Demodulation with Hilberttransformation

On Thu, 29 Aug 2013 22:06:34 +0000 (UTC), glen herrmannsfeldt
<gah@ugcs.caltech.edu> wrote:

>Eric Jacobsen <eric.jacobsen@ieee.org> wrote:
>
>(snip, someone wrote)
>
>>>> Converting from IF to baseband is, IMHO, more a tuning 
>>>> operation than "demodulation".   De-modulating implies 
>>>> undoing the modulation, which may not, and usually doesn't, 
>>>> have anything to do with frequency conversion.
>
>(then I wrote)
>>>Reminds me of the discussion about whether a device is, or is not,
>>>a modem based on whether it does, or does not, modulate and
>>>demodulate anything.
>
>>>To me, though IF to baseband is demodulating, where RF to IF is,
>>>in the systems I know, frequency conversion. 
> 
>> So does direct conversion do demodulation or frequency conversion? 
>
>My old favorite reference on such is the "ARRL Handbook."
>
>In the chapter titled "Mixers, Modulators and Demodulators"
>(and given that, you might guess that they are related).
>
>  "Translating information into radio form entails a process
>   we call modulation, and demodulation, is its reverse.
>
>The chapter then goes on to explain mixers and their use for
>modulation and demodulation.

And I think the ARRL deserves a whack in the head for promoting that
as a definition of what mixers do.


>> The issue is that the terms are overloaded and used ambiguously.
> 
>>>But OK, detection is the non-linear operation that demodulates
>>>an AM signal. So, you want to separate the conversion to baseband
>>>(as not demodulation) and detection. 
>
>>>Do you agree that AF to IF is modulation? So, IF to RF is not?
> 
>> What is AF?
>
>Must be that I read too many old radio books, when the output
>was AF, or Audio Frequency. I suppose now there are enough more uses
>for radios that doesn't make sense. Still, I believe it is commonly
>used in describing transistors not designed for higher frequencies.

In that case, no I don't agree that AF->IF or IF->RF is "modulation".
It's "tuning" or "frequency translation" or whatever, but by the usual
definition of "modulation"in this context, there's no information
added to the signal by the process.
 
>(snip on complex signals)
>
>>>> Unfortunately some do use the term "demodulation" the way you have,
>>>> but I think it is more confusing than helpful.
>
>>>Seems to me that the operation of a mixer is commonly modulation
>>>when going up, and it also could be going down.
> 
>> I think a mixer mixes, and the resulting frequency conversion is best
>> referred to as "frequency conversion" or "tuning" or "mixing".
>> Modulation, in this context, to me,  implies that something is being
>> modulated to carry a recoverable information signal.   Mixing doesn't
>> do that.   AM, FM, PM, MSK, etc., etc., does, and "demodulating" those
>> signals recovers the information used in the "modulation".
>
>But mixing does do that! You might want to separate balanced
>and double balanced mixers. Conventional AM comes from a singly
>balanced mixer, which allows the carrier through even with no
>input on the other port. AM-SC, like the FM stereo subcarrier,
>and NTSC chrominance subcarrier, from a double balanced mixer.

It is unfortunate that these limited (but historic) cases have
produced such a great and continuing source of confusion.   I think it
much better that the concepts or processes of adding and recovering
information from a signal and translating it in frequency would have
different nomenclatures.
 
>> The sad fact is that the terms are already ambiguous and in common use
>> in their overloaded cases, so it's all just opinion on both sides.
>
>That is true just about everywhere, and might just as well be here.

I think reducing confusion is a desirable goal.

>
>-- glen

Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com