One of the most significant single advancements in telecommunications technology was the development of microwave radio. Essentially an evolution of radar, the middle of the Second World War saw the first practical microwave telephone system. By the time Japan surrendered, AT&T had largely abandoned their plan to build an extensive nationwide network of coaxial telephone cables. Microwave relay offered greater capacity at a lower cost. When Japan and the US signed their peace treaty in 1951, it was broadcast from coast to coast over what AT&T called the "skyway": the first transcontinental telephone lead made up entirely of radio waves. The fact that live television coverage could be sent over the microwave system demonstrated its core advantage. The bandwidth of microwave links, their capacity, was truly enormous. Within the decade, a single microwave antenna could handle over 1,000 simultaneous calls. Microwave's great capacity, its chief advantage, comes from the high frequencies and large bandwidths involved. The design of microwave-frequency radio electronics was an engineering challenge that was aggressively attacked during the war because microwave frequency's short wavelengths made them especially suitable for radar. The cavity magnetron, one of the first practical microwave transmitters, was an invention of such import that it was the UK's key contribution to a technical partnership that lead to the UK's access to US nuclear weapons research. Unlike the "peaceful atom," though, the "peaceful microwave" spread fast after the war. By the end of the 1950s, most long-distance telephone calls were carried over microwave. While coaxial long-distance carriers such as L-carrier saw continued use in especially congested areas, the supremacy of microwave for telephone communications would not fall until adoption of fiber optics in the 1980s. The high frequency, and short wavelength, of microwave radio is a limitation as well as an advantage. Historically, "microwave" was often used to refer to radio bands above VHF, including UHF. As RF technology improved, microwave shifted higher, and microwave telephone links operated mostly between 1 and 9 GHz. These frequencies are well beyond the limits of beyond-line-of-sight propagation mechanisms, and penetrate and reflect only poorly. Microwave signals could be received over 40 or 50 miles in ideal conditions, but the two antennas needed to be within direct line of sight. Further complicating planning, microwave signals are especially vulnerable to interference due to obstacles within the "fresnel zone," the region around the direct line of sight through which most of the received RF energy passes. Today, these problems have become relatively easy to overcome. Microwave relays, stations that receive signals and rebroadcast them further along a route, are located in positions of geographical advantage. We tend to think of mountain peaks and rocky ridges, but 1950s microwave equipment was large and required significant power and cooling, not to mention frequent attendance by a technician for inspection and adjustment. This was a tube-based technology, with analog and electromechanical control. Microwave stations ran over a thousand square feet, often of thick hardened concrete in the post-war climate and for more consistent temperature regulation, critical to keeping analog equipment on calibration. Where commercial power wasn't available they consumed a constant supply of diesel fuel. It simply wasn't practical to put microwave stations in remote locations. In the flatter regions of the country, locating microwave stations on hills gave them appreciably better range with few downsides. This strategy often stopped at the Rocky Mountains. In much of the American West, telephone construction had always been exceptionally difficult. Open-wire telephone leads had been installed through incredible terrain by the dedication and sacrifice of crews of men and horses. Wire strung over telephone poles proved able to handle steep inclines and rocky badlands, so long as the poles could be set---although inclement weather on the route could make calls difficult to understand. When the first transcontinental coaxial lead was installed, the route was carefully planned to follow flat valley floors whenever possible. This was an important requirement since it was installed mostly by mechanized equipment, heavy machines, which were incapable of navigating the obstacles that the old pole and wire crews had on foot. The first installations of microwave adopted largely the same strategy. Despite the commanding views offered by mountains on both sides of the Rio Grande Valley, AT&T's microwave stations are often found on low mesas or even at the center of the valley floor. Later installations, and those in the especially mountainous states where level ground was scarce, became more ambitious. At Mt. Rose, in Nevada, an aerial tramway carried technicians up the slope to the roof of the microwave station---the only access during winter when snowpack reached high up the building's walls. Expansion in the 1960s involved increasing use of helicopters as the main access to stations, although roads still had to be graded for construction and electrical service. These special arrangements for mountain locations were expensive, within the reach of the Long Lines department's monopoly-backed budget but difficult for anyone else, even Bell Operating Companies, to sustain. And the West---where