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KU EECS 622 - B. The Super-Heterodyne Receiver

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10/9/2007 The Super Heterodyne Receiver notes 1/2 Jim Stiles The Univ. of Kansas Dept. of EECS B. The Super-Heterodyne Receiver The “super-het” is by far the most popular receiver architecture in use today. HO: The Super-Heterodyne Receiver Q: So how do we tune a super-het? To what frequency should we set the local oscillator? A: HO: Super-Heterodyne Tuning Another vital element of a super-het receiver is the preselector filter. HO: The Preselector Filter Q: So what should this preselector filter be? How should we determine the required order of this filter? A: HO: The Image and Third-Order Signal Rejection Q: I have heard of some receivers being described as up-conversion receivers, what exactly are they? A: HO: Up-Conversion There are many variants of the basic super-het receiver that can improve receiver performance. HO: Advanced Receiver Designs10/9/2007 The Superhet Receiver 1/9 Jim Stiles The Univ. of Kansas Dept. of EECS The Super-Heterodyne Receiver Note that the homodyne receiver would be an excellent design if we always wanted to receive a signal at one particular signal frequency (sf, say): No tuning is required! Moreover, we can optimize the amplifier, filter, and detector performance for one—and only one—signal frequency (i.e., sf). Q: Couldn’t we just build one of these fixed-frequency homodyne receivers for each and every signal frequency of interest? ()1sGf f= ()ˆit narrow-band detector/ demodulator narrow-band amplifier antenna narrow-band filter ()()10ssffff=≈≠≈TT A Fixed-Frequency Homodyne Receiver10/9/2007 The Superhet Receiver 2/9 Jim Stiles The Univ. of Kansas Dept. of EECS A: Absolutely! And we sometimes (but not often) do. We call these receivers channelized receivers. ()11sGf f= ()1ˆit ()11ff=≈T ()2ˆit ()3ˆit ()Nˆit A Channelized Receiver # # # ()1Nff=≈T ()31ff=≈T ()21ff=≈T ()21sGf f= ()31sGf f= ()1sNGf f=10/9/2007 The Superhet Receiver 3/9 Jim Stiles The Univ. of Kansas Dept. of EECS But, there are several important problems involving channelized receivers. Æ They’re big, power hungry, and expensive! For example, consider a design for a channelized FM radio. The FM band has a bandwidth of 108-88 = 20 MHz, and a channel spacing of 200 kHz. Thus we find that the number of FM channels (i.e., the number of possible FM radio stations) is: 20 MHz100200 kHz= channels !!! Thus, a channelized FM radio would require 100 homodyne receivers! Q: Yikes! Aren’t there any good receiver designs!?! A: Yes, there is a good receiver solution, one developed more than 80 years ago by—Edwin Howard Armstrong! In fact, is was such a good solution that it is still the predominant receiver architecture used today. Armstrong’s approach was both simple and brilliant: Instead of changing (tuning) the receiver hardware to match the desired signal frequency, we should change the signal frequency to match the receiver hardware!10/9/2007 The Superhet Receiver 4/9 Jim Stiles The Univ. of Kansas Dept. of EECS Q: Change the signal frequency? How can we possibly do that? A: We know how to do this! We mix the signal with a Local Oscillator! We call this design the Super-Heterodyne Receiver! A super-heterodyne receiver can be viewed as simply as a fixed frequency homodyne receiver, proceeded by a frequency translation (i.e., down-conversion) stage. ()1IFGf f= ()ˆit ()()10IFIFffff=≈≠≈TT sacos tω LOAcos tω IF s LOfff=− Fixed Heterodyne Rx (IF Stage) Frequency Translation (RF Stage) tuning A Simple Super-Het Receiver Design10/9/2007 The Superhet Receiver 5/9 Jim Stiles The Univ. of Kansas Dept. of EECS The fixed homodyne receiver (the one that we match the signal frequency to), is known as the IF stage. The fixed-frequency IFf that this homodyne receiver is designed (and optimized!) for is called the Intermediate Frequency (IF). Q: So what is the value of this Intermediate Frequency IFf ?? How does a receiver design engineer choose this value? A: Selecting the “IF frequency” value is perhaps the most important choice that a “super-het” receiver designer will make. It has many important ramifications, both in terms of performance and cost. * We will discuss most of these ramifications later, but right now let’s simply point out that the IF should be selected such that the cost and performance of the (IF) amplifier, (IF) filter, and detector/demodulator is good. * Generally speaking, as we go lower in frequency, the cost of components go down, and their performance increases (these are both good things!). As a result, the IF frequency is typically (but not always!) selected such that it is much less (e.g., an order of magnitude or more) than the RF signal frequencies we are attempting to demodulate. * Therefore, we typically use the mixer/LO to down-convert the signal frequency from its relatively high RF frequency to a relatively low IF frequency.10/9/2007 The Superhet Receiver 6/9 Jim Stiles The Univ. of Kansas Dept. of EECS Æ We are thus generally interested in the second-order mixer term RF LOff− . As a result, we must tune the LO so that sLO IFff f−= —that is, if we wish to demodulated the RF signal at frequency sf! For example, say there exits radio signals (i.e., radio stations) at 95 MHz, 100 MHz, and 103 MHz. Likewise, say that the IF frequency selected by the receiver design engineer is fIF = 20 MHz. We can tune to the station at 95 MHz by setting the Local Oscillator to 95-20=75 MHz: ()201Gf= ()ˆit ()()20 120 0ff=≈≠≈TT 75LOfMHz= tuning W/Hz f (MHz) 100 103 95 W/Hz f (MHz) 25 28 20 W/Hz f (MHz) 25 28 20 ()fT10/9/2007 The Superhet Receiver 7/9 Jim Stiles The Univ. of Kansas Dept. of EECS Or, we could tune to the station at 103 MHz by tuning the Local Oscillator to 103-20=83 MHz: Q: Wait a second! You mean we need to tune an oscillator. How is that any better than having to tune an amplifier and/or filter? A: Tuning the LO is much easier than tuning a band-pass filter. For an oscillator, we just need to change a single value—its carrier frequency! This can typically be done by changing a single component value (e.g., a varactor diode). ()201Gf= ()ˆit ()()20 120 0ff=≈≠≈TT 83LOfMHz= tuning W/Hz f (MHz) 100 103 95 W/Hz f (MHz) 17 20 12 ()fT W/Hz f (MHz) 17 20 1210/9/2007 The Superhet Receiver 8/9 Jim Stiles The Univ. of Kansas


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KU EECS 622 - B. The Super-Heterodyne Receiver

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