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Radio Receiver Intermediate Frequencies

 
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Re: Radio Receiver Intermediate Frequencies

Post by turretslug » Tue May 05, 2015 2:07 pm

The NTSC crystal hypothesis seems very plausible- a traditional free-running LC 2nd conversion oscillator at around this frequency could be a liability as regards drift- even if the 1st LO (i.e. main tuning) was adjusted to compensate, the wanted signal would now be unfavourably placed in the 1st IF passband (assuming, as would be hoped, that this at least provided initial or "roofing" selectivity). Nowadays, we're accustomed to crystals being cheap and ubiquitous in everything from alarm clocks through radios to computers, churned out by the umpteen million in a bewildering array of frequencies but it would have been a different matter then. Could 3.579545 MHz crystals perhaps have been the first widely-available and reasonably cheap type? I gather that they were also used, divided down in various ratios, as the basis for tone dialling. Additionally, "osc low" for 2nd conversion will preserve the sense of USB/LSB BFO setting (or sideband filter selection, if used) compared to the single-conversion bands.

I am a little surprised that the dual conversion wasn't switched in, say, a band lower though- whilst a set that switches across an array of broadcast bands of relatively narrow span may not be too affected by misleading image reception, a general coverage set of 455kHz IF, presumably here with three-gang tuning, would be showing misleading "ghost" reception in the images of 16 and 19m bands at least.

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Wed May 06, 2015 5:52 am

I had also wondered why double conversion was not used on the 7.5 to 18 MHz band as well as on 18 to 30 MHz. Heathkit quoted image rejection numbers of ≥35 dB for the 18-30 and 3-7.5 MHz bands, but only ≥25 dB for the 7.5-18 MHz band. So the last-mentioned would have benefitted from double-conversion. Now that I have thought about it some more, I wonder if there were lowish harmonics of the crystal oscillator that might have been problematical in the 7.5-18 MHz band without additional and serious screening. From what I have read about TV IF selection, it seems that the 5th harmonic of anything potentially nasty is about the highest that one has to worry about. So 18 MHz just clears the 5th harmonic of the crystal oscillator.

Perhaps too a mitigating factor was that both the Heathkit GR-78 and the Kenwood/Trio QR-666 had bandpsread tuning (of the fiddly kind that first required precise setting of the main tuning). The bandpsread drums could be marked optionally with the broadcast or amateur bands. That facility would have helped with avoiding accidentally tuning into a ghost, something that I know is all too easily done with non-bandpsread shortwave receivers. Kenwood/Trio must have been happy enough with its product (in relationship to its price point), as essentially the same arrangement was used on its later R300 model.

Cheers,

Steve

 
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Re: Radio Receiver Intermediate Frequencies

Post by turretslug » Wed May 06, 2015 2:14 pm

One thing that strikes me about the generation of HF PLL synthesised receivers that were extant about 10-40 or so years ago (thus hopefully qualifying as "vintage"!) is the remarkable stopband selectivity achieved by VHF 1st IF crystal filters, thus enabling 2nd conversion straight to a low final IF of the 455 or 1400kHz region without an intervening and intermediate IF (tautology unintended!). 1st IF frequencies I am aware of are scattered between 35.4MHz (Racal RA1772) and 70.455MHz (JRC NRD505/515) and I'm sure higher ones were used in some sets. To put this in perspective, imagine this 1st IF as a single-channel VHF receiver with a very low subsequent IF- with classical LC filtering, this would be asking for almost unusably severe image problems. Before crystal 1st IF filters became mainstream, the usual 1st:2nd IF ratio seemed to settle in the region of 15-20:1, presumably representing optimum compromise between effective 1st and 2nd IF image rejection capabilities without unmanageably complex and expensive 1st IF filters. Marconi's Mercury receiver is a good example of an exception to this rule, with 1st IF of 4.5MHz and 2nd IF of 85kHz- here, the 1st IF filter was an optimally-coupled module with 4 tuned circuits and the 2nd LC conversion oscillator presumably took some care over stability and temperature compensation for the reasons discussed above re. NTSC crystal usage. Examples of this approximate ratio include Eddystone 750 (1.62MHz:85kHz), 910 (1.4MHz (centre):85kHz), 830 (1.35MHz (centre):100kHz) and STC R4187 (2.15MHz:100kHz)- this latter having 4 tuned circuits at 1st IF. The Mk3 version of the G2DAF receiver featured 5-5.5MHz 1st IF and 455kHz 2nd IF (i.e. approx 11:1 ratio)- even here, considerable emphasis was placed on the need for careful isolation and screening to ensure good stopband suppression. Racal's RA17 featured 3-2MHz 2nd IF and 100kHz 3rd- another example of a wide ratio, here, a four-gang "kHz" tuning capacitor was used, presumably in the interest of 3rd IF image suppression on what was pitched as a "reference" standard receiver.

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Fri May 08, 2015 4:05 am

That certainly provides food for thought.

In respect of the Racal RA17 and its final conversion, one might look askance at a single-conversion receiver that tuned 2 to 3 MHz and had a 100 kHz IF. A four-gang rather than a three-gang front end would confer some credibility, though.

Amongst upconversion HF receivers, the highest 1st IF of which I am aware is 81.4 MHz for the Rhode & Schwarz EK-070, which I think was a 1980s model. Its 2nd IF was 1.4 MHz, which gave a ratio of 58:1. The Marconi Oceanic (a rebranded Dansk M3000) of circa 1984 had a 75.0 MHz 1st IF. The Eddystone 1650 of about the same time had a relatively ordinary 46.205 MHz 1st IF, but in the very wide AM mode (14 kHz bandwidth), the 46.205 MHz crystal filter provided all of the selectivity, there being no corresponding 1.4 MHz filter. So definitely those crystal filters were very good in-situ. One imagines that impedance matching and screening of possible bypass paths needed to be near-immaculate.