these difficult conditions were encountered the most---also contained some of the least profitable telephone territory, areas where there was no interconnected phone service at all until government subsidy under the Rural Electrification Act. Independent telephone companies and telephone cooperatives, many of them scrappy operations that had expanded out from the manager's personal home, could scarcely afford a mountaintop fortress and a helilift operation to sustain it. For the telephone industry's many small players, and even the more rural Bell Operating Companies, another property of microwave became critical: with a little engineering, you can bounce it off of a mirror. James Kreitzberg was, at least as the obituary reads, something of a wunderkind. Raised in Missoula, Montana, he earned his pilots license at 15 and joined the Army Air Corps as soon as he was allowed. The Second World War came to a close shortly after, and so, he went on to the University of Washington where he studied aeronautical engineering and then went back home to Montana, taking up work as an engineer at one of the states' largest electrical utilities. His brother, George, had taken a similar path: a stint in the Marine Corps and an aeronautical engineering degree from Oklahoma. While James worked at Montana Power in Butte, George moved to Salem, Oregon, where he started an aviation company that supplemented their cropdusting revenue by modifying Army-surplus aircraft for other uses. Montana Power operated hydroelectric dams, coal mines, and power plants, a portfolio of facilities across a sparse and mountainous state that must have made communications a difficult problem. During the 1950s, James was involved in an effort to build a new private telephone system connecting the utility's facilities. It required negotiating some type of obstacle, perhaps a mountain pass. James proposed an idea: a mirror. Because the wavelength of microwaves are so short, say 30cm to 5cm (1GHz-6GHz), it's practical to build a flat metallic panel that spans multiple wavelengths. Such a panel will function like a reflector or mirror, redirecting microwave energy at an angle proportional to the angle on which it arrived. Much like you can redirect a laser using reflectors, you can also redirect a microwave signal. Some early commenters referred to this technique as a "radio mirror," but by the 1950s the use of "active" microwave repeaters with receivers and transmitters had become well established, so by comparison reflectors came to be known as "passive repeaters." James believed a passive repeater to be a practical solution, but Montana Power lacked the expertise to build one. For a passive repeater to work efficiently, its surface must be very flat and regular, even under varying temperature. Wind loading had to be accounted for, and the face sufficiently rigid to not flex under the wind. Of course, with his education in aeronautics, James knew that similar problems were encountered in aircraft: the need for lightweight metal structures with surfaces that kept an engineered shape. Wasn't he fortunate, then, that his brother owned a shop that repaired and modified aircraft. I know very little about the original Montana Power installation, which is unfortunate, as it may very well be the first passive microwave repeater ever put into service. What I do know is that in the fall of 1955, James called his brother George and asked if his company, Kreitzberg Aviation, could fabricate a passive repeater for Montana Power. George, he later recounted, said that "I can build anything you can draw." The repeater was made in a hangar on the side of Salem's McNary Field, erected by the flightline as a test, and then shipped in parts to Montana for reassembly in the field. It worked. It worked so well, in fact, that as word of Montana Power's new telephone system spread, other utilities wrote to inquire about obtaining passive repeaters for their own telephone systems. In 1956, James Kreitzberg moved to Salem and the two brothers formed the Microflect Company. From the sidelines of McNary Field, Microflect built aluminum "billboards" that can still be found on mountain passes and forested slopes throughout the western United States, and in many other parts of the world where mountainous terrain, adverse weather, and limited utilities made the construction of active repeaters impractical. Passive repeaters can be used in two basic configurations, defined by the angle at which the signal is reflected. In the first case, the reflection angle is around 90 degrees (the closer to this ideal angle, of course, the more efficiently the repeater performs). This situation is often encountered when there is an obstacle that the microwave path needs to "maneuver" around. For example, a ridge or even a large structure like a building in between two sites. In the second case, the microwave signal must travel in something closer to a straight line---over a mountain pass between two towns, for example. When the reflection angle is greater than 135 degrees, the use of a single passive repeater becomes inefficient or impossible, so Microflect recommends the use of two. Arranged like a dogleg or periscope, the two repeaters reflect the signal to the side and then onward in the intended direction. Microflect published an excellent engineering manual with many examples of passive repeater installations along with the signal calculations. You might think that passive repeaters would be