Talk:Intermediate frequency

Untitled

From an email to Wikimedia:

"While no factual errors, the omission of the most common IF (Intermediate Frequency) for FM radios, 10.7 MHz, used since the 1940s when FM first came about is glaring. Virtually any FM broadcast radio you pick up today will have a 10.7 MHz IF in it. Many will also often have a second IF at 455 kHz.

True, the listed 455 kHz is also the most common IF in AM and single conversion radios, but the other two IFs listed are brand-specific random choices of frequencies. 10.7 MHz has been a staple of nearly every brand of modern FM radio ever produced, tube or certainly solid state, since FM's inception in the 1930s. So much so, NOT using 10.7 MHz specifically identifies some brands (e.g., 10.8 MHz was 'unique' by patent to Electra Bearcat scanner radios)."

Can someone review and make any edits neeed? This isn't my area. Thanks -- sannse (talk) 20:38, 21 January 2006 (UTC)

The 10.7 MHz IF for FM is used for a number of countries, but not all. There are two or three other commonly used IF Frequencies throughout the world for FM. The situation is similar for AM IF frequencies, but I think there are more different ones used. --71.38.170.27 (talk) 18:13, 12 November 2012 (UTC)

Other common intermediate frequencies ...

`Other common intermediate frequencies are 2400MHz (WLAN), 2000MHz (Cellular/WLAN), 850/900/1800/1900MHz (Cellular)'

Um, wouldn't these be the actual frequencies used by these applications, rather than intermediate frequencies? dougmc 22:40, 16 June 2006 (UTC)

Please note that these are the actual RF frequencies and not the IF frequencies. Make correction kindly.

Clean up 1/9/07

Cleaned it up a little bit since the sentences were repetitious. --SamMichaels 15:31, 9 January 2007 (UTC)

Revision 159334848 by 116.71.34.80

You changed the IMF of television from 30mhz to 41mhz. How can you explain this when common TV modules which use varying frequencies in 30mhz:

1. 36.00
2. 36.13
3. 36.16
4. 36.167
5. 38.9

--SamMichaels 13:50, 21 September 2007 (UTC)

System B

I added the intermediate frequencies of the international system B (G.H etc.) Nedim Ardoğa (talk) 18:17, 28 October 2009 (UTC)

Zero Intermediate Frequency

I would like to see a link to the article on "Zero Intermediate Frequency". Also, it would be good to see an explanation of why IFs different from zero are used. If I had to design one of these systems, f=0 comes to mind as the first candidate for an IF. So the question is: why is IF=0 not used? —Preceding unsigned comment added by Avalcarce (talk • contribs) 08:28, 2 March 2010 (UTC)

You get issues with DC offset, see Homodyne receiver for that topic --Moritz der Moralapostel (talk) 12:51, 28 September 2010 (UTC)

In addition, it is harder to build amplifiers with flat frequency response. At IF, the bandwidth is small compared with the center frequency, making a flat response easier. The design of a receiver involves choosing how much amplification to do at RF (input frequency), IF (one or more), and at baseband (what you call f=0). Note for an audio (stereo) amplifier, it is generally specified as 20Hz to 20kHz, a ratio of 1000. For an FM receiver with a 10.7MHz IF, and with the 200kHz bandwidth, the amplifier needs to be fairly flat form 10.6MHz to 10.8MHz, much easier to build. Gah4 (talk) 21:34, 28 March 2017 (UTC)

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filter bandwidth

The article says With all known filtering techniques the filter's bandwidth increases proportionately with the frequency. I believe this is reasonably obvious with analog RLC filters. I am not so sure about SAW filters. For digital filters, you have to say what is constant, and that isn't always so obvious. Gah4 (talk) 20:21, 5 February 2020 (UTC)

I believe that if one scales a SAW filter proportionally, then all frequencies (including bandwidth) will also scale proportionally. For digital filters, is IF even a meaningful concept in DSP? SpinningSpark 12:34, 11 February 2020 (UTC)

