4/24/2005 Automatic Gain Control notes 1/1 Jim Stiles The Univ. of Kansas Dept. of EECS 5. Receiver Gain and AGC We find that a detector/demodulator likewise has a dynamic range, a value that has important ramifications in receiver design. HO: Instantaneous Dynamic Range Q: We have calculated the overall gain of the receiver, but what should this gain be? A: HO: Receiver Gain Q: How can we build a receiver with variable gain? What microwave components do we need? A: HO: Automatic Gain Control (AGC) HO: AGC Dynamic Range Q: How do we implement our AGC design? A: HO: AGC Implementation11/3/2006 Instantaneous Dynamic Range 1/3 Jim Stiles The Univ. of Kansas Dept. of EECS Instantaneous Dynamic Range Q: So, let’s make sure I have the right—any input signal with power exceeding the receiver sensitivity but below the saturation point will be adequately demodulated by the detector, right? A: Not necessarily! The opposite is true, any signal with power outside the receiver dynamic range cannot be properly demodulated. However, signals entering the receiver within the proper dynamic range will be properly demodulated only if it exits the receiver with the proper power. The reason for this is that demodulators, in addition to requiring a minimum SNR (i.e., SNRmin), likewise require a certain amount of power. If the signals enters the receiver with power greater that the MDS, then the signal will exit the receiver with sufficient SNR. However, the signal power can be too large or too small, depending on the overall receiver gain G. Q: How can the exiting signal power be too large or too small? What would determine these limits?11/3/2006 Instantaneous Dynamic Range 2/3 Jim Stiles The Univ. of Kansas Dept. of EECS A: Recall that the signal exiting the receiver is the signal entering the detector/demodulator. This demodulator will have a dynamic range as well! Say the signal power entering the demodulator (i.e., exiting the receiver) is denoted inDP. The maximum power that a demodulator can “handle” is thus denoted maxDP, while the minimum amount of power required for proper demodulation is denoted as minDP. I.E.,: min in maxDDDPPP≤≤ Thus, every demodulator has its own dynamic range, which we call the Instantaneous Dynamic Range (IDR): () ( ) ( )maxmax minDDDminDPIDR or IDR dB P dBm P dBmP==− Typical IDRs range from 30 dB to 60 dB. Rx insinRxPSNR out insDoutRxinDPPSNRSNR==11/3/2006 Instantaneous Dynamic Range 3/3 Jim Stiles The Univ. of Kansas Dept. of EECS To differentiate the Instantaneous Dynamic Range from the receiver dynamic range, we refer to the receiver dynamic range as the Total Dynamic Range (TDR): () () ()satsatininPTDR or TDR dB P dBm MDS dBmMDS==− Q: How do we insure that a signal will exit the receiver within the dynamic range of the demodulator (i.e., within the IDR)? A: The relationship between the signal power when entering the receiver and its power when exiting the receiver is simply determined by the receiver gain RxG : in inRx sDPGP= We simply need to design the receiver gain such that PD lies within the IDR for all signals insP that lie within the TDR. Big Problem Æ We find that typically TDR >> IDR. This can make setting the receiver gain RxG very complicated!11/3/2006 Receiver Gain 1/6 Jim Stiles The Univ. of Kansas Dept. of EECS Receiver Gain Let’s consider each element of a basic super-het receiver: 1. LNA - Required to make the receiver noise figure F as small as possible, thus making the receiver very sensitive. 2. Preselector – Required to reject all spurious-signal creating frequencies, while simultaneously letting the desired RF bandwidth pass to the mixer. 3. Mixer - Required for down-conversion; often sets the receiver compression point. 4. IF Filter - Required to suppress all mixer IF output signals, with the exception of the one desired signal that we wish to demodulate. Also determines the noise bandwidth B of the receiver. 5. IF Amp - Q: Why is this device required? What receiver parameter does it determine? 1 2 3 4 5 inP outDPP=11/3/2006 Receiver Gain 2/6 Jim Stiles The Univ. of Kansas Dept. of EECS A: It is true that the IF amplifier does not generally affect receiver bandwidth, or sensitivity, or saturation point, or image rejection. Æ However, the IF amp is the component(s) that we use to properly set the overall receiver gain. Say that we have designed a receiver with some specific TDR (i.e., MDS and satinP). This receiver will be connected to a demodulator with a specific IDR (i.e., minDP and maxDP). All we have left to do is determine the proper gain of the IF amplifier to give us the required gain of the overall receiver. This gain must satisfy two requirements: Requirement 1 -We know that the overall receiver gain RxG must be sufficiently large such that the smallest possible receiver input signal (insPMDS=) is boosted at least to the level of the smallest required demodulator signal (minDP). Thus, the absolute smallest value that the receiver gain should be is minRxG: () () ()minmin minDRx minDPG or G dB P dBm MDS dBmMDS−11/3/2006 Receiver Gain 3/6 Jim Stiles The Univ. of Kansas Dept. of EECS Requirement 2 - Likewise, the overall receiver gain RxGmust be sufficiently small to insure that the largest possible receiver input signal (i.e., in satsinPP=) arrives at the demodulator with a power less than to the maximum level minDP. Thus, the absolute largest value that the receiver gain should be is maxRxG: () () ()maxmax max max satDRx Rx inDsatinPG or G dB P dBm P dBmP− Q: Seems simple enough! Just select an IF amplifier so that the overall receiver gain lies between these two limits: min maxRx Rx RxGGG<< Right? A: Not exactly. We are typically faced with a big problem at this point in our receiver design. To illustrate this problem, let’s do an example. Say our receiver has these typical values:11/3/2006 Receiver Gain 4/6 Jim Stiles The Univ. of Kansas Dept. of EECS 10206090satinmaxDminDPdBmPdBmPdBmMDS dBm=−=−=−=− Note then that 80TDR dB= and 40IDR dB=. Thus, this receiver must have a gain of at least: ()()()()60 9030min minRxDG dB P dBm MDS dBmdB=−=− − −= But likewise have a gain of no more than: ()()()()20 1010max max satRx inDG dB P dBm P dBmdB=−=− − −=− So here’s our solution! The receiver gain must be any value greater than 30 dB, as long as