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Old 4th Feb 2014, 4:13 am   #1
Synchrodyne
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Default ISB Receivers

What happened, in a general sense, with point-to-point ISB receivers following the general industry transition from valves to solid state devices?

In the valve era, ISB receivers appeared to have formed a well-defined subgroup amongst HF receivers generally, largely based upon precepts developed back in the 1930s. The attached block schematic of what I assume to be a Marconi receiver is I think representative. These receivers had three final IF channels, often at 100 kHz, one each for the upper sideband, lower sideband and the carrier, the transmissions at interest being of the reduced carrier type, not the fully suppressed carrier type. The carrier channel had a very narrow filter, typically 60 Hz, and fed the agc and afc circuitry, and via a limiter provided the “local” carrier for individual sideband product demodulation. The sideband filters were 100 to 3500 Hz or 100 to 6000 kHz, the first catering for voice traffic and the latter for broadcast relay traffic, or a pair of multiplexed voice channels. I should assume that the 100 Hz lower limit was derived from broadcast relay requirements, as 300 Hz would have been adequate for voice.

Such ISB receivers were also used for HF AM reception for broadcast relay purposes, given their ability to minimize selective fading distortion, this benefit derived from their use of product (quasi-synchronous or homodyne) demodulators. In fact some receivers of this type, such as the Marconi HR22 (discussed recently in this thread: https://www.vintage-radio.net/forum/...d.php?t=101833) seem to be been designed primarily for this purpose. ISB receivers could also be used for the reception of suppressed carrier SSB transmissions, although this was not their primary mission.

Comparable solid state dedicated ISB receivers seem to have been very scarce though, or at least not well-publicized in the usual places. One example from the late 1960s was the Marconi Hydrus, which does seem to have been a solid state successor to its previous valve models. Information on this is very scarce. Apart from the attached Wireless World advertisement, I have been able to ascertain that it was triple conversion, with IFs of 40 MHz, 5 MHz and 100 kHz, and that it might have been of the Wadley loop form.

Was it the case that dedicated ISB receivers were overtaken to some extent by a new generation of highly stable solid state general coverage receivers that had add-on or optional ISB facilities? The Eddystone EC958/12 was such an example, developed from the original EC958. The separate ISB section had the customary three 100 kHz IF channels, but in this case the recovered carrier was used to synchronize a PLL, thus sideband demodulation was fully synchronous. I am not sure what sideband filters were available, but voice bandwidth (300 to 3400 Hz) might have been the norm. The epoch-marking Racal RA1772 also had an ISB option, basically adding a second IF strip and product demodulator, and also an AFC option, which added a carrier IF and effectively locked the demodulator BFO to the incoming carrier on reduced carrier SSB and ISB transmissions.

That there was an ongoing need for the ISB-type receiver in the 1970s was apparent for example from BBC Engineering #84, 1970 October (http://downloads.bbc.co.uk/rd/pubs/a...neering_84.pdf) which notes that the BBC was expanding the use of HF SSB program links, having started in 1964, with use of ISB receivers for HF AM reception predating that. The BBC used a bandwidth of 90 to 6000 Hz.

So I am curious as to whether the Marconi Hydrus was a rare species, or whether there were other solid-state point-to-point ISB receivers that were distinct from the HF general coverage type.

The attached Marconi valve ISB receiver block schematic was taken from Radio & Television Engineers’ Reference Book, 3rd Edition. The model is not identified, but it might be or be close to the HR24.

Marconi evidently also supplied marine-oriented ISB receivers, as shown by the block schematic taken from Danielson & Mayoh, “Marine Radio Manual”.

Mullard also made a point-to-point ISB receiver in the 1950s, the GFR.552, per the attached Wireless World items.

The Marconi Hydrus advertisement notes that this receiver used field effect transistors. Given that it is from about the same period as the Eddystone EC958, that is not surprising, and being from Marconi, and in any event predating the era of bullet-proof high-level 1st mixers, I am guessing that it had reasonably good pre-mixer selectivity. Notwithstanding its significance in the ISB world, whatever that may be, it is also relevant to the current more general, although FET-oriented transition era thread: https://www.vintage-radio.net/forum/...ad.php?t=98398.

