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Old 20th Apr 2016, 6:19 am   #41
Radio Wrangler
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Default Re: ISB Receivers

The H2900 had one synthesiser, dual front ends, and four final IFs so it could do ISB and dual antenna diversity. The synthesiser could also be made to lock onto the carrier of full or reduced carrier signals.

The whole structure of the receiver was one big casting. In the prototype it seemed to have been milled out of a single block of aluminium,

One idiosyncrasy was the lack of connectors on the rear panel. There was a cable duct milled into the right-hand side of the receiver, the right-hand part of the control panel was released by two knurled screws and swung out on a double-hinged arm, carrying the S meter with it. In the cavity behind were the connectors for the antenna inputs and all the signal outputs. So they could be accessed from the front with the receiver in a rack.

The density of the thing is so high that the number mounted in any one rack must be limited in order to prevent collapse into a gravitational singularity.

Oh, and the synthesiser boards are a rat's nest, all wire-wrapped together.

David GM4ZNX
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Old 20th Apr 2016, 7:10 pm   #42
John KC0G
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Default Re: ISB Receivers

Hello David et al

You comments about the weight of the H2900 make me think about equipment density being measured in kg per U of rack height. I have had to pull audio power amps, which would measure quite highly, out of racks.

I have been reading Sosin's article on the H2900. (see post #35) It paints broad pictures of the technical detail but only goes so far. Pat Hawker, G3VA, wrote a nice summary in the RSGB's Radcom, Feb 1970, pp 92-93. He did include the block diagram, which does show a filter for separating the carrier at IF, but does not show how it is regenerated. He also noted Sosin's use of the term "roofing filter". Was that term in common use at the time (1970)?

G3VA briefly mentioned the frequency synthesizer, saying that it had little connection with amateur equipment. It uses a pulse subtraction technique, and looks to me as though it is a frequency, not phase, locked loop. It may be an extension of Sosin's earlier work, ie patent no. GB1085866,, also granted as US3349338, and elsewhere, with a priority date of Feb 3rd, 1965.

I note your comments about the wire-wrapping. The whole radio was designed not to have plug and socket connectors. Marconi were seriously concerned about reliability in solid-state equipment, in part due to soaring component counts. This is what I took away from a brief glance at:
....J.V. Beard, Factors Affecting Reliability, Point-to-Point Telecommunications, Vol. 13, no. 4, October 1969, pp 157-163
I have not read the article in detail.

The survey of communications receivers by Beard, and Sosin's article on receiver reception failure (see post #35) really help put the H2900 into context. The only other receiver with 1Hz tuning increments was a Racal set, and I don't know which one. The Racal RA1772 and two other receivers were shown as tuning in 10Hz increments, and many of the others tuned in 100Hz increments. The price was some 20% less than the MST point-to-point receivers, but ca. 4 times that of the Racal RA1772., and ca. 9 times that of the price reference, which is assumed to be the Racal RA17L.

it makes me wonder just how many of the H2900 series were sold. The market place can be very cruel to products which might be technically brilliant, but are simply too expensive. The Drake TR-7 amateur radio transceiver is but one example.

73 John KC0G

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Old 20th Apr 2016, 8:49 pm   #43
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Default Re: ISB Receivers

A friend told me that there was an H2900 at interpol in London, but that's the only one I've heard of.

I wonder who the market for them was? Probably some sort of fixed links. It was too expensive for shore stations and marine use. Spooks roam, but presumably want better tuning than thumb wheel switches.

An interesting exercise, a tour de force of technology and who knows where stuff learned from it turned up, but probably a smoking hole in the foot from the viewpoint of commerce.

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Old 24th Apr 2016, 7:12 am   #44
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Default Re: ISB Receivers

John, that’s certainly an impressive reading list!

I have a small number of the items listed, including the WW items and the Mellor & Sutton and Sutton Eddystone papers. I don’t have the Ford Eddystone paper, but the original EC958 development is also covered in “The Cooke Report”. I don’t have the infamous Sosin article, but I do have the review of it by Michael O’Beirne in Radio Bygones #87. Pat Hawker’s “Technical Topics” is still available and I have ordered a set. I suspect that “Point-to-Point Communications” would be hard to find in these parts.