Conversions to and from 10.7 MHz provided some high IF ratios, too. 10.7 MHz to 455 kHz was not too high, at 23.5:1. Still I suspect that the combination might have seen increased used for VHF R/T equipment after narrow bandwidth 10.7 MHz crystal filters became available towards the end of the 1950s. In some cases the 10.7 MHz filter might have been the primary selectivity element, with the 455 kHz filter playing a secondary role. 10.7 MHz to 100 kHz, 107:1, was done in the Plessey PR155 HF receiver, so the 10.7 MHz crystal filter must have been a very good one. Amongst VHF-UHF receivers, the ICOM R7000 converted from 778.7 to 10.7 MHz, and the Rhode & Schwarz ESM-1000 from 810.7 MHz, for a ratio of 76:1. Both I think had SAWF 1st IF filters. That in the R7000 was of wide enough bandwidth to accept an NTSC or CCIR TV channel.

Going back to the valve era, the Mullard GFR552 point-to-point ISB receiver had a 1st IF of 3.1 MHz and a 2nd IF of 100 kHz, for a ratio of 31:1. That would have been with LC filters at 3.1 MHz. The Marconi valve-era ISB receivers seem to have had 1st IFs in the range 1.6 to 2.9 MHz, although the solid-state Hydrus had 40 MHz, 5 MHz and 100 kHz, thus 50:1 for the final conversion. Probably this had a crystal filter at 5 MHz, though.

Some of the Drake receivers converted from a 1st IF of 5645 kHz down to 50 kHz, for a ratio of 113:1, but as far as I know these had crystal filters at 5645 kHz.

An extreme, albeit probably trivial example is provided by the small American weather band receivers, which covered 7 NBFM channels in the 162 MHz band. Some of those converted signal direct to 455 kHz, for a ratio of 356:1. But they served their purpose well enough.

Cheers,

Steve

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Tue Jun 23, 2015 5:11 am

I recently came across the attached item, from Wireless World 1958 January.

WW 195801 p.44 BREMA FM IF.jpg

WW 195801 p.44ex BREMA FM IF.gif


It confirms that BREMA did formally adopt the 10.7 MHz IF for Band II FM radio receivers.

By then 10.7 MHz was the de facto global standard, so for BREMA to have done otherwise could have been awkward for setmakers involved with exports, to say the least.

Nevertheless, Leak stayed with its own 12.5 MHz number, adopted for the original Troughline, through the Troughline II, Troughline III and Troughline Stereo iterations, all of which postdated the BREMA proclamation, not changing to 10.7 MHz until the solid-state Stereofetic of 1969.

On the other hand, Jason chose the 10.7 MHz IF for its JTV FM (Band II) and TV sound tuner (Bands I & III) of 1958, and stayed with this for the JTV2 version in 1960 and the Mercury II of the same year. There was American precedent for using 10.7 MHz for combined FM and TV sound tuners, at least where the apparent TV soudn coverage was Bands I & III.

Cheers,

Steve

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Tue Jun 23, 2015 7:13 am

Here is another variation on FM receiver IFs, though. I have come across one data point only that suggests that Russian FM receivers, which would have tuned the OIRT FM band of approximately 66 to 74 MHz, had a standard IF of 8.4 MHz, at least in earlier times.

This comes from a perusal of an article in Wireless World for 1961 August that describes a Soviet exhibition held in London. Pertinent parts of that article are attached.

WW 196108 p.399 Russian FM IF.jpg

WW 196108 p.399ex Russian FM IF.gif


In respect of Russian combined TV-FM receivers, it was noted that some use a second FM frequency changer to get from the 8.4 MHz FM IF to the 6.5 MHz TV intercarrier. At least that creates the impression that 8.4 MHz might have been the standard Russian FM receiver IF, which number appears to be reasonable for the transmission frequencies used. But one would need additional datapoints to confirm, so for now 8.4 MHz is a provisional number.

There are some other points in the article, more in the nature of obiter dictum in the context of this thread, but nonetheless interesting. It is mentioned that Russian TV receivers were often fitted with headphone and tape recorder outlets. Whether the latter were done “properly”, i.e. low-level direct from the demodulator and ahead of the AF stages, or by the “quick and dirty” method, i.e. by tapping the audio output transformer, is unknown, but I guess that there is some chance that the former was used at least in some cases. The picture tubes of the time evidently had a 4:3 aspect ratio, whereas I think that Western tubes were then still mostly 5:4, with the 4:3 type not arriving until the mid-1960s.

And the polar system for FM stereo broadcasting was mentioned. This does not seem to have had much mention in the West. Quite why the Russians adopted it rather than the GE-Zenith Pilot Tone system is unclear, although I recall reading somewhere that the latter was less suitable with the ±50 kHz deviation. The actual subcarrier frequency was 31.25 kHz, as far as I know arrived at because it was twice the 625-line TV line frequency, thus allowing the same system to be used for TV sound, although I am not sure it ever was.

Cheers,

Steve

 
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Re: Radio Receiver Intermediate Frequencies

Post by turretslug » Wed Jul 01, 2015 10:54 am

The set under discussion in this thread;

www.forum.radios-tv.co.uk/viewtopic.php?f=3&t=11446

is interesting, not merely as an early example of double superhet technique, but also of the "tunable IF" system that proliferated in the 'fifties in professional (typically, but not always, spanning 1MHz) and subsequently amateur (often spanning around 500kHz) receivers. In this case, the use of the MW band as the tunable IF was dictated by "what's already budgeted for" considerations, being essentially an SW converter in front of a MW/LW receiver but could be viewed as asking for trouble with breakthrough of strong MW stations, even in the mid/late 'thirties. I wonder how much of a problem it was in practice (given that aerial advice in those days was frequently something like "as long and as high as possible"!) and whether it was found necessay to incorporate any high-pass filtering in the SW part front end, in addition to any normal input tuning?