Cheers,
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Old 4th Feb 2014, 11:55 am   #2
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Default Re: ISB Receivers

You raise an interesting question. My own experience at the time was that the need for the "classic" 3-channel ISB receiver declined sharply around the time that commercial point-to-point receivers went solid state, because of other, related, developments in technology.
The need for the "carrier" recovery channel faded as the stability of receiver local oscillators improved. As virtually all commercial solid state receivers were designed as SSB receivers from the outset, they were sufficiently stable to resolve either sideband of an ISB transmission without the need for AFC. There were exceptions to this - so links carrying Lincompex circuits may well have still needed carrier based AFC systems to meet the extremely stringent requirements of the Lincompex control channel. By the time of the introduction of frequency synthesizers locked to high stability reference oscillators, the need for carrier AFC had really gone for good.
With the general reduction in size of equipment with the introduction of solid state circuitry, the provision of a second IF chain to support resolution of both sidebands of an ISB transmission simultaneously became less of a mechanical issue. A second IF channel could now be accomodated in a relatively small add-on unit or even as an additional module within the receiver enclosure. The improvement in performance and reduction in size and cost of the block crystal filters needed for sideband selection also made it easier to design ISB receivers, as did the improved performance and sophistication of the AGC systems used in solid state receivers.
Point-to-point circuits are invariably extended from the transmitting and receiving stations to some central traffic office over land lines. These tend to be relatively intolerant of large variations in signal amplitude, due to the prescence of balancing networks, line amplifiers and the like. Receivers designed for use in such applications typically had more "powerful" AGC systems than a receiver which would be used by an operator who could reach the volume control.
So the days of the 7ft. rack full of equipment came to an end. Motor-driven AFC loops no longer chase drifting transmitters up and down the band. Changing from the day to the night frequency is no longer a 20 minute exercise with many opportunities to get it wrong. Setting up an ISB circuit became a simple exercise of entering the frequency on the front panel, selecting ISB on the mode switch and making sure that both 600ohm lines are connected. The receiver was probably being used on a RTTY circuit earlier in the day, and may well be used on a simplex SSB circuit later on. Such was the flexibility and ease of operation which was the real benefit of the transition to solid state electronics.
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Old 5th Feb 2014, 8:20 am   #3
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Default Re: ISB Receivers

Thanks very much for the detailed and lucid summary.

That certainly would explain the apparent paucity of solid state “three-channel” ISB receivers. In that light, the pattern can now be seen quite early on with the Marconi group receivers. The Hydrus ISB and Apollo marine SSB receivers, both dedicated to their respective functions, appeared circa 1969-69, at the beginning of the serious solid state era, at least if one discounts the early false starts with germanium transistors. The Eddystone EC958 general coverage receiver appeared at about the same time. Quite soon the EC958/5 was being rebadged as the Marconi Nebula for marine applications, and in 1974 the EC958/12, with ISB capability, arrived. So the GC receiver, probably the least costly of the three, was taking over the specialist receiver duties. And the EC958 was something of stepping-stone model, largely following valve receiver topology and delivering proximate performance, except perhaps for higher stability, but well short of the RA1772 level. (Although at the time Marconi might have argued otherwise.)

I wonder if SSB and ISB broadcast relays still retain the pilot carrier, so that the receiver can track Doppler shifts that might be noticeable on music. The VOA used – might still use – ISB relays and I can recall finding these back in the 1990s. The Liniplex F2 receiver, with its tracking PLL, would lock on to them – it was quite tenacious in that regard – and then one could simply switch between the two sidebands. But the reduced carrier upset the agc system a bit, given that it was designed around conventional AM transmissions with full carrier, and the result was a much higher audio level.

Re the Marconi Hydrus, it would be interesting to ascertain whether it largely followed the preceding valve topology, using filtering and limiting for carrier recovery, or whether it had a PLL, like the EC958/12.

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Old 5th Feb 2014, 9:42 am   #4
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Default Re: ISB Receivers

Were there other ISB solid state receivers?

Yup.

Receivers were getting rather modular. Take the Racal RA1792. Usually thought of as a free-tuning general coverage receiver of high performance... but get the thumbwheel switch tuned version and it's really only good for point-to-point links. All versions have numerous choices of filter combinations, but there's also an ISB option with an AFC lock board.

Lots of parts in common with the general use receiver, but a full blooded ISB AFC machine for fixed links.

I used to have a Marconi H2900 - one of the prototypes to boot! Again thumbwheel tuned, had the full pack of 2MHz IF filters with USB and LSB in separate strips for ISB, with an AFC system for the synthesiser and a motor-tuned preselector inside it. This set was known in Marconi as 'Sosin's Folly' There's a lot of info on the web about B M Sosin and a very contentious comparison article in the Marconi company's journal. Litigation leading to letters from Marconi to librarians to destroy all copies, to be replaced by a redone version.