so inefficient as to be impractical, especially when more than one was required, but this is surprisingly untrue. Flat aluminum panels are almost completely efficient reflectors of microwave, and somewhat counterintuitively, passive repeaters can even provide gain. In an active repeater, it's easy to see how gain is achieved: power is added. A receiver picks up a signal, and then a powered transmitter retransmits it, stronger than it was before. But passive repeaters require no power at all, one of their key advantages. How do they pull off this feat? The design manual explains with an ITU definition of gain that only an engineer could love, but in an article for "Electronics World," Microflect field engineer Ray Thrower provided a more intuitive explanation. A passive repeater, he writes, functions essentially identically to a parabolic antenna, or a telescope: Quite probably the difficulty many people have in understanding how the passive repeater, a flat surface, can have gain relates back to the common misconception about parabolic antennas. It is commonly believed that it is the focusing characteristics of the parabolic antenna that gives it its gain. Therefore, goes the faulty conclusion, how can the passive repeater have gain? The truth is, it isn't focusing that gives a parabola its gain; it is its larger projected aperture. The focusing is a convenient means of transition from a large aperture (the dish) to a small aperture (the feed device). And since it is projected aperture that provides gain, rather than focusing, the passive repeater with its larger aperture will provide high gain that can be calculated and measured reliably. A check of the method of determining antenna gain in any antenna engineering handbook will show that focusing does not enter into the basic gain calculation. We can also think of it this way: the beam of energy emitted by a microwave antenna expands in an arc as it travels, dissipating the "density" of the energy such that a dish antenna of the same size will receive a weaker and weaker signal as it moves further away (this is the major component of path loss, the "dilution" of the energy over space). A passive repeater employs a reflecting surface which is quite large, larger than practical antennas, and so it "collects" a large cross section of that energy for reemission. Projected aperture is the effective "window" of energy seen by the antenna at the active terminal as it views the passive repeater. The passive repeater also sees the antenna as a "window" of energy. If the two are far enough away from one another, they will appear to each other as essentially point sources. In practice, a passive repeater functions a bit like an active repeater that collects a signal with a large antenna and then reemits it with a smaller directional antenna. To be quite honest, I still find it a bit challenging to intuit this effect, but the mathematics bear it out as well. Interestingly, the effect only occurs when the passive repeater is far enough from either terminal so as to be usefully approximated as a point source. Microflect refers to this as the far field condition. When the passive repeater is very close to one of the active sites, within the near field, it is more effective to consider the passive reflector as part of the transmitting antenna itself, and disregard it for path loss calculations. This dichotomy between far field and near field behavior is actually quite common in antenna engineering (where an "antenna" is often multiple radiating and nonradiating elements within the near field of each other), but it's yet another of the things that gives antenna design the feeling of a dark art. One of the most striking things about passive repeaters is their size. As a passive repeater becomes larger, it reflects a larger cross section of the RF energy and thus provides more gain. Much like with dish or horn antennas, the size of a passive repeater can be traded off with transmitter power (and the size of other antennas involved) to design an economical solution. Microflect offered as standard sizes ranging from 8'x10' (gain at around 6.175GHz: 90.95 dB) to 40'x60' (120.48dB, after a "rough estimate" reduction of 1dB due to interference effects possible from such a short wavelength reflecting off of such a large panel as to invoke multipath effects). By comparison, a typical active microwave repeater site might provide a gain of around 140dB---and we must bear in mind that dB is a logarithmic unit, so the difference between 121 and 140 is bigger than it sounds. Still, there's a reason that logarithms are used when discussing radio paths... in practice, it is orders of magnitude that make the difference in reliable reception. The reduction in gain from an active repeater to a passive repeater can be made up for with higher-gain terminal antennas and more powerful transmitters. Given that the terminal sites are often at far more convenient locations than the passive repeater, that tradeoff can be well worth it. Keep in mind that, as Microflect emphasizes, passive repeaters require no power and very little ("virtually no") maintenance. Microflect passive repeaters were manufactured in sections that bolted together in the field, and the support structures provided for fine adjustment of the panel alignment after mounting. These features made it possible to install passive repeaters by helicopter onto simple site-built foundations, and many are found on mountainsides that