This topic seems worthy of a separate thread, which I have entitled “Radio Receiver Intermediate Frequencies”, so as not to limit it to the FM case only.
Anyway, in respect of FM receivers, a look through the likely sources on hand does suggest that 10.7 MHz was adopted quite early on in respect of Band II reception, although there were some variations.
Rider & Ulsan (1) appears to be a good reference in terms of American practice. Therein is the commentary that a 2.1 MHz IF was used for the first FM receivers tuning the original 42 to 50 MHz band, but that this was moved up to 4.3 MHz to ensure that images were out-of-band. With the change to the 88 to 108 MHz band there was some use of 8.3 MHz, but as this allowed in-band images, 10.7 MHz was adopted and was expected to become an industry standard.
The change to the 88 to 108 MHz band was announced by the FCC in mid-1945, but evidently took a couple of years or so to implement; see: “FM Broadcasting Chronology” at: http://jeff560.tripod.com/chronofm.html.
One might reasonably infer that the first use of the 10.7 MHz IF was likely to have been somewhere in the 1946-1947 period.
Tibbs and Johnstone (2) simply stated that for the 88 to 108 MHz band, a 10.7 MHz IF would appear to be standard, although frequencies in the 8 to 8.5 MHz range had been used.
Langford-Smith (3) was also succinct: “Frequency modulation receivers covering the 40-50 Mc/s band generally use a 4.3 Mc/s i-f, and for the 88-108 Mc/s band they use 10.7 Mc/s. This latter value has been adopted as standard in the U.S.A., and in some other countries, for v-h-f receivers.” Perhaps more significantly though, in chapter 38 “Tables, Charts and Sundry Data”, it notes: ”The American RTMA has standardized the following intermediate frequencies (REC-109-B, March 1950): Standard broadcast receivers – either 260 or 455 Kc/s. V-H-F broadcast receivers – 10.7 Mc/s.” Also in that section is noted that the European Copenhagen Plan allocated 415 to 490 kHz and 510 to 525 kHz as IF bands.
In the UK it would seem that 10.7 MHz was well-entrenched by the time that manufacture of FM receivers started. That attached excerpt from Molloy & Hawker (4) would i think, capture most early British FM and FM-AM receivers, and one or two FM tuners as well, and shows a general adherence to 10.7 MHz.
The notable exception in that list is Bush, with 19.5 MHz for its VHF41 model. As 19.5 MHz was used as a TV sound IF for Band I-only receivers (corresponding vision IF being 16.0 MHz), it seems possible that Bush simply transposed this number, perhaps in order to make use of existing components or designs.
Not in that list, but also dating from the start of UK FM transmissions is the Leak Troughline tuner, with 12.5 MHz IF.
According to the Leak literature, the intermediate frequency was chosen as 12.5 MHz in preference to the usual value of 10.7 MHz, with the oscillator operating on the high side of the signal frequency. This ensured that the oscillator never operated in Band II, and so could not cause interference with other receivers; that no IF harmonics fell into Band II; and that there were no images in Band I.
Actually, the first two of those three were true only for a Band II range of 87.5 to 100 MHz. The tuning range of the original Troughline was quoted as 88 to 100 MHz. But I should not be too surprised if Leak, who was very export oriented, had offered an export model that covered 88 to 108 MHz. Anyway, one may see where the 12.5 MHz came from; it was simply the difference between the UK Band II limits, namely 87.5 and 100 MHz, which both happened to be multiples of their difference.
The Troughline II, Troughline 3 and Troughline Stereo all tuned 88 to 108 MHz, but retained the 12.5 MHz IF. Leak did amend its benefit claims to simply that of eliminating mutual interference with neighbouring FM and TV receivers. With the solid state Stereofetic 1969, Leak reverted to the standard 10.7 MHz, probably in part because this model used standard ceramic IF filters.
As far as I know, Armstrong, Chapman, Jason, Pye and Quad all used a 10.7 MHz IF for their early FM tuners, dating from 1955 or even earlier in some cases.
Another possible exception is Eddystone, which used 5.2 MHz for its 770R VHF receiver. Given that it was a VHF communications receiver, covering 19 to 165 MHz, then perhaps there is no good reason why 10.7 MHz might be expected. On the other hand, it did cover wideband broadcast FM, as well as narrow band (for the time) communications FM of up to ±15 kHz deviation. And there was some evidence that in the USA at least, 10.7 MHz had quite early on been adopted for non-broadcast VHF receivers. Certainly it did become an important number in VHF communications; by the end of the 1950s 10.7 MHz crystal filters were available for 50 and 25 kHz channelling, and I think in anticipation of 12.5 kHz channelling. That was a few years before ceramic filters became the norm in domestic FM receivers. Eddystone changed to the standard 10.7 MHz IF for its solid-state 990R VHF communications receiver, successor to the 770R. (The 990R appears to have been a tour-de-force in germanium pnp technology that appeared on the cusp of the silicon planar era, itself soon overtaken by the combination of mosfet front ends and IC-based IF strips.)
Anyway, a summation might be that 10.7 MHz became the standard (both actual in some territories and de facto elsewhere) for Band II FM receivers quite early on in their development, and before many countries started their respective FM broadcasting services. Nevertheless, there were certainly some interesting exceptions.
For the non-band II cases, as far as I know the 10.7 MHz IF was used in Japan, where the FM band was and is 76 to 90 MHz. I should guess that the OIRT countries used 10.7 MHz for their Band I FM broadcasts, except perhaps in the early days in Russia.
- (1) “FM Transmission and Reception”, John F. Rider and Seymour D. Uslan, Second Edition, November 1950.
(2) “Frequency Modulation Engineering”, Christopher E. Tibbs and G.G. Johnstone, Chapman and Hall, Second Edition 1956.
“Radio Designer’s Handbook”, F. Langford-Smith, Newnes, 1997 reprint of Fourth Edition 1953, ISBN 0 7506 3635 1.
(4) “Radio Servicing Pocket Book”, E. Molloy & J.P. Hawker, editors, Newnes, 1955.