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![]() John Byrns wrote: What exactly are the advantages of a 2.0 MHz IF from a selectivity/bandwidth point of view? Both pros and cons to this John. Since the bandwidth is a percentage of the center frequency, the shape of the bandwidth will change based on the distance from the center frequency (as a percentage.) Assuming just for the moment +/- 5 KHz. At 455 KHz that's about 1% above and below. At 2 MHz, that's now only .25% above and below. As you get further from the center frequency, percentage wise, the shape of the curve as it transitions from inside to outside of the band pass is going to look different at the upper frequency than it does at the lower frequency. Certainly a high IF frequency will have advantages in image response, but if the bandwidth is the same, the audio quality should be similar. With the notable exception of the difference in shape of the roll off above and below the center frequency. In the world of designing filters (and overall system performance) this is called group delay. A shorter, perhaps more recognizable term would be linear phase shift over the entire band pass of the filter. Wireless World is a hobbyist magazine and all their authors are not necessarily up to speed They were under the same constraints as the Weekly World News. "If it wasn't true, they couldn't print it." Note smiley face here. ;-) Yes, although I have some reservations about the use of the term "Q", that is obvious, but so what, what difference does it make? Back to the original comments about Q. In a perfect world, it would only be a matter of the LC ratio setting the bandwidth of a tuned circuit. Of courses, there are other things that get in the way to reduce the overall Q of a circuit. Nasty little things like the series resistance of the coils, dielectric losses in both the coil forms and capacitor insulation material. Back to the original "ideal" values of Q. 15 at 455 KHz and 67 at 2 MHz. It is physically "more challenging" to get higher Q at a higher frequency. All of the various losses of the components tend to get in the way. Wire losses, dielectric losses and any losses of the ferrite used in the core materials. Also, what does "damped" mean in this context? I would have to do some research, but I suspect that "damping" is more related to filter bandwidth than to the center frequency, and both filters are aiming for the same bandwidth. "Damped" means adding some form of resistance across the reactive components of a circuit. As an example, if you were to assemble a nice 455 KHz IF transformer and found that the bandwidth was too narrow, a fast method of widening it would be to place parallel resistors across the windings. Another point about "damped" is that if a tuned circuit has too high a Q, a sudden transient will tend to make it oscillate. In communications receivers, this is obvious that a signal sounds more like you're ringing a bell, than simply turning a tone on and off. (Kind of like using the sustain pedal on a piano.) Maybe, in what way are you suggesting I am confused? I would suggest to you that you don't understand how to design an IF filter, and don't understand what can be done at 455 kHz. Lets not go in that direction. An IF transformer is simply a two pole butter worth filter. That it can have different input and output impedance just makes it really convenient for taking the source from a plate and connecting it to a grid for a load. By definition, a butter worth filter has a smooth curve with only one peak (in the middle.) And the shape (steepness) of the band pass is related to the overall Q of the circuit. The next type of filter, would be Chebychev, This is no more than a "predistorted" butter worth filter network. By allowing a certain amount of ripple in the pass band, the shape of the rejection can be made sharper. The obvious trade off is the amount of distortion to the signal within the pass band. A simple example of this would be stagger tuned IF coils. Two or more peaks, and a dip (or dips), ripple, in the middle. While I can't claim to have designed the filter I used, I have actually built a transistor superhetrodyne AM tuner using a 455 kHz block filter with a 30 kHz IF bandwidth. Will the 2.0 MHz IF work better than this? Have you tried a properly designed wideband 455 kHz IF filter to see how it worked? The filter I used came out of a 2-way land mobile radio and I think it was about an 8 pole filter. The point you've probably overlooked in land mobile operations is that it was NEVER designed as a "hi-fi" system. There's a reason for the term "voice grade." Having as much a 3 dB of ripple in a band pass filter is meaningless especially when the filter is in the midst of a limiting IF strip for FM recovery, and on AM demodulation. What really matters here is limiting the bandwidth of the received signal to ONLY include that of the wanted (in channel) information and none of the unwanted (adjacent channel) information to get to the discriminator. So what it boils down to is that you haven't tried a wideband 455 kHz filter while I have, and I haven't tried a 2.0 MHz IF filter, which you may or may not have done. I at least have cited some concrete facts about IF filters, while you have only muttered about Q, without indicating how it actually relates to the problem. I am not a "filter jock" (tm) but I think it is generally desirable that the Q of the components used in a filter be high, especially when we get beyond simple double tuned transformers. What you are calling Q is more related to how the filter is terminated, which is a different matter than the Q of the components that make up the filter. You should take the time to read up on "filter jockeying" John. You're making a lot of incorrect assumptions on how they work. The primary requirement on the Q of individual components in filter design is only such that their value of Q be high enough to not materially effect the overall Q of the circuit. As an example, (and without getting into cryogenic treatments and styrofoam cups) A speaker system sounds better through 25 feet of #12 AWG wire than it does through 25 feet of #18 AWG wire. And that's strictly due to the resistive loss of the wire in comparison to the losses in the actually speaker design and implementation. I had a electronics instructor in college that would show you "The secret of electronics" that he kept hidden, and locked, inside a small jewelry box if you "caught on" during his course. With some fanfare, he would slowly open the box and you would see an inductor, a resistor and a capacitor. And it's really just that simple. What gets complicated is when you forget that all three items have hidden values of the others contained within them. (I.e. the difference between practical and theoretical parts.) Jeff -- "They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." Benjamin Franklin "A life lived in fear is a life half lived." Tara Morice as Fran, from the movie "Strictly Ballroom" |
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