I think so, but I haven't followed it so closely. There is Software-defined_radio where you do more radio signal processing in the digital domain. (I disagree with the name, as it is often done in programmable hardware, but ...) The ideal for SDR is that you digitize the antenna input, and in the digital domain do all filtering and demodulating. I believe that includes filtering, generating a digital IF, filtering more, and such, but I haven't tried to build one. For dynamic range reasons, one normally needs some analog filtering before the ADC. Some might use an analog mixer and digitize the IF. Maybe then do much of the IF filtering in the digital domain. But also, I don't know all the different way to design digital filters, over different frequency ranges and sampling rates. I suspect, though, that the simple rules for LRC filters don't always apply. As you say, if you scale a SAW filter it scales proportionally, but what if you don't scale it that way? If you keep the same size, but increase the frequency? Or use a material with different surface wave speed? There are more variables than for RLC. Gah4 (talk) 12:52, 11 February 2020 (UTC)

On that last bit, it's a bit like saying one is going to scale the frequency of an RLC circuit but not change the size of the capacitor. Obviously, that will not result in the fractional bandwidth remaining invariant. Scaling, in this context, means scaling everything that is frequency dependent in such a way that the transform is impedance invariant. Problematically, digital filters don't care much about impedance, so we would have to say that frequency scaling meant all frequencies scale in the same way, which would make the bandwidth statement in the article a simple truism for digital filters. SpinningSpark 17:34, 29 February 2020 (UTC)

Some scaling rules are more obvious than others. In varactor circuits, I suspect that it is usual to change the capacitor and not inductor, as voltage controlled inductors are hard to make. While Dennard scaling might be the more obvious MOSFET scaling law, it isn't the only one. For some years, scaling was done keeping the gate oxide thickness constant, as it was somewhat easier, and didn't cause too many problems. Also, keeping Vss at 5 volts was convenient for users. As you say, all frequencies should scale, so Superhet receivers should scale IF with RF, but they don't. It does make me wonder about the earliest thoughts on such receivers, if it might not have been so obvious to keep the IF frequency constant. Reminds me of systems like CB converters which downconvert the whole 27MHz CB band to the AM radio band, essentially keeping the LO constant, and varying the IF. In sampled digital systems, it isn't so obvious that sampling rate should scale with RF, for about the same reasons as not scaling IF. Gah4 (talk) 23:43, 29 February 2020 (UTC)

I wrote the sentence at top. When I wrote it I was thinking about ordinary analog filters, not digital radio. I know almost nothing about digital filters, but I think that for a given filter algorithm the bandwidth scales with the sampling frequency? The sampling frequency must scale with the input frequency to the ADC, so it seems to me that as with analog filters a narrower bandwidth can be achieved by converting the RF to a lower IF and filtering at that frequency with a lower sampling rate. Of course it is easy to implement a digital filter with many poles, so maybe getting adequate selectivity is not a problem at any frequency. Also, most modern radio applications are at microwave frequencies. Gah4, I don't know much about modern components; are microwave ADCs that cheap that you can digitize at RF? I suspect most current digital radio architectures, at least those used in consumer wireless devices, digitize at the IF frequency. --Chetvorno^TALK 21:12, 1 March 2020 (UTC)

I don't think that they are sampling at the GHz or 2GHz that many use now, but then again, those usually use some digital modulation method. The systems are designed to make receivers easier to build. I believe that the earlier cell phones used more or less ordinary analog modulation techniques, but converted to digital after not so long. One is extracting a bit stream, so no ADC at that point. If one does actually want to do it, though, there is bandpass sampling where you use enough analog filtering to get a (relatively) narrow passband, and then sample that at an appropriate rate for no aliasing. Reminds me, though, I had the idea some years ago of building an AM radio, maybe car radio, that would sample and store the whole AM band. Then you could switch stations, and back up to the beginning of a song or news segment. A little more bandwidth to sample the whole FM band, but I think that can be done, too. But yes, I don't know the details of modern digital radio systems enough to say more about them. Gah4 (talk) 22:58, 1 March 2020 (UTC)

The information "The bandwidth of a filter is proportional to its center frequency" should really be explained properly. Do considerations need to be made about any constraints, such as a fixed value of C, or fixed value of L, and fixed value of R? KorgBoy (talk) 01:25, 15 September 2020 (UTC)

