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)

3 ERRORINTEGRATORSIGNALINPUTRSSI OUTPUTSETPOINT INPUTSIGNALOUTPUTDETECTORRinRstab +RF ATTENC integFig. 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.

4 Occasionally, it is desirableto cascade two VGAs in order to ame-liorate this behavior or simply to ob-tain more gain control range. 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.

5 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. 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.

6 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. The issue of excessive loop band-width deserves a bit more explana-tion.

7 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.

8 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.

9 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.

10 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.