The Marconi H2900 is certainly difficult to categorize, and does look more like a “concept” or “technology demonstration” unit. Hawker, in his WW 1970 communications receiver survey, said that the H2900 had synchronous demodulation (WW 1970 June p.260). The block schematic shown in Short Wave Magazine 1970 April shows filter separation of the sidebands and carrier, but not the origin of the demodulating 2 MHz carrier. That suggests that its origins were complex than simply limiting the output of the carrier filter and amplifier, which would have been easy to show on a basic diagram.

The above-mentioned O’Beirne article includes the relative price comparisons. One might say that Marconi seemed to have miscued on the Apollo SSB marine receiver, with a relative price of 2.5 as compared with the 1.9 of the Redifon R551. Thus it can be seen why it also offered the Nebula (Eddystone EC958/5), which I’d expect to be no more costly than the H2311, with a relative price of 1.8, and which was essentially the Eddystone EC958/7, the improved version of the EC958.

Where the R550 with the ISB option of the EC958/12 (ISB version of the EC958/7) would fit is hard to say, but perhaps in the 2.5-to-3.0 range. Then the Hydrus, at 3.8, looks logical as an economical purpose-designed point-to-point ISB receiver, more costly than optioned or “added-on to” communications receivers but well below the traditional “full works” models. I suspect that Redifon, Eddystone and others were probably not expecting more than a small proportion of their HF receivers to be specified with the ISB option, hence the “add-on” approach, albeit with designed-in capability rather than as an afterthought, but it was nevertheless worth doing.

In fact the “add-on” approach to SSB and ISB seems to go back quite a way, to the late 1940s GE YRS-1 at least. Racal had several SSB and ISB adaptors, including at least one, the RA98, with motor-driven AFC.

Peter, thanks for the detail on the Redifon R408 variable bandwidth system. I imagine that tuning two sets of LC IF filters over a small range whilst preserving their shape and symmetry could have been easier than varying the bandwidth of similar filters over quite a wide range.

Cheers,
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Old 24th Apr 2016, 7:31 am   #45
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Default Re: ISB Receivers

The synchronous demodulation feature is on the H2900 block, but it's hidden in full view.

The synthesiser is more than a little bit weird

For minimum phase noise, it employs a huge bank of crystal oscillators mounted in a thermal mass box. One oscillator is enabled at a time and is free-running and acts as the first local oscillator. The incoming signal is mixed into an IF around 80MHz, 1 or 2 MHz wide, which uses a helical filter. A sample of the crystal oscillator signal goes to the synthesiser and is compared to the frequency standard, so the 2nD LO synthesiser can be offset to compensate for errors and drift in the crystal oscillator frequency ( Ha! shades of Wadley!) For synchronous demod, all they had to do was to extract the carrier from the final IF and compare that to the frequency standard. The frequency standard provided the reinserted carrier, and the 2nd LO synthesiser was slewed to put the signal on the right frequency for the fixed inserted carrier at the demod. The crystal oscillators no longer needed compensation, any and all causes of offset got taken into account.

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Old 25th Apr 2016, 1:06 am   #46
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Default Re: ISB Receivers

Thanks much, RW. Yes, it was indeed hidden in full view.

The 2 MHz reference feed into the AFC section must come from the frequency standard section, as nothing else would make sense. Then of course it follows that the same 2 MHz feed was used for the sideband demodulators. And as you say, the AFC system, working back through the frequency control system, served to slew and lock the recovered carrier to the 2 MHz reference.

So the Marconi H2900 system was a bit different to an “independent” PLL demodulator system, in which the demodulator oscillator is locked to the incoming carrier, and to some small extent at least moves with it as it wanders, whether inherently or because of receiver conversion oscillator drift.