If fixed 1st IF was always going to be something of a Hobson's choice as far as finding a quiet spot in the spectrum was concerned, tunable IF must have been rather worse, given that the usual LC filter technology of the time pretty much dictated choice of a span in the lower HF bands, frequently awash with strong RTTY/fax type transmissions even when away from the broadcast/amateur bands. For some of their models, Eddystone even used a narrow (100kHz (910) or 200kHz (other, both thermionic and semiconductor sets)) slice of the upper MW bands. Perhaps I'm envisaging ogres where only gnats exist, but the necessity for good screening/trapping/filtering seems to have become more prominent over the evolution of these sets, possibly users became less tolerant of spurios/breakthrough.

 
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Re: Radio Receiver Intermediate Frequencies

Post by Cathovisor » Wed Jul 01, 2015 11:34 am

turretslug wrote:I wonder how much of a problem it was in practice (given that aerial advice in those days was frequently something like "as long and as high as possible"!) and whether it was found necessay to incorporate any high-pass filtering in the SW part front end, in addition to any normal input tuning?

The circuit is at http://www.murphy-radio.co.uk/diagrams/A36.jpg for study but by this time, "as long and as high as possible" for aerials was already in its death throes; many manufacturers suggested limiting the aerial to just forty feet total by the time the A36 came out.

 
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Re: Radio Receiver Intermediate Frequencies

Post by turretslug » Wed Jul 01, 2015 12:23 pm

Thanks for that! So no added MW rejection, other than the LF-attenuating action of the tapped-down aerial coupling arrangement itself. Fixed gain on the SW front end too- I can see that the reflexed RF stage would need to be left optimally biased for low AF distortion but I'm surprised that there wasn't even potted down AGC on the mixer. A few other interesting points but I won't distract from the thread title...

Forty feet is quite short, really- a small garden, or not even two-thirds of a cricket pitch!

 
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Re: Radio Receiver Intermediate Frequencies

Post by turretslug » Wed Jul 01, 2015 3:43 pm

To be fair, there were quite a few car radio SW converters over the years that used the "MW IF" principle- as car radios had to be inherently well screened and filtered devices anyway, the principle here started on a sound practical footing and aerials were necessarily short. Always assuming that the installer was reasonably competent and scrupulous re. soundness of interlinking screening.

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Sun Jul 12, 2015 1:49 am

I have recently come across a couple of articles in “Radio News” magazine for 1947 which provide useful background information on American domestic receiver IF choices. This magazine is available at the excellent American Radio History site, at: http://www.americanradiohistory.com/Rad ... _Guide.htm.

The two articles are in fact part of a long series entitled “Practical Radio Course” by Alfred A. Ghirardi, and the two at interest here are: Part 53, in the 1947 May issue, and Part 56, in the 1947 November issue. Part 53 discusses IFs for AM, FM and TV receivers. Part 56 elaborates on FM IFs.

Looking at FM first, the original American FM band was 42 to 45 MHz, early receivers for which typically had an IF of 2.1 MHz. Later receivers for this band had an IF of 3.2 or 3.3 MHz. Then the FM band was extended to cover 42 to 50 MHz. For this, an IF of 4.3 MHz was used and recommended by the RMA. Apparently this number satisfied the criterion that it be high enough that all images were out-of-band, and was also adequately clear of the 3.5 to 4.0 MHz amateur band.

With the upward movement of the FM band to 88 to 108 MHz, as announced in 1945, the RMA recommended an IF of 10.7 MHz, with oscillator high, rather than oscillator low as had previously been the norm for FM receivers. 10.7 MHz ensured that all images were out-of-band, and oscillator-high ensured that the images were above the FM band, and not below it where they would be in the TV channel allocations. 10.7 MHz was in the aero band, 10.2 to 11.3 MHz, which was considered not likely to create an interference problem, particularly as 10.7 MHz would not itself be allocated to a transmitter.

Apparently IFs of 8.25, 8.3 and 8.6 MHz were also used for early receivers for the 88 to 108 MHz FM band. Possibly there was some delay before the RMA number of 10.7 MHz was promulgated, and during that time setmakers had to choose their own IFs. Be that as it may, it does look as if the 10.7 MHz number was arrived during 1946, and so very early in the history of Band II FM. In fact it probably predated the Band II terminology, which I think came out of the 1947 Atlantic City frequency allocation meeting.

In the AM case, there was a migration to higher IFs as improved valve and IF transformer technology allowed, and as better suited “all-wave” receivers. Apparently IFs in use included, but were not limited to 450, 455, 456, 465 and 470 kHz, although many car receivers used 260 kHz. Following this the RMA studied the subject and came up with the standard number of 455 kHz. Although not stated in the Radio News articles, I imagine that this happened in the later 1930s. It was noted that it was not possible to have a below-band IF that completely avoided in-band images, and that 455 kHz was in a marine band, so that interference from marine transmissions was possible.

The foregoing generally confirms what has gone before in this thread, with some elaboration.

I’ll also add a few comments to the TV IF thread based upon those Radio News articles.

Cheers,

Steve

 
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Re: Radio Receiver Intermediate Frequencies

Post by turretslug » Fri Jul 17, 2015 10:05 pm

A refreshingly straightforward explanation then for several of the FM IFs! (including 10.7MHz), i.e. that they were slightly more than half the span of the "currently mooted" band, allowing for the image to be out-of-band.

