TO 2.5GHz AD8315 VSET VIN = kVRF CFLT RESPONSE-SHAPING OF OVERALL CONTROL- LOOP (EXTERNAL CAP) VAPC Figure 4. Idealized Control Loop for Analysis Finally, using the loop time constant for these parameters and an illustrative value of 2 nF for the filter capacitor CFLT: TV V T snF s OGBC SLP = =¥ = (/ ) (/ . ) . ( ) . 10 48 3 072 12 8 mm (17) Practical Loop At the present time, power amplifiers, or VGAs preceding such amplifiers, do not provide an exponential gain characteristic. It follows that the loop dynamics (the effective time constant) will vary with the setpoint, since the exponential function is unique in providing constant dynamics. The procedure must, therefore, be as follows. Beginning with the curve usually provided for the power output versus the APC voltage, draw a tangent at the point on this curve where the slope is highest (see Figure 5). Using this line, calculate the effective minimum value of the variable VGBC and use it in Equation 17 to determine the time constant. Note that the minimum in VGBC corresponds to the maximum rate of change in the output power versus VAPC. For example, suppose it is found that, for a given drive power, the amplifier generates an output power of P1 at VAPC = V1 and P2 at VAPC = V2. Then, it is readily shown that: VV V P P GBC = 20 21 2 1 (– )/(– ) (18) This should be used to calculate the filter capacitance. The response time at high and low power levels (on the “shoulders” of the curve shown in Figure 5) will be slower. Note also that it is sometimes useful to add a zero in the closed-loop response by placing a resistor in series with CFLT. For more about these matters, refer to the Applications section. VAPC – V 33 0 23 13 3 0.5 1.0 1.5 2.0 2.5 –7 V2, P2 V1, P1 Figure 5. Typical Power-Control Curve A Note About Power Equivalency In using the AD8315, it must be understood that log amps do not fundamentally respond to power. It is for this reason that dBV (decibels above 1 V rms) are used rather than the commonly used metric of dBm. The dBV scaling is fixed, independent of termi- nation impedance, while the corresponding power level is not. For example, 224 mV rms is always –13 dBV (with one further condition of an assumed sinusoidal waveform; see the AD640 data sheet for more information about the effect of waveform on logarithmic intercept), and this corresponds to a power of 0 dBm when the net impedance at the input is 50 W. When this impedance is altered to 200 W, however, the same voltage corresponds to a power level that is four times smaller (P = V 2/R) or –6 dBm. A dBV level may be converted to dBm in the special case of a 50 W system and a sinusoidal signal by simply adding 13 dB (0 dBV is then, and only then, equivalent to 13 dBm). Therefore, the external termination added ahead of the AD8315 determines the effective power scaling. This will often take the form of a simple resistor (52.3 W will provide a net 50 W input), but more elaborate matching networks may be used. The choice of impedance determines the logarithmic intercept, that is, the input power for which the VSET versus PIN function would cross the baseline if that relationship were continuous for all values of VIN. This is never the case for a practical log amp; the intercept (so many dBV) refers to the value obtained by the minimum error straight line fit to the actual graph of VSET versus PIN (more generally, VIN). Where the modulation is complex, as in CDMA, the calibration of the power response needs to be adjusted; the intercept will remain stable for any given arbitrary waveform. When a true power (waveform independent) response is needed, a mean-responding detector, such as the AD8361, should be considered. The logarithmic slope, VSLP in Equation 1, which is the amount by which the setpoint voltage needs to be changed for each decibel of input change (voltage or power), is, in principle, independent of waveform or termination impedance. In practice, it usually falls off somewhat at higher frequencies, due to the declining gain of the amplifier stages and other effects in the detector cells (see TPC 13). |
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