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#22
September 9th 06, 07:42 PM
posted to rec.radio.amateur.homebrew
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Intermediate Frequency
Hamateur wrote:
wrote: On 7 Sep 2006 13:17:19 -0700, "radio_rookie" wrote: Hello, I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Images of the same signal may be a source of interference. Does the IF stage conditions the incoming signal? Yes, in many ways. The most significant are amplification and selectivity. What are the advantages of the IF stage? Less amplification needed at the recieved frequency. Gain at a frequency removed from the recieved frequency. Selectivity is easier to obtain at lower frequencies. Gain control can be applied if needed. Allison I can easily agree that an IF amp's job is to cleanly and efficiently amplify a specific, modulated, carrier frequency and to allow for gain control feedback. But I don't see how "selectivity" should be considered a function of an IF amp (other than they're not amplifying what they shouldn't amplify). It seems easier to think of "selectivity" as a property of a tuner or several tuner stages. It's hard for me to think of IF "stages" as improving tuner selectivity when my homebrew 40m DC recvr seems to be selective enough so that when I listen to CW the pitch will not change audibly. The frequency may fluctuate a little, but certainly not enough to loose a signal, and it does not drift monotonically enough to worry about (except perhaps as a matter of pride). this is not selectivity, this is stability. selectivity is filtering nearby strong signals, which direct conversion has more trouble with, especially as they get closer. the IF allows cheap, narrow, lower freq filters, which will have great side skirts to remove the nearby strong signals. I tend to think of "stability" more in terms of random fluctuations. Instability may or may not effect selectivity. As long as my desired selection remains decipherable, I would say that selectivity has been accomplished regardless of whether there's any kind of instabilty. Filtering strong nearby signals seems more about "exclusivity" than "selectivity". I would rather say that IF stages maintain selectivity while they are excluding undesired mixing products and other signals. As long as a recvr includes my desired frequency and it does not drift out of my receiver's bandwidth requiring me to retune, then I would say that the recvr is maintaining selectivity even if I get *more* than what I want. The superhet's conversion mixers/filters/amps seem to be considered sub-steps of "IF stages", but I find it easier to think of the mixer/filter steps as "stages of tuners interlaced with IF amplifier stages". ok, then. Since the final conversion step may represent a detection, the idea of "selectivity" as being interlaced with IF amps has a more tidy representation in my mind. Comments, criticisms, corrections, caveats - are always welcome. |
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