Synchrodyne's mention of the Heathkit Mohawk on another forum (re. its usage of the 6CS6 (EH90) as first and second mixer, rather than the typically used 6BE6 (EK90)) prompted a quick search, revealing that it used another curiously precise 1st IF: 1.682MHz. This doesn't ring any bells here as allowing for any immediately obvious 2nd LO crystals (50kHz 2nd IF) and, moreover, it used sideband selection by switching between 1.632 and 1.732MHz types for 2nd conversion- any ideas, anyone? :aab

There were a miscellany of receivers that resorted to IFs in the region of 1.6-1.7MHz, hoping to strike a compromise between image rejection and selectivity in the higher HF regions without having to resort to four- (or more) gang tuning with its expense and complexity and consequent receiver size and expense by using a relatively quiet spot between the top of the MW band and the commercial/amateur allocations (though there is an account by a member on another forum of a Star amateur bands rceiver with IF of 1650kHz being blighted by a nearby shore station on just this frequency). Eddystone's 640 was probably one of the earlier examples made in significant numbers and featured one RF stage and two 1.6MHz IF stages, probably representing a good performer for the keen SWL/amateur of the time. There were murmurs of "barn-door" selectivity, though. The subsequent 750 model effectively represents (at least electronically) a revisiting of the design with miniature valves and a re-appraisal of the weak points- a separate HF oscillator valve was now used, together with a shunt voltage regulator and what would have been the 6BA6 1st 1.62MHz IF amp was now an ECH42 converting to 2nd IF of 85kHz. Perhaps Eddystone were slightly stung by criticism of the 640's selectivity shortcoming and there was an element of "we'll show 'em" in choosing a second IF all the way down at 85kHz, hopefully now giving good selectivity with just two IFTs at this frequency (four tuned circuits). Certainly, Eddystone dispensed with the 1.6MHz crystal filter that had featured in the 640, claiming that the variable-coupling 85kHz IFTs in the 750 would be sufficient for CW reception.

This 85kHz is another IF that featured in an eclectic selection of receivers, the BC453 being perhaps one of the best-known, it also featured in a number of Marconi designs. Not sure of the origins of this choice- I'm aware of early superhets that used in the region of 110-130kHz, perhaps some of the pre-war gurus here might shed some light. Possibly, it was too low for "fidelity" bandwidths and was more of a "communications" design choice. As well as the Drake receiver mentioned upstream, the previously mentioned Mohawk featured 50kHz IF- again, this looks like an attempt to realise both extreme and relatively easily variable selectivity without excess expense or complexity. With respect to the 1.682MHz/50kHz ratio, perhaps it's unsurprising that the Mohawk featured two double-tuned IFTs at 1st IF to keep 2nd IF image suppression acceptable. A 6BA6 sat between them, operating at low gain with 1k cathode and 68k screen-grid resistors- presumably not much more than was necessary to overcome IFT loss and prevent the second 6CS6 multigrid from detracting significantly from noise performance.

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Fri Aug 07, 2015 6:12 am

Unlike the case of the Heathkit GR-78, for whose 4.034 MHz 1st IF one may derive a plausible rationale, the 1.682 MHz 1st IF of the Heathkit Mohawk defies post facto analysis, at least in its precision.

Similarly not obvious are the reasons for choosing 85 kHz as a final IF, as compared with say 50 or 100 kHz. A working hypothesis might be that a 2nd IF as low as reasonably possible was required, but that calculation of the possible spurs in a given situation indicated that around 85 kHz was the lower limit. Spurs aside, I am not sure what would have been the practicable lower limit for an IF strip back in the days of valve technology, but I imagine that 50 kHz was getting close to it.

An interesting commentary on the relative merits of the circa 1.6 MHz IF are provided in Amalgamated Wireless Valve (AWV) “Radiotronics” #145 of 1950, October. This is available at: http://frank.pocnet.net/other/AWV_Radiotronics/. It does not though provide precise IF recommendations within the 1.6 MHz range.

AWV Radiotronics #119 of 1946 May helps establish a timeline for the American FM receiver IF of 10.7 MHz. On page 54, it is noted: “It has been proposed by the R.M.A. Standards Committee that an intermediate frequency of 10.7 Mc/s be adopted for use with v-h-f receivers.” Allowing that this happened a few months ahead of publication date, then the RMA worked fairly fast, as the decision to move FM up to the 88 to 108 MHz band was announced mid-1945.

Also from AWV Radiotronics, #117 of 1946 January, records that in Australia, 455 kHz had been kept as a clear channel for use as a radio receiver intermediate frequency. Evidently Australia had followed US practice, not so surprising as at the time, it also used 10 kHz channelling for the MF broadcast band.

Cheers,

Steve

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Fri Oct 30, 2015 1:59 am

Also worth recording here is the renaissance of the 450 kHz for domestic AM (MF and MF/LF) receivers, including the AM sections of hi-fi tuners. I am not sure exactly when or why this happened, but I think that it was in connection with the introduction of electronic tuning, particularly for car radios. So that would have made the transition period late 1970s and early 1980s. Given that 450 kHz, being a multiple of 10 kHz, was probably a less desirable IF than 455 kHz in those areas that had 10 kHz MF channelling, one assumes that it was sufficiently a preferable number for use with electronic tuning systems (at least in their early incarnations) that the change was justified. Thus the filter industry made available an appropriate range of ceramic 450 kHz IF bandpass filters and the like.

Of course, 450 kHz as an AM receiver IF was not new. As noted upthread, Eddystone had used it for some of its valve-era single-conversion HF receivers, although it had also used the standard 455 kHz for some of its later solid-state single-conversion designs. And 450 kHz was one of the several IFs that had been in use in North America prior to the standardization on 455 kHz.

The widespread introduction of 450 kHz as an AM receiver IF did not seem to carryover to HF receivers though, where 455 kHz remained the norm, at least where a number in that general vicinity was required. Two mid-1980s Sony portable receivers provide examples. The SRF-A100 was a small FM stereo/AM stereo model, and had a 450 kHz AM IF, even though it was conventionally tuned not electronically tuned. The well-known ICF-2001D/2010 HF receiver was double-conversion on AM, with a 455 kHz second IF. That was notwithstanding the fact that for AM synchronous demodulation, it used the same stereo AM IC set as was deployed in the SRF-A100. Thus for the included PLL, which operated at 8x IF, the SRF-A100 had a 3.6 MHz oscillator crystal, whereas the ICF-2001D/2010 had a 3.64 MHz crystal.