Cheers,
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Old 29th Apr 2016, 6:30 am   #47
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Default Re: ISB Receivers

I also looked more closely at the Eddystone EC958/12 ISB circuitry. This was a single-box receiver, but the ISB section was largely separate in what was called the “top tray”. Here the reference 100 kHz carrier for sideband demodulation was generated locally and phase-locked to the recovered carrier, obtained via the usual very narrow filter. There was also a second “loop” that was essentially an AFC system that fed back to the incremental tuning oscillator in the main receiver section, thus “steering” the tuning system to keep the 100 kHz carrier centred in its narrow filter.

I don’t know whether there was any Marconi influence in the EC958/12 design. It was a derivative of the EC958/7, released in 1973, which was the improved version of the original EC958, including digital readout and enhanced stability. The original I think, although released after the Marconi buyout of Eddystone, was essentially a legacy-Eddystone product. The EC958/5 was the marine GP-SSB version of the original, also (or only) rebadged as the MIMCO Nebula. And the EC958/7 was rebadged as the Marconi W/T H2311. So Marconi might have been involved to some extent with the rework that produced the EC958/7.

Marconi had previously rebadged a variant of the Eddystone 880 as its H2301 specifically for SSB (but not ISB) work. Although how it sat relative to Marconi’s own HR120 single-box receiver is unknown. In a broad sense, the EC958 series was the successor to the 880, so the H2311 might have been seen as following on from the H2301. To some extent the 880 had been Eddystone’s offering in some of the same markets at which the Racal RA17 was aimed, but unlike the latter, it does not appear to have had a corresponding ISB adaptor developed for it.

Cheers,
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Old 9th May 2016, 9:24 am   #48
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Default Re: ISB Receivers

Quote:
Originally Posted by G8BBZ View Post
I don't recollect ever hearing of any coast station being equipped for ISB telephony - maybe others know more. The norm for passenger phone calls was VHF with the fitting of the "private deck" addition to the standard VHF ship-to-shore transceiver, which provided the additional frequencies and duplex capability to support phone calls. Most coast stations had the capability to link the VHF into the telephone network, I think. This facility obviously depended on being in VHF range of a coast station - once you were out of range then it was "Ship's Telegrams" which were the passenger's means of communication.
The attached item from 1960 provides some background here. It mentions that four GPO coast stations were then being equipped to handle public radiotelephone calls to and from long-range ships. The equipment included Redifon receivers with ISB adaptors.

Cheers,
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Old 9th May 2016, 10:44 am   #49
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Default Re: ISB Receivers

While there has been a reasonable market for SSB receivers, that for ISB has been much much smaller. So any development of an ISB machine needs to be based on that of an SSB machine for simple economies. However, the ISB stuff needs to be sufficiently separate that it can be left out of the SSB version without any significant impact on the cost of making the SSB version. The SSB version will be lower cost, higher volume and much more critical on financial margins. Using an internal tray makes it simpler. Using an external ISB adaptor saves the base model receiver having to have too big a case.

We don't know how many ISB links worked with a pair of SSB machines. This would enable better standardisation and allow the versatility of splittint their frequencies to handle different SSB channels etc... and if one broke down, you still had an entirely separate receiver to use.

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Old 9th May 2016, 7:41 pm   #50
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Default Re: ISB Receivers

Re. #45, thanks to RW from me too re. the operation of the H2900 synchronous demodulator

Re. #46, The 2MHz oscillator was derived from the H2900 frequency standard system.

The synchronous detector in the Lowe HF-150 worked by adjusting the 2nd LO frequency, via a PLL. The synchronous detector injection frequency was derived from a crystal oscillator.

Re. #47, and the Eddystone EC958/12, I have read that the EC959/9 also had ISB capability. How it was done, and how it compared to the EC958/12, I do not know.

