ESIGN AND OPERATION OF AUTOMATIC GAIN …

1 This article is intended to provide insightinto the effective OPERATION of variablegain amplifiers (VGA) in AUTOMATIC gaincontrol (AGC) applications. Figure 1is a gen-eral block diagram for an AGC loop. The in-put signal passes through the VGA to producethe output level to be stabilized. The detec-tor s output is compared against a setpointvoltage to produce an error signal, which isthen integrated to produce a gain control volt-age. This is applied to the control input of theVGA. The attenuator shown be-tween the VGA and the detec-tor is used to align the maxi-mum output level of the VGAwith the maximum input levelof the the course of this articleseveral key issues will be ad-dressed, including VGA types,loop dynamics, detector types,the operating level of VGA andthe operating level of the detec-tor.

2 Then an example applica-tion revolving around anAD8367 VGA will be presentedfor further discussion of TYPEST here are two major classes of VGA in usetoday. The first is the so-called IVGA (inputVGA), which can be regarded as a passivevariable attenuator followed by a fixed-gainamplifier. The second type is the output VGA(OVGA), which is essentially equivalent to afixed-gain amplifier followed by a passive IVGA is the preferred choice for a re-ceive AGC system because the available out-DESIGN ANDOPERATIONOFAUTOMATICGAINCONTROLLOOPSF ORRECEIVERS INMODERNCOMMUNICATIONSSYSTEMSTUTORIALDAN AWHITLOWA nalog DevicesBeaverton, ORThis article is intended to provide insight into theeffective OPERATION of variablegain amplifiers (VGA) in AUTOMATIC gain control(AGC) (LINEAR IN dB)ERRORINTEGRATORSIGNALINPUTRSSI OUTPUTSETPOINT INPUTSIGNALOUTPUTDETECTORRinRstab +RF ATTENC integFig.

3 1 VGA-based AGC loop block diagram. put level at low distortion is relativelyindependent of the gain setting. Thisis the desired trait for an AGC sys-tem, whose very object is to maintaina constant output in the face of vary-ing input signal amplitude. The OVGA is generally ill suitedto AGC applications because of its re-duced output signal handling capabil-ity at low gain settings and thereforewill not be discussed further a single IVGA is used in asituation in which the VGA sets thesystem noise floor, the output SNR isessentially independent of the inputsignal; it does not improve as is oftenpreferred. Occasionally, it is desirableto cascade two VGAs in order to ame-liorate this behavior or simply to ob-tain more gain control range.

4 Doingso requires proper coordination ofthe gain control inputs of the two de-vices. If the gain control of only the sec-ond stage VGA is manipulated in theweak signal regime, the signal level tothe first stage VGA s amplifier in-creases with increasing input level, sothe output SNR improves with in-creasing input level. It is necessary tohand off the gain control from thesecond stage to the first stage onlywhen overload of the first stage s am-plifier is imminent. Alternatively, the two gain controlinputs may simply be driven in paral-lel, in which case the output S/N (ex-pressed in dB) improves at half therate at which the input level (also ex-pressed in dB) rises. In cases wherethe VGAs used have residual ripple intheir gain control functions, an addi-tional benefit of this approach can beobtained if the two gain control inputsignals are intentionally offset by halfthe period of the ripple.

5 This can pro-vide considerable reduction of of the benefits of using anIVGA in an AGC loop is that theVGA s gain control voltage bears anaccurate logarithmic relationship tothe input signal level when the loop isin equilibrium. This means that thegain control voltage may also be usedas an excellent received signalstrength indicator (RSSI).LOOP DYNAMICSR esponse time is an important is-sue when designing any AGC is usually a compromise be-tween having the loop respond to un-desired input level fluctuations asrapidly as one would like, and havingit undesirably modify amplitude mod-ulation on the signal. Additionally,large and/or abrupt changes in the in-put level may lead to unacceptablerecovery behavior, necessitating fur-ther adjustments of the responsetime.

6 The issue of excessive loop band-width deserves a bit more explana-tion. If the loop responds too quickly,it will introduce undesired gain mod-ulation arising from the loop s effortsto stabilize the output level of a signalcontaining legitimate amplitude mod-ulation. This is referred to as gainpumping. In the context of digitalmodulation, the presence of apprecia-ble gain pumping can result in signifi-cant modulation errors and perhapseven noticeable spectral re-growth inextreme cases. A tolerable value ofgain pumping would generally be onlya fairly small fraction of 1 TYPES (DETECTORLAW)One convenient aspect of an AGCloop is that the detector need not nec-essarily have a very wide dynamicrange over which it obeys any particu-lar law.

7 This is because the detectoroperates at a constant average levelwhen the AGC loop is in equilibrium;thus, the detector should only need tocope accurately with the level rangeassociated with a modulated , as mentioned earlier, thedetector s response law (that is linear,log, square law, etc.) can play a signifi-cant role in determining the loop s dy-namic response during large, abruptchanges in signal level. Perhaps moreimportantly, the detector s responselaw influences the dependency of theloop s equilibrium level on the input swaveform or crest detector types will be consid-ered here: envelope detector; square-law detector; true-RMS detector; andlog DETECTOR(RECTIFIER)The output voltage of the enve-lope detector is proportional to themagnitude of the instantaneous RFinput voltage.

8 Assuming that suffi-cient low pass filtering is applied atits output to eliminate RF ripple, thisdetector produces a voltage propor-tional to the envelope amplitude ofthe RF signal. Assuming that the loop s band-width is made sufficiently small as toavoid significant gain pumping, theeffect of the loop using an envelopedetector is to stabilize the averagerectified voltage of the signal. The re-sulting power is therefore dependenton the RF signal s envelope wave-form. Such a loop acting on a con-stant-envelope signal such as GSMwill produce an average output powerwhich is different than that for aheavily-amplitude-modulated signal,such as CDMA or output of the envelope detec-tor cannot go negative no matter howweak the input signal, but may reachextreme positive values in response tovery strong signals.

9 Starting with theAGC loop in equilibrium, a suddenlarge increase in input amplitudecauses a very large initial increase indetector output, which very rapidlydrives the loop towards lower the other hand, an abrupt reduc-tion of the input signal level (no mat-ter by how many dB) cannot reducethe detector output below zero, andthe loop s best response is to slew to-wards equilibrium at a fairly low rateuntil the detector output begins tochange by a significant fraction of thereference voltage, at which point therecovery trends towards an exponen-tial decay. In the slew rate limited re-gion, the gain of the signal path isvarying at a constant number of dBper second.

10 Figure 2shows the behavior ofsuch a loop for a large input levelstep (note that curves for all four de-tector types are superimposed on thisplot). These results were obtainedTUTORIAL+20+100 10 20 30 40 5030252015 TIME (ms)LOG LINBNVRMS SQLAW1050Po (dB) Fig. 2 Simulated response of AGC loopto large amplitude steps for various simulations in which the VGAhas representative limits on the gainrange and on the maximum outputlevel. The detectors contrived forthese simulations have no particularlimits, on the grounds that in mostpractical situations the designer willscale the circuit so that the detectordoes not limit appreciably before theVGA DETECTORThis type of detector has an in-stantaneous output which is propor-tional to the square of the instanta-neous RF input voltage, which isequivalent to saying that its output isproportional to input power.