450 kHz may have been seen as something of a norm by the time AM stereo arrived in the USA and elsewhere. The Motorola MC13020 was a very early C-QUAM stereo decoding IC, and the datasheet quotes 450 kHz as a typical IF, which in turn required a 3.6 MHz PLL oscillator crystal.

Interestingly, the datasheet for the Motorola MC13030 AM dual-conversion IC mentioned upthread referred to both 450 and 455 kHz for the 2nd IF. Other AM receiver IC datasheets provide some additional clues about the transition to 450 kHz. That for the well-known Hitachi HA1197 refers to a 455 kHz IF. The Sanyo clone of the HA1197 was the LA1240. The Sanyo LA1245, presumably a later development, accommodated electronic tuning, but still referred to a 455 kHz IF. But the LA1247 data sheet referred to a 450 kHz IF. National had also entered the electronically-tuned AM radio IC field in the early 1980s with its LM1863, and the corresponding data sheet mentioned a 450 kHz IF.

Cheers,

Steve

 
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Re: Radio Receiver Intermediate Frequencies

Post by turretslug » Fri Oct 30, 2015 11:45 am

I had assumed (and I'll readily admit that I haven't exhaustively researched the subject, either in its practical implementation or its underlying maths) that, post-WARC '78 (?), 450kHz represented a convenient common multiple in both 9kHz and 10kHz channeliised broadcast receivers with 900kHz and 1350kHz IF harmonics zero-beating (or at least, given production tolerances and drift, not whistling too obtrusively) with allocated channels in both systems.. I've also encountered 459kHz and 468kHz ceramic filters in synthesized car radios and domestic tuners, sometimes the instructions will make reference to this being the IF for "European" market version, evidently with 9kHz spacing in mind. Presumably, these offered sufficient advantage over 450kHz filters for the MW/LW spectrum as it existed in this region to make it worth producing non-"universal" front end/IF strip manufacturing lines.

As for the continuing usage of 455kHz in HF receivers, maybe 5kHz channel spacing and its IF harmonic implications/other possible spur products were in mind and the existing 455kHz product array was "good enough". Or could filter manufacturers have been reluctant to produce a 450kHz line of high-quality multi-element communications-spec filters when there was already a range of such items long established at 455kHz? Such a market at any frequency is probably much smaller than that for simpler filters for less demanding mass-market consumer broadcast receivers and choosing/sticking to 455kHz perhaps made it easier to offer selectivity options at "enthusiast" level spec and above- and anyone who more than casually listened to HF broadcasting soon becomes "enthusiast" at least as far as an awareness of the need for half-decent selectivity is concerned!

As for Eddystone's choice of 450kHz, it is a bit off the by-then-accepted "beaten track" of 455kHz. One thing that occurred to me is that they typically liked to offer MW coverage down to 480kHz, rather than the usual "broadcast" 520-530 or so kHz of others. (Not sure why, possibly they liked to pitch their receivers at professional, as well as well-heeled domestic usage, and this would have offered watch capability at 500kHz and other marine frequencies.) Tuning IFTs down to 450kHz might have offered a bit more safeguarding against IF breakthrough or other spurs resulting from input RF and high level IF getting too close in frequency. I know from my own tinkering with a CR100 (IF 465kHz, input RF coverage gap between 420kHz and 500kHz) that slight mis-tweaking of oscillator frequency can produce a shriek in this region.

 
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Re: Radio Receiver Intermediate Frequencies

Post by turretslug » Sat Oct 31, 2015 12:05 am

Beg pardon- it was actually the Geneva Frequency Plan of 1975, coming into force in November 1978, that put European MW channels onto integer multiples of 9kHz.

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Mon Dec 14, 2015 8:27 am

Thanks much for that. I had completely forgotten that the 1978 changes could have had a corresponding effect on IFs. Not just in Europe, either, as that was approximately when countries like New Zealand, Australia and Japan changed from 10 kHz to 9 kHz MW channelling.

Thus I suspect that the 450 kHz idea might have come from Japan, given that the OEMs then needed to accommodate 9 kHz channelling for its domestic and many export markets, and also 10 kHz for the all-important North American market.

I have had a look through the Ambit Catalogues 1, 2 and 3 (I do not have #4), as these were issued at about the time of the change and just after. None mention the 450 kHz IF or filters for it. Amongst the AM IFs and filters mentioned, 455 kHz was the most common, with some references to others, including but not limited to numbers such as 460 and 470 kHz. I don’t recall any discussion in Wireless World of the period to the effect that the 1978 changes required changes in British (or European) receiver IFs. And I think that some makers continued to use 470 kHz. That said, the British post-Copenhagen IFs of 422 and 470 kHz were not, as far as I know, universal for Europe, which probably explains the existence of 460, 468 kHz and so on. So it is possible that some parts of Europe needed different IFs after the 1978 changes.

Some data on hand for several Sansui tuners provides a clue. The TU-X1 “supertuner” of 1979-to-1986 was analogue-tuned and had a sophisticated AM section with PLL demodulation. It had a 455 kHz IF.

The TU-919 tuner, also of 1979-to-1985, was analogue-tuned but was also “digitally quartz locked”, with a digital was as well as an analogue readout. The data shows two alternative AM IFs, namely 450 and 455 kHz. Whether these were “either/or”, perhaps according to destination market, or sequential, with say a change from 455 to 450 kHz during the production run is not clear. But it does seem to have been an early use of 450 kHz by a Japanese OEM. The TU-919 used a Hitachi HA1197 AM IC.

The TU-S77AMX (released 1983) and TU-D99AMZ (1985) were both digitally synthesized, with 450 kHz IF. The former used a Sanyo LA1245 AM IC, the latter either the same or the LA1247.

The American OEMs, probably less concerned about export markets than the Japanese, on the face of it had less reason to change away from 455 kHz than did the Japanese, but 450 kHz was appearing as an IF by 1985 at least. Digital tuning may have been a part of this. The American “car radio” IF seemed to have moved from 260 and 262 to 262.5 kHz in the digital tuning age, although that is based upon casual observation.