Mc/s

Wondering (but not complaining) about the notation Mc/s. Since superheterodyne was developed before the Hz unit, it makes sense to use the older units when discussing it. (At least I believe so.) Though, as well as I know, the common unit was Mc with the /s implied. (No-one measured frequencies per minute, hour, or day.) In paragraphs actually referencing older citations, it does make some sense to for the units to be consistent with that in the reference. Is there a WP:MOS on this? Gah4 (talk) 23:11, 9 July 2021 (UTC)

I would leave direct quotes alone, and use hertz for everything else. A long list of redundant synonyms is an unlovely hallmark of Wikipedia composition and is one of the reasons many of our articles are impenetrably obscure. Plus it's ugly. --Wtshymanski (talk) 21:17, 10 July 2021 (UTC)

It seems to be discussed in WT:Manual_of_Style/Dates_and_numbers/Archive_160#Historic_unit_name,_usage. Gah4 (talk) 00:19, 11 July 2021 (UTC)

I hate ISO for abolishing Mc/s, apparently on the grounds they did not like people abbreviating it to Mc, but I think we have to bite the bullet and accept that nowadays everyone expects SI units to be used. Think of our poor children busy catching up with their schoolwork after the covid lockdown. They don't have time to learn the ancient language of the electronics tribe.

"Cycles per second" is what it says on the tin. So are feet and ergs and dynes were pretty self-explanatory too. Now we have to remember which scientist is associated with which quantity. I'm going to propose a new system of units called the foot-dyne-erg system. Any seconders? SpinningSpark 13:21, 15 August 2021 (UTC)

Crossword editors might. Erg and dyne are more popular than joule and newton in crossword puzzles. I am not sure about feet. Gah4 (talk) 23:47, 15 August 2021 (UTC)

Long quotations in refs

@Matthiaspaul: Do we really need these massive quotations in the references? They really are cluttering the ref section. Also, why link so many pdfs in one reference? Just link to the main page, readers are quite capable of clicking through pages themselves. SpinningSpark 07:47, 15 August 2021 (UTC)

Yes, it does look pretty strange. If it is important enough, it should go in the article. Gah4 (talk) 08:48, 15 August 2021 (UTC)

Some editors include a quotation from the source that verifies the fact being cited to make it easy for reviewers checking the referencing. This is mostly done in historical articles and especially if FA is being sought. Citation templates even have a field for it. But it is over the top here; half a page of text to verify a particular frequency is used is really unnecessary. SpinningSpark 09:02, 15 August 2021 (UTC)

Well, Wikipedia is WP:NOTPAPER, we don't have a space restriction here, in particular not in appendices such as the reference section.

As you correctly point out, our citation templates have a |quote= parameter for this and it is even considered good style to list specific page numbers and quote relevant sections from the original source as accurate as possible - after all, not everyone might have the source readily available, and even if they have, always having to look up the specific bits of information is inconvenient when it is distributed over multiple places in a publication.

I agree, in this case the quotes are rather long, but my intention including them was to compile and provide the "essence" of these publications in regard to IF frequencies (the actual frequencies used as well as the timeframe, type of receivers and other contexts they were used in) in order to make it easier for readers to check the context. I came to this article expecting a comprehensive and detailed overview of all IF frequencies ever used in receivers of different types - and found almost nothing but a very incomplete and unsourced list, leading to more questions than answers. That's why I started to retrofit sources and list more IFs if I found them mentioned in the publications. If we continue this for a couple of years we will hopefully have nearly all IF frequencies properly referenced and listed with their specific context, and can then search for specific references supporting the few remaining unsourced frequencies listed (and if no sources can be found for them over the years, they can be removed). However, in this endeavour, we cannot expect to reread all the small snippets of information distributed over whole books all the time, so having the "essence" of each book in regard to IF destilled in the quotations, this will become much easier. Since I was just starting this effort, I didn't know in advance how much context would be needed in the quotations later on and what could be avoided. I expect that the quotations will still remain on the longer side but they could probably be trimmed down a bit over time, as the article contents gets expanded accordingly.