73 John KC0G
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Old 13th May 2016, 7:49 pm   #51
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Default Re: ISB Receivers

Hi All,
Thanks to Synchrodyne for the info on Redifon eqpt. in coast stations. I am impressed that you found this item. The Redifon eqpt. listed is new to me and will duly get added to my Redifon Equipment List.
Picking up on Radio Wrangler's comment about the use of SSB receivers for ISB reception, I would add further that most of the ISB circuits I have come across used fixed tuned receivers, generally with multi-channel capability, rather than their much more expensive fully tuneable cousins, Communications circuits of this type would generally be assigned 8 frequencies if duplex opoeration is required, 4 in each direction, 2 for Day use and 2 for Night use. These frequencies rarely changed once issued so the need for a fully tuneable receiver is not obvious.
As an example, the Redifon R499 receiver shared the same RF, IF and AGC circuitry as the R550 family of tuneable receivers, but was single conversion using dedicated RF coilpacks for each tuned frequency and the same IF block crystal filters as the big boys, With crystal controlled Local Oscillators and simple switched channel selection the receiver was very simple to remote control using Ledex motors to rotate the switch shafts. While not exactly "cheap as chips" these receivers would often be fitted up to 10 in a rack cabinet destined to spend their life in an un-manned receiving station, being remotely switched twice a day from Day frequency to Night frequency and vice versa.
The R499 supported the same ARU10 ISB Adapter as used by the R550 family so could be set up for ISB operation, or two receivers could be used to achieve the same end, Aviation ground -to-ground and HF ground-to-air circuits tended to separate receivers as they commonly used separate receivers for the Day and Night frequencies so that all frequencies were monitored all the time.
I would mention an earlier Redifon product introduced in thr early 60's. This was Rediplex, a packaged 6 RF channel, 4 speech channel ISB terminal with 100W RF output. All solid state except for the PA's, germanium transistors and based on the Weaver (3rd method) of SSB generation and demodulation. I know little of the Rediplex equipment but assume it was based on the Redifon GR410 transceiver which appeared at the same time and also used the Weaver method.
An advert for Rediplex appears on page 37 in:
http://www.americanradiohistory.com/...ld-1963-03.pdf

cheers
Peter G8BBZ
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Old 15th May 2016, 1:54 am   #52
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Default Re: ISB Receivers

I found the information on the Redifon equipment for UK coast stations in a “Google Books” search. This sometimes finds information that does not come up in a general search, although it is usually, but not always, in snippet form.

On the use of SSB receivers for ISB links, the Marconi HR24 was effectively an SSB version of the HR21 ISB receiver (1954 advertisement attached). It was apparently generally similar to the HR21, including the AFC system, but had only one sideband path. Either USB or LSB could be selected, and this was done by switching the second local oscillator between the high and low sides of the 1st IF. I imagine that the modular construction easily allowed these variations. There was also an HR23 in that family, about which I know little except that it was a triple-diversity receiver.

Before the HR21 et seq there were the HR92 and HR93, which I think were Marconi’s immediate post-WWI point-to-point receivers. I am not sure, but I think that the HR93 was the ISB version, and the HR92 the SSB version.

Marconi also offered the HR22, which was a much smaller ‘single-box’ pilot-carrier SSB receiver (with AFC), which like the HR24 could be switched between USB and LSB, and which followed point-to-point receiver precepts. I understand that the HR22 was used on some shipboard installations for passenger telephone circuits, judging by the attached item that came up on a Google Books search..

Anyway, it does appear that Marconi offered equipment for those stations who preferred to use pairs of SSB receivers rather than single receivers for ISB work, but these were derived from ISB practice.

From the other end of the scale as it were, came the adaptation of ‘single-box’ communications receivers with both internal and external SSB/ISB units. I imagine that this became more attractive as receiver stability improved through the 1950s and 1960s, partly in response to the needs of suppressed carrier SSB communications and the growing use thereof. Racal was an interesting case in that for use with its RA17 it offered both SSB adaptors, without AFC, and ISB adaptors, with AFC.

Redifon clearly offered that approach, as shown in the UK coast station example. Whether Marconi also did that with any of its single-box general-purpose communications receivers is unknown. If it did, the HR120 and H2301 (derived from the Eddystone 880) were possible candidates.

