3/31/2005 The Preselector Filter.doc 1/14 The Preselector Filter Say we wish to tune a super-het receiver to receive a radio station broadcasting at 100 MHz. I the receiver uses and IF requency o IF = 30 MHz, and uses high-side tuning, we must adjust the local oscillator to a requency o = 130 MHz. 0 = 100MHz G ( 30MHz) 1 T ( MHz ) 30 1 î ( t) = 130MHz tuning Thus, the desired signal will be down-converted to the IF requency o 30 MHz. But BEWARE, the desired radio station is not the only signal that will appear at the output o the mixer at 30 MHz! Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 2/14 Q: Oh yes, we remember. The mixer will create all sorts o nasty, non-ideal spurious signals at the mixer IF port. Among these are signals at requencies: 1 st order: = 100MHz, = 130MHz 2 nd order: 2 = 200MHz, 2 = 260MHz, + = 230MHz 2 = 70MHz, 3 rd order: Right? 2 = 160MHz, 3 = 300MHz, 3 = 390MHz, 2 + = 330MHz, + 2 = 360MHz A: Not exactly. Although it is true that all o these products will exist at the IF mixer port they will not pose any particular problem to us as radio engineers. The reason or this is that there is a narrow-band IF ilter between the mixer IF port and the demodulator! Look at the requencies o the spurious signals created. They are all quite a bit larger than the ilter center requency o 30MHz. All o the spurious signals are thus rejected by the ilter none (eectively) reach the detector/demodulator! Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 3/14 Spurious mixer products 0 = 100MHz 30MHz G ( 30MHz) 1 T ( MHz ) 30 1 î ( t) = 130MHz tuning T ( ) ω 30MHz Look again at the statement I just made: But BEWARE, the desired radio station is not the only signal that will appear at the output o the mixer AT 30 MHz! In other words, there can be spurious signals that appear precisely at our IF requency o 30 MHz. The IF ilter will not o course ilter these out (ater all they re at 30 MHz!), but instead let them pass through unimpeded to the demodulator. Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 4/14 The result demodulated signal î ( t ) is an inaccurate, distorted mess! Q: I m just totally baled! Where do these unilterable signals come rom? How are they produced? A: The answer is a proound one an incredibly important act that every radio engineer worth his or her salt must keep in mind at all times: The electromagnetic spectrum is ull o radio signals. We must assume that the antenna delivers signals operating at any and all requencies! In other words, we are only interested in a signal at 100 MHz; but that does not mean that other signals don t exist. You must always consider this act! Q: But I m still conused. How do all these signals cause multiple signals at our IF requency? Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 5/14 A: Remember, each o the signals will mix with the drive signal, and thus each signal will produce its very own set o mixer products (1 st order, 2 nd order, 3 rd order, etc.) Here s the problem some o these mixer products might lie at our IF requency o 30 MHz! * To see which input signal requencies will cause this problem, we must reverse the process o determining our mixer output products. * Recall earlier we started with known values o (100 MHz) and (130 MHz), and then determined all o the spurious signal requencies created at the mixer IF port. * Now, we start with a know (130 MHz), and a know value o the spurious IF signal requency (30 MHz), and try to determine the requency o the signal that would be required to produce it. For example, let s start with the 3 rd order product 2. In order or this product to be equal to 30 MHz, we ind that: 2 130 = 30 2 130 =± 30 2 = 130± 30 130 ± 30 = 2 = 50, 80 Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 6/14 Thus, when attempting to tune to a radio station at 100 MHz, we ind that radio stations at both 50 MHz and 80 MHz could create a 3 rd order product at 30 MHz precisely at our IF ilter center requency! But the bad news continues there are many other mixer products to consider: 2 2(130) = 30 260 = ± 30 = 260 30 = 290, 230 + 2 Q: What?! A radio station operating at a negative requency o -230 MHz? Does this have any meaning? 2(130) + = 30 260 + = 30 = 30 260 = 230 A: Not in any physical sense! We ignore any negative requency solutions they are not a concern to us. Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 7/14 2 + 2 + = 30 2 + 130 = 30 30 130 = 2 = 50 Again, a negative solution that we can ignore. 