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Of all possible IF frequencies, why did so many consumer radio and amateur HF radio manufacturers (and filter component vendors, etc.) end up converging on 455 kHz as the IF frequency, from the vacuum tube era, and onward? Is there something special about the number 455000?

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    \$\begingroup\$ It's about halfway between the top of the Long Wave band (extended by the Aircraft band) and the bottom of the Medium Wave band. Go back to the early 1930s and you'll find a lot of IFs around 110kHz, below both AM bands. \$\endgroup\$ Jan 28 at 13:15
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    \$\begingroup\$ Not all had 455 kHz. I have a Stromberg-Carlson 58-T (mid 1930's) radio with a 465 kHz IF. \$\endgroup\$
    – John Doty
    Jan 28 at 14:27

1 Answer 1

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Piecing together information from a number of sources on the web, I believe this is the correct combination of circumstances:

  • Early amplifiers were not stable much above 500 kHz, so the IF had to be less than that frequency.

  • Also, the broadcast band itself begins around 530 kHz (in the US), and you don't want any chance of having a station on or near the IF.

  • On the other hand, making the IF as high as possible improves image rejection.

  • In addition to the desired station mixing with the local oscillator to produce an IF signal, two strong stations whose frequencies happen to be separated by the IF will also produce an interfering signal.

  • In the US, stations are spaced by multiples of 10 kHz.

  • Elsewhere, stations are spaced by multiples of 9 kHz.

Therefore, 455 kHz was chosen because it is not possible to have stations separated by that amount in either system: 455/10 = 45.5 and 455/9 = 50.555. This is the highest frequency less than 500 kHz for which this is true.

Specifically, we're looking for frequencies that are as close as possible to odd multiples of both 10/2 = 5 kHz and 9/2 = 4.5 kHz. This puts the beat notes produced by this particular phenomenon as high as possible in the audio output of the mixer, where they can be filtered out. As it happens, 445 kHz would have worked equally well. The nearest frequencies to these two that would have worked just as well are +/- 90 kHz away -- 355/365 kHz or 535/545 kHz.

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    \$\begingroup\$ en.wikipedia.org/wiki/… \$\endgroup\$ Jan 28 at 7:24
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    \$\begingroup\$ Concise summary, great question. Perhaps some references could be included in the answer itself, like the link by @BruceAbbott ? \$\endgroup\$
    – P2000
    Jan 28 at 17:34
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    \$\begingroup\$ 455 isn't quite the highest integer frequency that doesn't divide evenly by both 10 and 9, even 499 qualifies. But if you're trying to optimize for being dead center between two adjacent frequencies on both systems, 455 is the optimum choice between 400 and 500. \$\endgroup\$ Jan 29 at 2:51
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    \$\begingroup\$ @MarkRansom: It isn't just a matter "not dividing evenly" -- we're looking for frequencies that are as close as possible to odd multiples of both 10/2 = 5 kHz and 9/2 = 4.5 kHz. This puts these particular beat notes as high as possible in the audio output of the mixer, where they can be filtered out. As it happens, 445 kHz would have worked equally well. The nearest frequencies to these two that would have worked just as well are +/- 90 kHz away -- 355/365 kHz or 535/545 kHz. \$\endgroup\$
    – Dave Tweed
    Jan 29 at 4:11
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    \$\begingroup\$ Obviously, the problem this addresses is only an issue in the broadcast band, but then hams would have adopted the same IF because equipment and parts (transformers, crystal filters, etc.) based on that frequency were readily available. \$\endgroup\$
    – Dave Tweed
    Jan 30 at 4:47

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