--Matthiaspaul (talk) 11:44, 17 August 2021 (UTC)

Sounds like it needs its own article. This article is about the concept of IF, and listing some popular frequencies used make sense. It is not meant as an index of all the IFs ever used. There are WP pages with titles like List of ..., and I suppose there could be one. Note that the article on crystal oscillators indicates some of the popular frequencies, but no-one expects a table of all the crystal frequencies ever made. I suspect that one is already bigger than it should be. Gah4 (talk) 00:45, 18 August 2021 (UTC)

OK, it seems that there is now Crystal oscillator frequencies as its own article, and none are listed in Crystal oscillator. Maybe that is what we need here, too. Gah4 (talk) 00:53, 18 August 2021 (UTC)

Yeah, regarding listing all crystal frequencies ever used might be an impossible task to do, because there are so many. Still, it might have some encyclopedic value to find out some context or background about the purpose and properties of some resonator or oscillator frequencies of some part spotted in the box or mentioned somewhere.

But in regard to IF frequencies, the list would be magnitudes shorter, so it seems to be a quite realistic task to be able to achieve near-completion on an encyclopedic level in a couple of years - and IMO there is also quite a bit to tell about many of them, why and where they were or are used, etc.

I agree, splitting out the list or table might be a good idea at a certain point in time, but do you really see that point reached now already? After all, the article is less than 27 KB in size (and that is including all the new references and long quotations), with the body of the article on intermediate frequencies being less than 13 KB including just 9 references - that's tiny. There is so much more that could and should be said about intermediate frequencies. I would start to consider splitting out this stuff when the article reaches a size and has references ten times as much and many as it has now. But if you think it should better happen now, that would be an option I could agree with as well.

--Matthiaspaul (talk) 11:00, 18 August 2021 (UTC)

@Matthiaspaul: Yeah, I don't see a need for a lot of the quotes. You have 3 or 4 quotes just saying that 455 kHz is a popular IF. "The intermediate frequency in general use is 455 Kc/s.", "A frequency of 455 Kc/s. is receiving universal acceptance as a stanard frequency...". Totally unnecessary. Another issue is that your citations are so disorganized they are almost impossible to read. It looks like you are trying to cram a lot of separate references to a single source (such as the Langford-Smith book) into a single inline citation. It is less confusing to use separate citations for separate pages of a single source. One way is to use {{rp}} or {{r}} templates after a cite of the source document to list the pages. A better way is to go to Harvard or shortened citation templates (WP:OPCIT). --Chetvorno^TALK 06:15, 18 August 2021 (UTC)

There is also {{multiref}} which works well with shortened citations. SpinningSpark 07:03, 18 August 2021 (UTC)

I deliberately did not split single publications into multiple references, because that is what adds redundancy and clutter. If I have one book in my hands, I want to know all the cited pages dealing with a particular topic in one place, not having to manually sift through dozens of distributed short references trying to find out what they actually refer to.

Using {{rp}} would certainly be a useful addition here (I was already thinking about adding it), but this would still require all the cited pages to be list in the core citation.

I certainly would not want to use the {{sfn}} style, because this adds an unnecessary extra level of indirection and does not provide backlinks, so it makes it impossible to easily jump from one quoted page to another in the same source.

Regarding having multiple citations supporting a frequency such as i.e. 455 kHz as a popular IF, if you read my reply further above, it was the whole point of this undertaking to ideally have multiple reliable references backing up each of the various frequencies in order to find out which frequencies are really common, more common, used a couple of times, and which frequencies were used in one-off-its-kind solutions only, and about their contexts. I deliberately did not want to rely on some single editor's (including my own) accumulated knowledge or single book on this because (except for some über-common frequencies globally agreed upon today) what is or was used very much depends on the time frame you are looking at and the locale you were or are in. As an encyclopedia we have to document what is (or was) without a bias on present-day solutions or a focus on specific countries like the US or UK only.

At some point in the future, that is, when we would have a few dozen citations more backing up the various frequencies, the table could be rearranged and some of the redundancy be removed because then it would no longer be needed. But IMO that point hasn't been reached yet, as we have just started the effort to properly reference the frequencies, so in my judgement that would probably be years in the future.

If we trim this out now, we'd go back to near-zero and it would only slow down the process to accumulate this info in one place in a properly referenced way. As Gah4 suggests, split it off into a kind-of-list article would be an option, however, although personally I do not think it would be necessary at this early stage already.

--Matthiaspaul (talk) 11:00, 18 August 2021 (UTC)

@Matthiaspaul: Just wanted to say I appreciate your commitment to sourcing this content, there are many editors who omit that part of the process. --Chetvorno^TALK 19:05, 19 August 2021 (UTC)