3 3 = 30 30 = 3 = 10 OK, that s all the 3 rd order products, now let s consider the second-order terms: 130 = 30 130 = ± 30 = 130 30 = 100, 160 Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 8/14 * Note that this term is the term created by an ideal mixer. As a result, we ind that one o the signals that will create a mixer product at 30 MHz is = 100 MHz the requency o the desired radio station! * However, we ind that even this ideal mixer term causes problems, as there is a second solution. An signal at 160 MHz would likewise result in a mixer product at 30 MHz even in an ideal mixer! * We will ind this second solution to this ideal mixer (i.e., down-conversion) term can be particularly problematic in receiver design. As such, this solution is given a speciic name the image requency. For this example, 160 MHz is the image requency when we tune to a station at 100 MHz. + No problem here! 130 + = 30 130 = 30 130 = 100 2 2 = 30 30 = 2 = 15 Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 9/14 Finally, we must consider one 1 st order term: = 30 In other words, an signal at 30 MHz can leak through the mixer (recall mixer isolation) and appear at the IF port ater that there s no stopping it until it reaches the demodulator! In summary, we have ound that that: 1. An signal (e.g., radio station) at 30 MHz can cause a 1 st -order product at our IF ilter requency o 30 MHz. 2. signals (e.g., radio stations) at either 15 MHz or 160 MHz can cause a 2 nd -order product at our IF ilter requency o 30 MHz. 3. signals (e.g., radio stations) at 10 MHz, 50MHz, 80 MHz, 230 MHz, or 290 MHz can cause a 3 rd -order product at our IF ilter requency o 30 MHz. Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 10/14 Many other spurious signals at other req. are likewise created, but not shown! 30 30MHz 50 80 100 160 230 G ( 30MHz) 1 T ( MHz ) 30 1 î ( t) = 130MHz tuning T ( ) ω 30MHz Q: I now see the problem! There is no way to separate the spurious signals at the IF requency o 30 MHz rom the desired station at 30 MHz. Clearly, your hero E.H. Armstrong was wrong about this Super-Heterodyne receiver design! A: Armstrong wrong!?! NEVER! Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 11/14 There is an additional element o Armstrong s super-het design that we have not yet discussed. The preselector ilter. The only way to keep the mixer rom creating these spurious signals at our IF ilter center requency is to keep the signals that produce them rom the mixer! O course, we must simultaneously let the desired station reach the mixer. Q: Hmmm A device that lets signals pass at some requencies, while rejecting signals at other requencies sounds like a microwave ilter! A: That s correct! By inserting a preselector ilter between the antenna and the mixer, we can reject the signals that create spurious signals at our IF center requency, while allowing the desired station to pass through to the mixer unimpeded. Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 12/14 30 50 80 100 160 230 30 50 80 100 160 230 T ( MHz ) 100 1 = 130MHz Preselector Filter tuning 30MHz Q: So how wide should we make the pass-band o the preselector ilter? A: The pass-band o the preselector ilter must be wide enough to allow any and all potential desired signals to pass through. Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 13/14 * Consider our example o 0 = 100 MHz. This signal is smack-dab in the middle o the FM radio band, and so let s assume it is an FM radio station (i it were, it would actually be at requency 100.1 or 99.9 MHz). * I we are interested in tuning to one FM station, we might be interested in tuning into any o the others, and thus the preselector ilter pass-band must extend rom 88 MHz to 108 MHz (i.e., the FM band). * Note we would not want to extend the pass-band o the preselector ilter any wider than the FM band, as we are (presumably) not interested in signals outside o this band, and those signals could potentially create spurious signals at our IF center requency! As a result, we ind that the preselector ilter eectively deines the bandwidth o a superheterodyne receiver. Q: OK, one last question. When calculating the products that could create a spurious signal at the IF center requency, you neglected the terms, 2 and 3. Are these terms not important? Jim Stiles The Univ. o Kansas Dept. o EECS
3/31/2005 The Preselector Filter.doc 14/14 A: They are actually very important! However, the value o is not an unknown to be solved or, but in act was (or our example) a ixed value o = 130MHz. Thus, 2 = 260MHz, and 3 = 390MHz none o these are anywhere near the IF center requency o 30 MHz, and so these products are easily rejected by the IF ilter. However, this need not always be true! * Consider, or example, the case were we again have designed a receiver with an IF center requency o 30 MHz. This time, however, we desire to tune to radio signal operating at 60 MHz. * Say we use low-side tuning in our design. In that case, the signal requency must be = 60 30 = 30MHz. * Yikes! You must see the problem! The Local Oscillator requency is equal to our IF center requency ( = IF ). The signal will leak through mixer (recall mixer isolation) and into the IF, where it will pass unimpeded by the IF ilter to the demodulator (this is a very bad thing). Thus, when designing a receiver, it is unathomably important that the requency, along with any o its harmonics, lie nowhere near the IF center requency! Jim Stiles The Univ. o Kansas Dept. o EECS