The H2900 was a deep, rack-wide unit about 9 inches tall. The one I had was milled out of a solid aluminium block. Marconi priced this above the ICS3.

There have been plenty of fixed link ISB receivers, and exciters (I used to use a Redifon GK203) but they lived quiet, isolated lives without much human intervention, and when sold off their switch tuning killed off most interest, though some enterprising traders made some front panels to convert them to the more desirable version.

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Old 5th Feb 2014, 4:45 pm   #5
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Default Re: ISB Receivers

Picking up on a comment by Radio Wrangler, improvements were not limited to receivers. The improvements achieved in frequency setting accuracy and stability in transmitter exciters was also a major driver of change.
In the good old bad old days, it was not unknown for a transmitter to drift by anything up to 1 kHz after a frequency change, before finally settling somewhere near the nominal channel frequency as everything finished warming up. If the link was in traffic at the time, the transmitter would have to be carefully chased up and down the band until it settled. Crystal controlled transmitters were obviously better, but so much more limited in what they could be used for due to the finite number of crystal positions and the inability to "qsy hf +5" temporarily to avoid an interfering signal.
The introduction of synthesized local oscillators in transmitter drive units changed all of this overnight. The GK203 mentioned by Radio Wrangler had a setting accuracy of 3Hz at 30MHz and could be expected to stay on frequency +/_1Hz for long periods. So now it was possible to tune the receiver without having to wait for the transmitter to come up, simply by setting the channel frequency, in the confident expectation that the transmitter would be on channel to within a few hertz - good enough for everything except the most demanding of compression and multiplexing systems.
The logical outcome of all of these improvements was, of course, the conversion of most point-to-point transmitter and receiver sites to unmanned operation, all of the frequency setting, antenna selection and service selection activity being achieved by remote control from the traffic office. Such is progress.
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Old 9th Feb 2014, 10:21 am   #6
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Default Re: ISB Receivers

I know it's only a domestic receiver, but where does the Sony ICF2001D/2010 fit in this narrative? It has buttons for USB/LSB reception.

It has a synchronous detector, so the USB/ISB demodulation is available for AM signals, not just SSB.

Stuart

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Old 9th Feb 2014, 1:39 pm   #7
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Default Re: ISB Receivers

Quote:
Sony ICF2001D/2010 fit in this narrative? It has buttons for USB/LSB reception
As it is a 'domestic' radio I would think it is there to reduce co-channel interference on one side. It would be a bit of fun to have two and a stereo ISB broadcast to receive, as far as I know there have been none.
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Old 10th Feb 2014, 12:50 am   #8
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Default Re: ISB Receivers

The Sony ICF-2001D/2010 I think falls into a different group of receivers, namely the domestic or semi-professional type fitted with phase-locked loop fully synchronous AM demodulation, and often with a selectable sideband facility, using the phasing and matrix method to separate the sidebands, rather than doing it with filters as in the professional ISB receivers.

Sony was an early adopter of PLL synchronous AM demodulation, but not the first. Sansui had used it on the AM side (MF only) of its TU-X1 hi-fi “supertuner” in the late 1970s. I think that Technics did something similar with one of its tuners, but I am not sure of the details. A nodal point for the technique was the release of the Phase Track Liniplex F1 in 1983. This receiver was designed expressly for SW listening (not DX’ing), and had only a PLL synchronous demodulator, extremely well executed. But Sony’s move led to PLL synchronous AM becoming de rigeur for upper end domestic and semi-professional shortwave receivers. The Lowe HF-150 had it, albeit without selectable sidebands. JRC introduced it with its NRD-535 model, and Drake with its R8.

In the ICF-2001D/2010 case, Sony used the same ICs that it had developed for multi-system stereo AM decoding, and as for example deployed in its SRF-A100 portable FM-AM receiver. Either sideband was selectable, but not double-sideband synchronous demodulation. As MerlinMaxwell has said, it seemed that Sony had used the PLL AM facility primarily to obtain the sideband selection facility, and not necessarily to obtain lower distortion AM per se. I never tried to ascertain whether the Sony would lock to VOA reduced carrier ISB transmissions, but basis its general performance, I suspect that it would have been unlikely. Its PLL unlocked very easily. On the other hand, the Liniplex F1 and F2 were designed for minimum distortion HF AM broadcast demodulation, and were probably best used as tuners for feeding hi-fi systems.