Moving to more modern receivers that had built-in ISB convertibility and in particular the way it was done, in the RA1772, both ISB and AFC were optional, and as best I can determine, independently so. The AFC circuitry used the 1.4 MHz carrier to provide a nominally 1 MHz output that was used in place of that derived from the frequency standard to control the 34 MHz second frequency change oscillator. The nominal 1 MHz AFC output tracked any deviations from 1.4 MHz in the incoming carrier, and so in turn did the second conversion oscillator, thus keeping the 2nd IF centred on 1.4 MHz.

Finally a bit more information on the Redifon R499 and Rediplex system is attached. Given that Rediplex was four voice channels on one pilot carrier, I imagine that two of these were respectively USB and LSB, 300 to 3000 Hz, and the second pair were themselves each suppressed carrier SSB in the 3000 to 6000 Hz band, with the suppressed carrier at one or other end of that band, then multiplexed on to the pilot carrier, one USB and the other LSB.

Cheers,
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Old 16th May 2016, 12:30 pm   #53
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Default Re: ISB Receivers

Hi All,
Once again, well done Synchrodyne, to find the Rediplex ad. I have seen another ad in the past which I can no longer find, which was directed at oil exploration/off-shore comms as being the perfect application for Rediplex.
Looking back over the course of this thread (which I see has been running for over two years now!) we have traced the changing techniques used in the generation and demodulation of ISB signals, but we haven't given the same consideration to the disappearance of space diversity receiving techniques in the reception of ISB (or any other) signals.
While early ISB receivers like the Marconi monsters featured dual or triple space diversity with appropriate combiners or switches, this capability does not appear to be present in any of the more modern receivers we have considered.
There was the Racal MA168 diversity switch for use with the RA17/117 family of receivers as an example of an outboard unit for use with two standard receivers - but what did everybody else do?
cheers
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Old 18th May 2016, 12:05 am   #54
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Regarding space diversity reception, that was apparently possible with the Eddystone EC958/12. In the manual it stated: “Two receivers can be installed with independent aerials to operate as a versatile Dual Diversity Terminal.”

For that purpose, the IF and SSB/ISB AGC lines were interconnected, and if desired, one could be slaved to the other’s oscillator. At AF, combination was done simply by series-summing half each of the USB and LSB 600R line outputs.

I imagine that similar arrangements were possible with other :single-box” communications receivers of the era.

It would that more complex signal combining processes were used in the past, though, for example the Crosby system used both on Crosby and TMC (Technical Materiel Corporation) receivers. For example the TMC TDRS triple-diversity unit was based upon three modified Hammarlund SP600X receivers. Each was fitted with a Crosby ECC exalted-carrier demodulation unit and they were followed by a Crosby DCB combining unit. The ECC unit was of the quasi-synchronous type, using filtered and limited incoming carrier as the demodulation reference for both the I and Q channels, whose outputs fed a Norgaard phasing circuit to separate the sidebands. So it could receive ISB and SSB, although its primary raison d’être was to minimize selective fading distortion on AM (DSB) signals. In the DCB, the three AF outputs were then each remodulated on to a locally generated carrier whose amplitude was controlled to reflect the amplitude of the incoming signal on that particular receiver. The remodulated signals were then fed to diode demodulators sharing a common load. Because of diode demodulator behaviour, this approach was said to allow the strongest signal at any time to substantially suppress the weaker signals, something not possible with simple summing of the outputs. The remodulation was kept low – at 30% - to avoid diode demodulator distortion, and so not lose the advantages of the low-distortion distortion product demodulators in the exalted-carrier section only. (More details at: http://www.virhistory.com/tmc/tmc_pa...mc.ssb_143.pdf.)

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Old 18th May 2016, 12:21 am   #55
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Default Re: ISB Receivers

I also checked the Racal RA1772 manual, which confirmed that pairs of this receiver could be used for space diversity reception, in the master/slave configuration, including for SSB and ISB. Audio combining looks to have been a bit more sophisticated than in the Eddystone case, but then the RA1772 probably cost a lot more than the EC958/12.