Starting in 1982 December, the Australian magazine “Electronics Australia” ran a construction project for a hi-fi AM tuner. This had analogue tuning with a digital readout and a 455 kHz IF. Starting 1984 October, it ran another construction project for an add-on C-QUAM AM stereo decoder, for use with the above or other tuners. This was based upon the Motorola MC13020 IC. In the detailed commentary, after noting that the VCO in that IC ran at eight times IF, it was said: “Typically the IF will be 455kHz so the VCO runs at 3.64MHz. Note, however, that many synthesized tuners use a 450kHz IF and this requires a VCO frequency of 3.6MHz.”

So a possible pathway to 450 kHz might have been that the Japanese OEMs found this to be a convenient and workable number for both 10 kHz channelling and the new variant of 9 kHz channelling, which had the channel frequencies as integral multiples of 9 kHz. Then it was also found to be convenient for digital tuning purposes, which thus pushed it into the mainstream. Quite recently, somewhere I have seen that 450 kHz was included alongside 455 kHz in the appropriate American standard, but right now I can’t find it. The standard would have been ANSI/CEA-109-D of 2010, which I think was the first revision since CEA-REC109-C of 1955, although it was later renamed as ANSI/CEA-REC109-C.

Cheers,

Steve

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Mon Jan 25, 2016 2:35 am

Discussed upthread was the use of 1.4 MHz as a second IF in professional upconversion HF receivers, which practice may be dated back at least back to the Redifon R550 of early 1969.

The following item from Wireless World 1970 May, about some new Cathodeon IF filters, provides some additional background.

WW 197005 p.249 Cathodeon.gif


The subject Cathodeon filter range was aimed at the marine market, in anticipation of the then-forthcoming introduction of suppressed carrier SSB for marine R/T communications.

The range included 1.4 MHz USB, LSB and DSB filters, from which it may be inferred that Cathodeon saw this as at least a de facto standard that was likely to be chosen by a reasonable number of marine R/T equipment makers. One assumes that Cathodeon had reviewed the matter with its potential customers before proceeding with design and production.

Also included was an RF bandpass filter covering the range 1.6 to 3.8 MHz. This, I should guess, was intended for the simpler R/T equipments used on coastal and fisihing vessels, etc., where coverage of the higher HF marine bands was not required. This filter also had 70 dB rejection at 1.4 MHz.

The logical deduction here is that 1.4 MHz was seen as a suitable IF for single-conversion receivers that covered the 1.6 to 3.8 MHz marine HF band, as well as the marine MF band below say 520 kHz. On the other hand, 1.6 MHz or thereabouts would not have been workable. Then also, 1.4 MHz was suitable for dual-conversion receivers where 1.6 MHz might otherwise have been used. So, one set of IF filters would cover both cases.

Confirmation of that deduction is provided in this item from WW 1970 October, announcing the introduction of a similar range of 1.4 MHz IF filters from Plessey, including one suitable for CW, as well as a 1.6 to 3.8 MHz bandpass filter with 1.4 MHz rejection.

WW 197010 p.519 Plessey Marine IF Filters.gif


So with 1.4 MHz established as at least a quasi-standard number and with a full range of professional-quality filters available, it is not so surprising that Racal, Plessey, Eddystone, Marconi and others also adopted it.

Cheers,

Steve

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Mon Jan 25, 2016 3:05 am

Here is another case where the products of an IF filter maker provide an historical reference point. Here it is Salford with 37.3 and 21.4 MHz filters, in a WW 1968 October item.

WW 196810 p.378 Salford Crystal Filters.gif



37.3 MHz was the 1st IF used by Plessey in its PR155 professional HF receiver. I am not sure, though, that it was a widely used IF.

The 21.4 MHz filter was mentioned in connection with HF receiver applications, but with a 30 kHz bandwidth, it looks more like a VHF/UHF receiver filter. At the time, 10.7 MHz filters for VHF/UHF communications receivers were usually available in 30, 15 and 7.5 kHz bandwidths or thereabouts. This is the earliest mention I have seen of the 21.4 MHz number.

Cheers,

Steve

 
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Re: Radio Receiver Intermediate Frequencies

Post by Michael Watterson » Mon Jan 25, 2016 11:54 am

30KHz is fine for a 1st IF in a dual conversion receiver. Even 200KHz wide first IF is fine on a dual conversion with 450 to 480 KHz second IF. The main purpose of the high first IF is for image rejection, secondarily intermodulation, so 30KHz is better than 200KHz for intermodulation performance / noise on the 2nd mixer.
It's only on a single conversion IF you need the 9KHz (or whatever) bandwidth.

The Sony ICF5900 uses the 10.7MHz FM IF filters as 1st IF on dual conversion for Shortwave. That became a common approach. The ICF2001D is also dual conversion, but I don't remember if it uses same filters for FM IF and AM 1st IF.

Some cable receivers and modems receive 80MHz to 870MHz. They use a quite wide 1.2GHz first IF. The second IF is often approximately 44MHz (USA TV IF I think) and before SDR versions was 2MHz, 4MHz, 6MHz or 8MHz wide. Europe usually is 8MHz DOCSIS modem/ Digital Cable and USA 6MHz (to be compatible with Analogue channels on same cable) but data only wireless linked systems using DOCSIS cable modem indoors and 2.5GHz, 3.5GHz, 6GHz, 10GHz or 12GHz outdoor full duplex radio sets, or 700MHz, 800MHz, 900MHz directly can be using 2MHz, 4MHz, 6MHz or 8MHz channels depending on country and band in use. Only the 10.5GHz band 8MHz version is much in use in UK and Ireland, but other bands used in Canada and Caribbean.