As previously mentioned, PLL techniques were used in professional receivers by 1974, if not earlier. The Eddystone EC958/12 ISB section, and I think the ISB option for the Racal RA1772 both followed the valve-era pattern of having a very narrow bandwidth 3rd IF channel for carrier acquisition, as well as the two sideband channels, but instead of simply limiting the filtered and amplified carrier and using it as the reconditioned carrier for demodulation, they used a PLL to lock the local carrier to the incoming carrier. Whether the earlier Marconi Hydrus did this, or simply replicated valve ISB receiver topology I do not know. I still need to track down a brochure or other information on this one.

Quote:
Originally Posted by merlinmaxwell View Post
It would be a bit of fun to have two and a stereo ISB broadcast to receive, as far as I know there have been none.
Whilst I ended up with a pair of Sony ICF-2010, I never tried this. Even in the early days of AM stereo, before Motorola C-QUAM became dominant, stations using the Khan ISB system were scarce.

Quote:
Originally Posted by Radio Wrangler View Post
Take the Racal RA1792. Usually thought of as a free-tuning general coverage receiver of high performance... but get the thumbwheel switch tuned version and it's really only good for point-to-point links. All versions have numerous choices of filter combinations, but there's also an ISB option with an AFC lock board.

Lots of parts in common with the general use receiver, but a full blooded ISB AFC machine for fixed links.
Which I think illustrates the trend that ISB receivers were becoming derivatives of or related to general coverage receivers, rather than being purpose-designed and standalone.

Quote:
Originally Posted by Radio Wrangler View Post
This set was known in Marconi as 'Sosin's Folly' There's a lot of info on the web about B M Sosin and a very contentious comparison article in the Marconi company's journal. Litigation leading to letters from Marconi to librarians to destroy all copies, to be replaced by a redone version.
That would be the reception failure factor parameter business. I am sure that the folks at Racal would not have been happy.

Cheers,
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Old 10th Feb 2014, 1:16 am   #9
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Default Re: ISB Receivers

I think that Marconi and Racal weren't so much creating ISB receivers out of general purpose jobs so much as when any new receiver was being planned, it was intended to take wide ranges of options and ISB was just part of the mix.

Doing this sort of work caries the risk that you could lock-out something you later need to do, so the safest approach is to design everything in at the beginning and then to progressively introduce things to the catalogue later as needed.

Of course, when our librarian got the request to destroy a copy of a journal, the journal interest factor went through the roof and lots of personal photocopies got made. Racal had to object, Marconi had to try to retract and the publicity made everything so much more prominent. Rather like banning a record boosts sales (I like playing 'Je t'aime' on ecclesiastical organs)

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Old 19th Mar 2014, 1:55 am   #10
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Default Re: ISB Receivers

The Marconi H2900 may well have been a nodal point in the convergence of point-to-point ISB and general coverage receivers. And I have now managed to find some information on the H2900, namely the (brief) article in SWM 1970 April.

As Radio Wrangler has already said, it was clearly of the traditional “3-IF-channel” ISB form. According to a comment in Hawker’s HF receiver article in Wireless World 1970, it had PLL AM demodulation. Thus it might have been one of the first to use this approach for reconditioned carrier generation rather than the established quasi-synchronous technique using filtered and limited carrier.

Frequency coverage of the H2900 was said to be 1.5 to 30 MHz. This was consistent, even slightly better than established ISB receiver practice, examples from the valve era being the Marconi HR21 family at 3 to 27.5 MHz, and the Mullard GFR 552, at 4 to 30 MHz. The then-recent solid-state Marconi Hydrus covered 1.5 to 30 MHz.

The H2900 was not quite “general coverage”, the norm for which one might take as being 0.5 to 30 MHz. But on the other hand, there were earlier Marconi general-purpose HF receivers that did not tune below the top of the MF broadcast band. For example, the HR120 of 1960 covered 2.1 to 30 MHz. And the “Radio & Television Engineer’s Reference Book, 3rd Edition, chapter 22, includes a description of an unidentified HF receiver that was probably a Marconi model. The chapter was written by a Marconi staffer, and the valve line-up is very Marconi, with for example a Z77 (EF91) 1st RF stage, W77 (EF92) 2nd RF stage, X77 (6BE6) 1st & 2nd mixers, W77 IF stages, just like the HR24. This receiver covered 1.5 to 30 MHz, and was dual-conversion, with IFs of 1.2 MHz and 100 kHz. So the H2900 frequency coverage was aligned with long-established Marconi precedent for general-purpose HF receivers.

