[NJARC] Why 455?

[Jim Whartenby]

Al
Just playing with the calculator and using approximate frequencies for
the old AM band limits such as 500kcs and 1500kcs (600 to 200 meters)
you get a geometric mean of 866kcs.  Divide this by 2 gives 433kcs.  So
depending on what the frequencies actually were for the old AM band,
you end up quite close to 455kcs.
Just a thought,
Jim

--- Al Klase <al at ar88.net> wrote:

> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Every so often the question comes up: Why are all the IF’s 455 KHz?
> I’d 
> like to get an article together that solves this riddle while the
> people 
> who know are still with us. I know parts of the story, but I need
> help 
> with a couple of issues.
> 
> There are two major consideration is the choice of the intermediate 
> frequency used in a superheterodyne receiver. The lower the
> frequency, 
> the easier it is to attain high selectivity. Also, in the early days,
> 
> before tetrode and pentode tubes, it was easier to achieve a high
> degree 
> of amplification at lower frequencies. Conversely, a higher IF
> frequency 
> results in better image rejection.
> 
> Early superhets had the IF at 100KHz or lower in order to get
> adequate 
> gain from the available triode tubes. They suffer severely from 
> “two-spot tuning” (images). By the early 1930’s, broadcast set had 
> settled in at 175KHz, and automobile receivers would later adopt
> 262KHz 
> as a standard.
> 
> The advent of the short-wave craze, and multi-band broadcast
> receivers 
> dictated a higher IF frequency to achieve adequate image suppression
> on 
> the short-wave bands. The broadcast band occupied 550-1500KHz at this
> 
> time, and the designer encounters sever problems if his radio tunes 
> across it’s own IF. Some shortwave sets used 1600-1700KHz for better 
> image rejection, but one couldn’t go higher if the 160-meter ham band
> 
> (1800-2000KHZ) was to be covered. Most multi-band receiver settled in
> 
> near 450KHz, a comfortable distance from the first broadcast channel
> at 
> 550KHz.
> 
> Questions:
> 
> Odd multiples of 5KHz, 455, 465, etc., were usually chosen so that
> the 
> image of the carrier of a broadcast-band station could be zero-beat
> with 
> the carrier of the station being tuned to achieve minimal
> interference. 
> (This assumes 10KHz channel spacing. Did the Europeans (9KHz) do 
> something else?)
> 
> The Radiotron Designers Handbook, Third Edition, p. 159, states “A 
> frequency of 455 Kc/s is receiving universal acceptance as a standard
> 
> frequency, and efforts are being made to maintain this frequency free
> 
> from radio interference.”
> 
> (1) Do FCC and international frequency allocations reflect this?
> 
> (2) I’ve heard the term “Clear-Channel IF.” Can anyone cite
> references?
> 
> (3) At lease one news group posting claims that broadcast frequencies
> in 
> a particular market are assigned to prevent strong inter-modulation 
> products from falling near 455KHz. Is this factual? Need reference.”
> 
> (4) Was this (3) at least part of the reason for “Radio Moving Day”
> in 
> 1941? See:
> http://www.dcmemories.com/RadioMovingDay/032341WINXFreqChange.jpg
> 
> (5) Many National Radio sets used a 456KHz IF’s and I think I
> remember a 
> 437 somewhere. Why? Are there different considerations for short-wave
> CW 
> operation?
> 
> Further input, corrections, and elaborations are greatly appreciated.
> 
> Scolarly reference will be looked upon with great favor.
> 
> Regards,
> Al
> 
> -- 
> Al Klase - N3FRQ 
> Flemington, NJ 
> http://www.skywaves.ar88.net/
> 
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