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Tue Jan 26, 2016 2:53 am

In respect of the 1st IF filter bandwidth for an HF receiver, I guess that it depends upon the desired performance level. The folks at Racal provided a succinct summary when writing in 1974 about their then-new high-performance RA1772 HF receiver, which was I think at least epoch-marking if not epoch-making.

“Although it is sometimes beneficial to frequency selection it is never advantageous to the receiver performance if the first i.f. bandwidth is wider than the final output bandwidth. The highest possible amount of single-signal and dynamic selectivity are required both of which are obtained if the bandwidth is made narrow as soon as possible. It can be arranged for all frequency selection processes to be made in the first mixer, with fixed frequency injection in the subsequent mixer(s), so that a narrow first i.f. filter can be used. This filter can also be a crystal type so that its bandwidth need only be wide enough to pass the widest i.f. bandwidth envisaged, normally ± 6kHz. This allows protection to subsequent stages against signals farther off-tune than 10kHz and considerable pro-tection at 20kHz off-tune. Having such protection we may concentrate on providing a very high linearity in the stages which are wide-band, particularly the first mixer and r.f. amplifier.”

As well as Racal (35.4 MHz 1st IF in the RA1772), others used a 12 kHz bandwidth 1st IF filter, examples being found in the Plessey PR155 (37.3 MHz 1st IF), Marconi Oceanic (75 MHz 1st IF) and JRC NRD-525 (70.455 MHz 1st IF) to name just one or two. For upconversion HF receivers, 1st IFs seemed to be very much an individual manufacturer choice.

Presumably wider bandwidth filters were chosen on cost grounds for lower-priced, lower-performance receivers in the consumer class, or, as you say, because a given IF strip, typically 10.7 MHz, was required to handle both wideband FM and AM signals. Also, wider bandwidth was required where the 1st IF was variable in order to accommodate fine tuning or interpolation tuning.

Regarding 21.4 MHz, I can’t say that I have seen evidence of its use as an IF in HF receivers, but then I am hardly guilty of rigorous research in this regard. It would seem to be a bit of an awkward number though, as it is just below the lower edge of the 13 metre broadcast band, 21.45 to 21.75 MHz. On the other hand, 10.7 MHz fitted neatly between the 25 and 31 metre bands. My best guess is that 21.4 MHz, as a doubling of 10.7 MHz, was chosen to provide better image rejection in professional VHF equipment, particularly that which tuned beyond 200 MHz. The range of filters available for 21.4 MHz seems to parallel that for 10.7 MHz. The Eddystone 1990R VHF receiver of the 1970s, which could be equipped to tune from 25 to 500 MHz, had a 21.4 MHz IF. The available IF filters had bandwidths of 250, 30, 15 and 7.5 kHz. The last three were said to be respectively for 50, 25 and 12.5 kHz channeling.

The earlier Eddystone 990R VHF receiver, covering 27 to 240 MHz, had a 10.7 MHz IF. In a data sheet dated 1967 May it was said: “…bandwidths of 30 kHz and 200 kHz are provided as standard, the former employing a crystal filter, alternative filters being available to order.”

A data sheet of nearly a decade later, 1976 November, stated: “It can be supplied with crystal filter to suit 12.5, 25 or 50 kHz channel spacing, the latter [sic] being fitted as standard. A wide selectivity position of 200 kHz allows the receiver to be used as a monitor for wide-band FM transmissions.” In the tabulation of specifications, the standard IF bandwidths were quoted as 30 and 200 kHz, thus connecting the 30 kHz number with 50 kHz channeling et seq.

Anyway, it would appear that in the late 1960s, 30 kHz IF bandwidth (for 50 kHz channeling) was the norm for VHF communications, whereas by the late 1970s, narrower bandwidths for closer channeling had become important. If we assume that the 21.4 MHz IF came into use in VHF equipment in the late 1960s, then the 30 kHz bandwidth filter would likely have been the first crystal type made available. And it could be that Salford had this application in mind – amongst other possible uses – with its 1968 filter release.

Cheers,

Steve

 
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Re: Radio Receiver Intermediate Frequencies

Post by turretslug » Tue Jan 26, 2016 12:01 pm

I had been mildly curious (though hardly enough as to lose sleep) as to why the Pye (originally Rees-Mace) CAT619 maritime general coverage receiver (I think introduced around 1953, so presumably an early adopter of the ECH81 over the ECH42) had adopted 1.4MHz as 1st IF, rather than anything somewhat higher- especially considering its main 460kHz IF strip. However, it makes sense if you consider that, as a marine-orientated receiver, there would have been need to offer uninterrupted (and, hopefully, largely sprog-free) coverage from the top of the MF broadcast band across the lower HF bands. The established circa 1.6MHz IFs would, as said, have risked breakthrough in this environment and the chance of broadcast stations around 1.4MHz giving trouble was presumably felt to be low.

Whilst this example used LC filtering at 1.4MHz (a half-lattice crystal filter featuring at 460kHz), perhaps it showed in service that this frequency was a reasonably trouble free-spot to adopt crystal filtered IF technology later on- I think Redifon were a largely marine-market orientated outfit. As mentioned upthread, this frequency also suited uninterrupted receiver coverage between LF and MF broadcast bands.

Perhaps by time 21.4MHz filters came along, the electronics had advanced far enough that generating a synthesized LO in the 35-85MHz region was sufficiently economic (at least in professional terms!) that an elegant, effective and potential low-sprog solution for high dynamic range receiver front ends was a steep 30MHz low-pass filter with 35MHz+ IF, thus keeping front-end and IF strip out of each others' hair, so to speak, and 21.4MHz (or any other in-band frequency) would have involved more awkward work-arounds. I'd agree that 21.4MHz seems more VHF/UHF receiver orientated.

 
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Re: Radio Receiver Intermediate Frequencies

Post by Michael Watterson » Tue Jan 26, 2016 12:58 pm

Synchrodyne wrote:Presumably wider bandwidth filters were chosen on cost grounds for lower-priced, lower-performance receivers in the consumer class, or, as you say, because a given IF strip, typically 10.7 MHz, was required to handle both wideband FM and AM signals. Also, wider bandwidth was required where the 1st IF was variable in order to accommodate fine tuning or interpolation tuning.