The Marconi H2900 delivered its multi-mission capability in just one box, albeit a very expensive one. It arrived on the scene about two years after the Hydrus solid-state ISB receiver. The trade press advertisements for the latter show it to occupy four rack bays in dual-diversity form, so presumably the basic non-diversity option would have taken three bays. And the Hydrus itself was much more compact than its valved predecessors such as the HR21 family.

On the question of frequency coverage, and pertinent to the convergence theory, is the fact that hitherto, marine main receivers had been somewhat separate from general coverage receivers because of their need to tune down to 15 kHz or thereabouts, as well as their need for certification against the pertinent GPO and other regulations. But this distinction had started fading by the late 1960s. The Plessey PR155 general coverage receiver of 1967 tuned down to 15 kHz, although I do not know whether it was ever qualified as a marine main receiver (or whether it had an ISB option). The Eddystone EC958 of 1968-69 covered 10 kHz to 30 MHz, and was designed with marine applications in mind. The Marconi Nebula marine receiver was a rebrand of the EC958/5; this might have provided a lower cost alternative to its own Apollo marine receiver.

The Apollo seems to have arrived on the scene at about the same time as the Hydrus ISB receiver, so at that time Marconi was still treating the marine and point-to-point cases as being separate. Whether it also had a contemporary solid-state general coverage receiver, perhaps as a successor to the HR120, I do not know. But the Apollo was also described as being a general-purpose receiver, so it might also have been sold into non-marine applications. (I think that might also have happened with its Atalanta valved predecessor, which apparently had a model number in the main Marconi series as well as in the marine series.)

So the H2900 did not quite connect all of the dots. That step appears to have come in 1974, with the Racal RA1772, which covered 15 kHz to 30 MHz, and for which a 3-IF-channel ISB variant was offered from the start. And at least judging by the Radio Bygones “Marconi Mishap” article, the RA1772 was about a quarter of the price of the H2900. The Eddystone EC958/12 also I think dated from 1974 or thereabouts. This was an EC958/7 (an update of the original) with “add-on” ISB equipment, so that it had four IF channels in all.

Subsequent general-purpose HF receivers, such as the Plessey PR2250, and the Redifon R551, appeared to include ISB options almost as a matter of routine.

An assumption here is that the general-purpose HF receivers with ISB options also came with a range of ISB filters according to customer requirements. The Marconi valve-era point-to-point receivers seemed to have offered both 100 Hz to 3.5 kHz and 100 Hz to 6 kHz passbands, the former for voice traffic (and whatever else used a voice channel) and the latter either for broadcast relays or to accommodate two frequency-multiplexed voice channels. The 100 Hz lower limit might well have been determined by broadcast relay needs, which would point to go as low as reasonably feasible whilst still adequately rejecting the carrier. I imagine that anything much above 100 Hz would cause a significant decline of “naturalness: for voices and be prejudicial to music. On the other and data for the Eddystone EC958/12 indicate that the baseline sideband filters were of the voice traffic type, and probably followed the normal voice bandwidth, 300 Hz to 3.4 kHz, or something close to it.

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Old 19th Mar 2014, 10:06 am   #11
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Default Re: ISB Receivers

I used 'General coverage' as the opposite of 'amateur bands only'

I didn't just have a prototype H2900, I had the full manuals as well, so I know it fairly well. Being professionally involved in high performance measuring receivers in the same frequency region at around that time, I found the measures to combat phase noise interesting. Sosin also knew about intermod effects in quartz filters. Racal were a jump ahead on mixers, Sosin was a jump ahead in a few other areas.

The H2900 slewed the main synthesiser to lock the IF onto a residual carrier using a phase detector at 2MHz IF.

The marine market had already got far cheaper radios which met their certification requirements. I think this thing was targeted at point-to-point use, military and spooks. Ships wouldn't have the budget.

Oh, the RA1772 has a crystal filter in the LO feed to allow sensitivity at VLF without the phase noise sidebands of the synth leaking into the IF.

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Old 20th May 2014, 5:05 am   #12
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Default Re: ISB Receivers

The Redifon R550 series might have been a better and earlier nodal point in the commercial HF receiver “convergence” than the Marconi H2900.

