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Thursday, March 26, 2020

Radio History Question: Why 455 kHz as the IF frequency?


My work on the S-38Es, on the HRO-dial receiver, on the Mate for the Mighty Midget,  and on various mechanical filters has caused me to think (once again) about why we ended up with 455 kHz as the  IF frequency for so many radios.  I've heard many explanations for this, but unfortunately I've forgotten the explanations and lost the sources.  I started digging into this again today.  I found the below e-mail from Al N3FRQ on the Boatanchors mailing list (2008). 

I contacted Al to find out if he had learned anything else on this topic.  He has not.  So if anyone out there has answers to Al's questions, or anyother info that would shed light on why they went with 455, please let us know. 

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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/

6 comments:

  1. Real men know that the real world is analog. Almost anybody can use a DSP chip to make a software defined, low power, ultra small, state of the art radio. It takes a real man to step back in time, to use discrete parts and these shiny, vintage,hard to find IF cans, to build an incredibly chunky, but high performance AM or shortwave radio. If this is not your cup of tea, step away now.

    https://www.bgmicro.com/ACS1667.aspx

    Ed KC8SBV

    ReplyDelete
  2. Re 9kHz European stations.

    Australia used to be 10kHz spacing but changed on 23rd November 1978. I believe Europe and many other countries changed on the same date.

    Peter VK2EMU

    ReplyDelete
  3. https://en.wikipedia.org/wiki/Geneva_Frequency_Plan_of_1975

    ReplyDelete
  4. In this country the largest company manufacturing them would probably hold sway for the others to follow. Standards have been established this way as well.

    ReplyDelete
  5. Question: What's the earliest dual conversion SW receiver? The oldest one I remember coming across is the Crosley 86CR from 1946, which uses (from memory) 5865 and 165 KHz on the AM/SW bands and the usual 10.7 MHz for FM.

    ReplyDelete
  6. Ray Moore's book indicates that dual conversion was post WWII. Halli's first was the SX-17 in 1950. 2075 kc then 455 kc. Hammarlund SP600 also 1950 - 3955 and 455 kc IFs. 1952: National HRO-60 2020 and 456 kc; NC183D 1720 and 455 kc.

    ReplyDelete

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