Purely cost on non-professional gear, about 100th of cost, yet removes the plague of approx 910kHz image on HF, which is quite severe on some single conversion sets with no third RF tuning section from 1935 to the cheap Tesco World receiver today!

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Sat Apr 09, 2016 7:13 am

One may wonder whether the reasonably widespread adoption of 1.4 MHz as a final IF for professional upconversion HF receivers, apparently from 1969, was in any way influenced by the fact that Rees Mace had used this number as a 1st IF for its CAT receiver, designed to an admiralty specification. By the way, that receiver was described in Wireless World (WW) 1954 July, p.333ff.

As previously mentioned, Redifon, with its R550, announced in WW 1969 March, p.136, looks to have been the first upconversion model to use the 1.4 kHz final IF, and soon thereafter this number was supported by the filter makers. Redifon was a major supplier of marine HF receivers, and perhaps unusually during the valve era, used common designs for marine and general-purpose models, whereas for example IMR and Marconi designed dedicated marine HF receivers. With Redifon, the commonality goes back at least to the R50, which was advertised at least as early as 1947 (WW 1947 November p.41), and which was described in WW 1949 July p.251ff. The marine version of the R50 was the R50M; the marine version of the R550 was the R551, announced in WW 1970 January p.41. So Redifon would have had marine applications in mind when it was designing the R550/R551, and like Rees Mace, would have wanted an IF that was outside of (below) the upper MF marine band in which vessels might transmit.

I don’t think that Racal was much concerned about the marine market – although it certainly had sold its RA17 receiver into marine applications, albeit for ancillary purposes. Rather it said that the choice of 1.4 MHz as final IF for its RA1772 was due to the ready availability of suitable filters; an example of using an established and generally suitable number rather than starting from scratch.

More-or-less a contemporary of the Redifon R550 was the Eddystone EC958, announced in WW 1969 July p.322 (and I understand earlier in other trade journals). Whereas previously Eddystone had stayed away from the marine HF receiver market (except for designing and initially building the IMR54), the EC958 was designed with marine applications in mind. The marine version was the EC958/5, also rebranded as the Marconi Nebula, which thus competed with Marconi’s own Apollo design. Eddystone had opted for a more traditional design rather than upconversion, albeit one that incorporated what was more-or-less a narrow-band Wadley loop. It was triple conversion with IFs of 1235 to 1335 kHz (tuneable), 250 kHz and 100 kHz. The tuneable 1st IF was well established in Eddystone practice, having been used in the valved 830 series, where the range was 1250 to 1450 kHz, 1350 kHz nominal, evidently chosen to allow its use on incoming frequencies down to 1.5 MHz.

In the case of the EC958, the tuneable IF was used on incoming frequencies down to 1.6 MHz. It was 1335 kHz nominal, tuneable in the downwards direction only. It looks as if it may have been a carryover from the 830; perhaps the 5 kHz offset was something to do with the Wadley Loop implementation. Anyway, it was in the right place, that is below 1.6 MHz, for the marine application. Also probably connected with the Wadley Loop was the need for an intermediate IF of 250 kHz, whereas the 830 had converted directly to the 100 kHz final IF from the 1st 1350 kHz (nominal) IF. 250 kHz looks to have been an ad hoc number, perhaps chosen on the one hand to keep the 2nd mixer local oscillator injection frequency – which was on the high side - below 1.6 kHz, and on the other to keep any images from the tuneable 1st IF far enough away, something that may have been more difficult with conversion from 1335 kHz (nominal) direct to 100 kHz.

But even 250 kHz had a precedent. Recently I read an article about the use of SSB in aircraft HF communications, in WW 1958 October, p.460ff. The transmitter and receiver block schematics shown had IFs of 250 kHz. I suspect that these block schematics were representative of Marconi equipment, given that the article was written by a Marconi staffer. Whether 250 kHz was a norm in aircraft HF practice I don’t know. The only other reference point I have is that of the earlier Marconi AD108 aircraft HF receiver, which was single-conversion with a 600 kHz IF.

All of the above-mentioned WW items and articles are available at: http://www.americanradiohistory.com/Wir ... gazine.htm.

Cheers,

Steve

 
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Re: Radio Receiver Intermediate Frequencies

Post by Synchrodyne » Sun Apr 17, 2016 11:03 pm

The use of 21.4 MHz as a 1st IF in VHF receivers may go back earlier than I first thought, which was the late 1960s.

Wireless World 1966 April, in a survey of communications receivers, listed the Rohde & Schwarz ESM180 VHF receiver, covering 30 to 180 MHz, with IFs of 21.4 and 3.4 MHz.

It turns out that there is quite a bit of information about this model on the web, including an Instruction Book at: http://bee.mif.pg.gda.pl/ciasteczkowypo ... ESM180.PDF. Rmorg dates it from 1956, which seems reasonable considering that its valve complement included some Rimlocks.

In wideband mode, it is single conversion with an IF of 21.4 MHz. In narrow-band mode, a second conversion to 3.4 MHz is used. There is a similar model of apparently the same vintage, the ESM300, which covered 85 to 300 MHz and had the same pair of IFs.

One may surmise that Rohde & Schwarz thought that the standard 10.7 MHz IF was not high enough to avoid image problems at the higher end of the tuning range, and simply doubled it to 21.4 MHz. Even then, the ESM180 had a four-gang front end. The 2nd IF of 3.4 MHz appears to be a very individual choice.

The 21.4 MHz IF transformers in the ESM180 were wideband; if the previously quoted Salford case is indicative, then it was not until the late 1960s that the filter makers offered package narrow-band 21.4 MHz units, say about a decade after they did the same for 10.7 MHz.

Cheers,

Steve

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