I think that it dated from 1969. The R550 was the base model for point-to-point use, etc. The R551 was the marine version, qualified as a marine SSB main receiver. There was an optional ISB unit available for. So Redifon was covering most of the bases with one design; commercial point-to-point, all modes, and merchant marine SSB (and ISB if required). Quite possibly it also offered an R550 variant for aviation use, base stations at least.

It would seem that Redifon had taken a bigger step at the beginning of the marine SSB era than had Marconi Marine with its Apollo. Whereas the later more-or-less followed traditional HF receiver layout, the R550 series was of the synthesized upconversion form. The R550 series also seems to have been an early example of a commercial HF receiver that used a 1.4 MHz final IF, a number later made well-known by the Racal RA1772 of 1974, and subsequently used by others.

Meanwhile I have not succeeded in finding any material information on the Marconi Hydrus, which with looks to have been an interesting transitional design, both looking back to the large dedicated three-channel valve-era ISB receivers and looking forward to the more compact solid state multipurpose designs. With a 40 MHz 1st IF it was at least upconversion, and one uncorroborated reference suggests that it was of Wadley Loop form. I have a suspicion that the H2900, which was a couple of years later than the Hydrus, might have been the first synthesized receiver from any Marconi division.

Does anyone know what kinds of ISB receivers were used by the BBC World Service for broadcast relay purposes, both from HF broadcasts per se and in the 1970s, from its own ISB relay transmitters? I should guess that in the valve era that it might have used receivers such as the Marconi HR21 and the Mullard GFR.552, and possibly even the “small” Marconi HR22. But it seems likely that in the 1970s it would have acquired solid state ISB receiving equipment as well.

Cheers,
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Old 20th May 2014, 7:25 am   #13
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Default Re: ISB Receivers

Have you tried asking the ex-Quartz Hill people what they used?

I think the world service technique received broadcast full carrier DSB AM on one site and fed a transmitter site miles away. If ISB receivers were used, it would have been for sideband diversity reception, not for different material on each sideband.

I gather all the radio masts got demolished to make way for wind mills in the past few years which seems a shame. A radio society had the use of the place for a while. Our local club had the use of 'Outhmuir' a moor-top HF site used by the RAF as the Eastern receiving end of a group of transatlantic links. It had large wire arrays from lots of masts and a small building stuffed with RA17 racks once upon a time. We got it for an affordable rent but only the mast foundations were left along with the empty buildings. Still we got some aerials up and it was a fabulous site for us for 15 years, then a political sort of change brought in agents who thought it was usable as a vhf site and cellphone site and ought to have a commercial rent to suit. The rent multiplied dramatically, the club disbanded. There is a good hilltop VHF site just across the road with 360 degree coverage and all mod cons, so outh was not marketable. I last went past a couple of years ago. It's all derelict, the buildings vandalised. They never got another penny in rent from anyone.

THis site had antenna and frequency diversity, but all the signals were FSK teleprinter traffic, encrypted, for offsite decryption. So there was no ISB or sideband diversity.

Oh, yes, the RAF people drank rainwater collected from the roof. The only water treatment was a storage tank, a kettle and a teapot!

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Old 20th May 2014, 9:11 am   #14
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Default Re: ISB Receivers

Hi David:

Thanks for that.

The BBC situation was described in BBC Engineering #84, 1970 October (http://downloads.bbc.co.uk/rd/pubs/a...neering_84.pdf) which noted that the BBC was expanding the use of HF SSB program links, having started in 1964, with use of ISB receivers for HF AM reception predating that. But the article deals mainly with the SSB transmission, and does not mention the receivers that were in use.

I imagine that the ISB receivers were used both to provide sideband diversity when receiving AM broadcasts, and perhaps more so to mitigate selective fading distortion. As far as I know the BBC did not use ISB program links, although the VOA did (and perhaps still does). Way back then sometimes I used to listen to them using the Liniplex F2.

The Quartz Hill operation is described here: http://www.zl6qh.com/000468.html. From the 1960s, the primary receivers for the BBC World Service were of the Marconi HR21 ISB type. To quote from that website: “Two Marconi HR21 independent-sideband HF receivers were the mainstay of the station’s receiving equipment. They were used primarily for maintaining a round-the-clock feed of the BBC World Service to Broadcasting House. Designed for use on international radio-telephone circuits, they were purchased in the early sixties when the advantages of using the sideband reception technique for short-wave broadcasts were finally recognised. These were the ability to select the sideband with the least interference (especially useful in eliminating heterodynes or whistles), and a substantial reduction in the audio distortion caused by selective fading. Each HR21 had separate 6kHz bandpass filters and amplifiers for the upper and lower sideband, and its two audio output channels were fed to faders on the control panel, thus enabling a smooth changeover to the better channel. Each receiver employed about 65 valves (vacuum tubes), and comprised several separate units occupying a complete rack about two metres high. They were relatively difficult to tune and to service, but their shortwave reception quality was superior to that of the nine GEC BRT-402 general-coverage receivers that continued to be used for medium-frequency and less important short-wave reception.”