“The audio diversity combiner circuit consists basically of an electronic switch which selects either channel A or channel B dependent on which channel has the highest level of AGC voltage. An audio signal zero-crossover detector circuit ensures that switching between channels occurs only at the zero-crossover point, to avoid the introduction of switching distortion and clicks.”

So in the 1970s, if you wanted space diversity, you installed a pair of multi-purpose HF receivers and fitted the appropriate accessories and interconnections. Or perhaps you stepped up to the Marconi Hydrus or something similar, which was available in diversity versions but still a lot more compact than the older Marconi HR21 and like monsters.

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Old 18th May 2016, 1:00 am   #56
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Default Re: ISB Receivers

That H2900 i had was a prototype, so it was fully stuffed. There was one synthesiser. There were two front-ends and early IFs.... the motor driven preselectors for the two diversity paths were mechanically ganged. Then in the final IF there were USB and LSB filters on each diversity path, so the one box did 2 channel diversity (space, or polarisation) of ISB signals.

All four utputs were populated so presumably an external scorer controlled audio switching.

There were also filters for FSK.

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Old 8th Aug 2016, 1:24 am   #57
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Regarding the putative general move to the “one box” receiver concept, there was an interesting reference to that in an article on HF receivers in WW 1980 written by R.F.E. Winn of Racal. In a look-back at the RA1772 development, he said: “Another innovation was to provide a complete receiving terminal in a single case instead of extending facilities with add-on adaptor units, which, in the past, had frequently resulted in a 6ft high rack of equipment.”

I imagine that was a reference to the RA17 era and the external adaptors, including SSB and ISB units, that were developed for use with it. Also of course the classic point-to-point ISB receivers such as the Marconi HR21 & co., typically occupied a full rack.

The accompanying block schematic for the RA1772 showed the optional ISB board. The article also referred to the then new “Anglo-American” HF receiver, which I think was the RA1792, and one of whose rationale was much lower cost….As an aside, I wonder if that receiver’s 455 kHz second IF (as compared with 1.4 MHz for the RA1772) was to meet American preferences.

Cheers,
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Old 8th Aug 2016, 1:52 am   #58
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Default Re: ISB Receivers

It was touted for a while as having Collins mechanical filters. I'm not sure of how this got put around, but I came across the statement from an owner of a surplus one in a certain treasure cave called M&B Radio in an archway under Leeds city station.

I've got a 1792 in the attic and the manual for it, I'll take a look sometime.

The Racal fan club in the 1980s were ecstatic over a newer receiver appearing on the surplus market and were convinced that as a newer model it had to be even better than the 1772

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Old 9th Aug 2016, 11:01 pm   #59
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Default Re: ISB Receivers

The use of Collins filters certainly sounds plausible for a design aimed at the American market. The 1.4 MHz filters in the RA1772 might have been Cathodeon or Plessey. Both companies issued a comprehensive 1.4 MHz filter range circa 1970 in anticipation of the needs of marine receivers that were compliant with the incoming SSB requirements. Although there was some prior use of 1.4 MHz, Redifon was probably the first to use it in a “modern” receiver, and it suited the marine application being outside of any of the marine bands. And it looks to be the case that once 1.4 MHz filters became a standard industry item, that IF was also used for general-purpose receivers, at least in the UK and Europe.

This is pertinent to the core thread theme, in that as ISB receivers migrated from specialist to variations on the single-box general-purpose concept, the hitherto commonly used 100 kHz final IF often moved up to 1.4 MHz. Presumably suitably narrow 1.4 MHz filters were available for carrier extraction, but then the solid state era also brought with it PLL-type carrier recovery systems, which may have eased the carrier filter requirements.

Cheers,
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Old 9th Aug 2016, 11:31 pm   #60
Radio Wrangler
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Default Re: ISB Receivers

Cathodeon
Plessey
Salford Electrical Instruments (a subsidiary of GEC)
ITT
Crystal Electronics

There were loads of firms into 1.4 MHz crystal filters. I have quite a collection! USB, LSB, vaarious CW and RTTY bandwidths as well as carrier recovery filters. Tons of them cropped up on the surplus market.

David
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