I drove past the Quartz Hill site a few times back in the old days when it was fully operational, and once I think up the hill to the gate, but never saw inside it. It certainly had a bleak, remote feel about it, even though it was maybe only 40 minutes from the Wellington suburbs.

Cheers,
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Old 20th May 2014, 11:53 am   #15
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Default Re: ISB Receivers

Hi All,
Racal also had an offering which was specifically targetted at the HF re-broadcast market - the RA133.
This was a dual diversity HF receiving terminal, comprising:
2 x MA197B Preselector
2 x RA117C Receiver
2 x RA98C ISB Adaptor
1 x MA299A DSB demodulator & Audio Combiner
1 x MA343A distribution & Patch panel.
Unfortunately, the only documentation I have for this equipment provides very little information on the MA299A. No mention is made of what type of DSB demodulator is used, or how the diversity combining is achieved. Interesting that synthesizers are not included in the package.
I have no knowledge of who Racal may have sold this equipment to, somebody with deep pockets, I guess!
cheers
Peter G8BBZ
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Old 20th May 2014, 1:52 pm   #16
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Default Re: ISB Receivers

The RA117, through having the external VFO capability could have it supplied by a crystal oscillator rather than a synthesiser if it is to sit staring at one channel.

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Old 21st May 2014, 4:30 am   #17
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Default Re: ISB Receivers

Peter G8BBZ, thanks for mentioning the Racal RA98C ISB adaptor. Whilst I was vaguely aware of its existence, I had not correlated it with this thread. It is significant, though, in that it shows that back in the valve era, general-purpose, general coverage HF receivers were adapted for ISB, including broadcast relay purposes. I imagine that the Racal ISB assembly would have been more compact and lower cost than say a Marconi HR21/23/24, although above the HR22 in cost and complexity.

Anyway, a (web) search under “RA98” has brought up some useful information.

The RA98 (D-version) is described at: http://www.pdp-11.nl/racal/ra98/startpage.html.

And this, http://www.qth.net/pipermail/premium...q2/002874.html, provides some useful additional commentary, including:

"7. For some years the BBC World Service (and I think CBC) used the Racal RA98C ISB adaptor with the RA17L for long range HF feeder circuits at the distant end for rebroadcast of BBC programmes there on local MW or FM. The normal -3dB filter bandwith of the RA98 is 300Hz to 6,000Hz. Using a 6.5 kHz bandwidth in the receiver reduces the overall bandwidth to about 300 -
3250Hz. In the RA98C version the sideband filter bandwidth is widened to about 90Hz - 6,000Hz to give a much enhanced bass response, necessary for rebroadcasting quality."


The 90 Hz number is covered in the previously mentioned BBC Engineering #84. (I think that the Marconi ISB receivers ran 100 to 6000 Hz.) Anyway, we now have at least a partial answer as to what ISB receivers the BBC used for program relay purposes during the 1960s.

The RA98D version, with 300 to 6000 Hz audio bandwidth, looks as if it were designed for multiplexed voice channels, say 300 to <3300 Hz and >3300 to 600 Hz. Nominal voice channel bandwidth is 300 to 3400 Hz (as I understand it established in the 1930s on the basis that intelligibility in the presence of noise did not further improve by going below 3400 Hz), but the 3400 Hz number seems to have been more honoured in the breach than the observance. (perhaps with good reason, in which case the original (and more obscure) meaning of that phrase would apply).

The RA98C version and its use by the BBC also gets a mention here: https://uk.groups.yahoo.com/neo/grou...ions/topics/17.

As to how the MA299A DSB demodulator & Audio Combiner worked, I can only speculate.

At minimum I think that it would have provided for simple summation of the two sideband audio signals. This would then make the combined output more-or-less equivalent to DSB synchronous demodulation. In situations where both sidebands are relatively “clean”, this would reduce the frequency response “bending” effects of selective fading. I noticed this with the Liniplex F2. Where usable, DSB was the best option. Where interference to one or other sideband made necessary the use of USB or LSB, the frequency response effects were more noticeable, although several orders of magnitude less distracting than the harmonic distortion caused by rectifying demodulators dealing with selectively fading signals.

That the MA299A was also described as a demodulator is intriguing. Perhaps it took an 18 kHz DSB signal from the RA98C (i.e extracted ahead of the sideband filters), along with an 18 kHz “carrier” signal, and used the latter (after any needed phase adjustment) to synchronously demodulate the former. But then the summation function would appear to be superfluous. So it remains an open question.

Possibly the MA299A also included some AF conditioning circuits. Such might have included a 5 kHz adjacent channel notch filter, and possibly low pass filter(s) restricting the AF response to say 3.5 or 5 kHz, to mitigate interference effects.

It does look as if Racal was capitalizing upon the general success, frequency setting ease and stability of its RA17 series receiver by extending its capability not only into the ISB realm, but also into the broadcast relay subset of that realm. One wonders whether the same was done for any other valve-era HF general-purpose receivers, or whether the RA17 was uniquely suited, stability-wise, to that role. Possibly it could have been done with the crystal-controlled Eddystone 880, but in that case there surely would have been some information about it amongst the wealth of Eddystone data available. The apparently obscure Marconi HR120, also crystal controlled, might have been another candidate.

Presumably the Racal Wadley Loop system was clean enough, in terms of oscillator phase noise to handle broadcast-quality SSB demodulation. Phase noise was an issue with synthesizers at one time, though. The Liniplex F1 was crystal-controlled only, for that reason. The F2 made provision for using an external synthesizer, which was made available (as the OSC-1) once such could be made with low enough phase noise at acceptable cost (bearing in mind that the Liniplex was basically an upper consumer level receiver.) The original OSC-1 only went to 22 MHz (receive frequency); its designer told me at the time (1988, as I remember), extending it to 26 MHz would have been too costly. But within a couple of years or so, costs had come down enough to allow that extension.

Looking into the solid-state era, where ISB versions of general-purpose receivers became perhaps commonplace, I imagine that broadcast-quality ISB sections were a subset, possibly not available in all cases. Certainly the data that I have seen for the Eddystone EC958/12 suggests that its ISB section was communications-oriented, although one may suppose that Eddystone would have built a broadcast relay version had it been wanted by a customer.

Cheers,
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Old 21st Jan 2016, 8:04 pm   #18
John KC0G
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Default Re: ISB Receivers

Re: post #10, Pat Hawker discussed the H2900 in his Technical Topics column in Radio Communication, February 1970, pp 92-93. It references an article by B.M Sosin, "A breakthrough in hf receiver design", Point-to-Point Communications, January 1970.

73 John
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Old 10th Apr 2016, 12:01 am   #19
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Default Re: ISB Receivers

Moving back a couple of decades, the attached items from Wireless World 1949 cover what was claimed to be the first shipboard SSB/ISB installation, on the vessel Caronia, using STC equipment. This was used for passenger telephone communications; the vessel being fitted with conventional equipment for regular marine R/T and W/T MF/HF communications (which did not change to SSB until the 1970s).

I imagine that at the time, point-to-point communications was changing over to SSB/ISB, and that would have included those shore stations that handled international telephone calls, including those to/from ships.

From the STC advertisement:

“The R.X.9 Independent-sideband receiver. Capable of receiving either or both channels of a double-channel circuit. Whilst one channel is busy with ‘subscriber’ calls, the other can be used as an ‘order wire’. Suitable for reception of high-fidelity double-sideband telephony. Automatic frequency control.”

From that one could deduce that the R.X.9 was typical of ISB receivers of its time, and might even have been a point-to-point model adapted for the marine application. In respect of its mention of double-sideband telephony, STC might have been saying that its ISB receiver did a better job of receiving incoming DSB telephone calls – by minimizing selective fading distortion – but it could also have recognized that it was a better way of receiving HF broadcasts for shipboard redistribution.

Cheers,
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Old 10th Apr 2016, 12:15 am   #20
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Default Re: ISB Receivers

And judging by the attached advertisement from 1965, STC was very early with the introduction of an all-solid state ISB receiver, namely its RX.11. This, in its tall rack, looked more like a “traditional” valved point-to-point receiver. I imagine that its circuitry was based mostly upon germanium transistors.

Cheers,
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