are difficult to reach even on foot. Even in less difficult locations, these advantages made passive repeaters less expensive to install and operate than active repeaters. Even when the repeater side was readily accessible, passives were often selected simply for cost savings. Let's consider some examples of passive repeater installations. Microflect was born of the power industry, and electrical generators and utilities remained one of their best customers. Even today, you can find passive repeaters at many hydroelectric dams. There is a practical need to communicate by telephone between a dispatch center (often at the utility's city headquarters) and the operators in the dam's powerhouse, but the powerhouse is at the base of the dam, often in a canyon where microwave signals are completely blocked. A passive repeater set on the canyon rim, at an angle downwards, solves the problem by redirecting the signal from horizontal to vertical. Such an installation can be seen, for example, at the Hoover Dam. In some sense, these passive repeaters "relocate" the radio equipment from the canyon rim (where the desirable signal path is located) to a more convenient location with the other powerhouse equipment. Because of the short distance from the powerhouse to the repeater, these passives were usually small. This idea can be extended to relocating en-route repeaters to a more serviceable site. In Glacier National Park, Mountain States Telephone and Telegraph installed a telephone system to serve various small towns and National Park Service sites. Glacier is incredibly mountainous, with only narrow valleys and passes. The only points with long sight ranges tend to be very inaccessible. Mt. Furlong provided ideal line of sight to East Glacier and Essex along highway 2, but it would have been extremely challenging to install and maintain a microwave site on the steep peak. Instead, two passive repeaters were installed near the mountaintop, redirecting the signals from those two destinations to an active repeater installed downslope near the highway and railroad. This example raises another advantage of passive repeaters: their reduced environmental impact, something that Microflect emphasized as the environmental movement of the 1970s made agencies like the Forest Service (which controlled many of the most appealing mountaintop radio sites) less willing to grant permits that would lead to extensive environmental disruption. Construction by helicopter and the lack of a need for power meant that passive repeaters could be installed without extensive clearing of trees for roads and power line rights of way. They eliminated the persistent problem of leakage from standby generator fuel tanks. Despite their large size, passive repeaters could be camouflaged. Many in national forests were painted green to make them less conspicuous. And while they did have a large surface area, Microflect argued that since they could be installed on slopes rather than requiring a large leveled area, passive repeaters would often fall below the ridge or treeline behind them. This made them less visually conspicuous than a traditional active repeater site that would require a tower. Indeed, passive repeaters are only rarely found on towers, with most elevated off the ground only far enough for the bottom edge to be free of undergrowth and snow. Other passive repeater installations were less a result of exceptionally difficult terrain and more a simple cost optimization. In rural Nevada, Nevada Bell and a dozen independents and coops faced the challenge of connecting small towns with ridges between them. The need for an active repeater at the top of each ridge, even for short routes, made these rural lines excessively expensive. Instead, such towns were linked with dual passive repeaters on the ridge in a "straight through" configuration, allowing microwave antennas at the towns' existing telephone exchange buildings to reach each other. This was the case with the installation I photographed above Pioche. I have been frustratingly unable to confirm the original use of these repeaters, but from context they were likely installed by the Lincoln County Telephone System to link their "hub" microwave site at Mt. Wilson (with direct sight to several towns) to their site near Caliente. The Microflect manual describes, as an example, a very similar installation connecting Elko to Carlin. Two 20'x32' passive repeaters on a ridge between the two (unfortunately since demolished) provided a direct connection between the two telephone exchanges. As an example of a typical use, it might be interesting to look at the manual's calculations for this route. From Elko to the repeaters is 13.73 miles, the repeaters are close enough to each other as to be in near field (and so considered as a single antenna system), and from the repeaters to Carlin is 6.71 miles. The first repeater reflects the signal at a 68 degree angle, then the second reflects it back at a 45 degree angle, for a net change in direction of 23 degrees---a mostly straight route. The transmitter produces 33.0 dBm, both antennas provide a 34.5 dB gain, and the passive repeater assembly provides 88 dB gain (this calculated basically by consulting a table in the manual). That means there is 190 dB of gain in the total system. The 6.71 and 13.73 mile paths add up to 244 dB of free space path loss, and Microflect throws in a few more dB of loss to account for connectors and cables and the less than ideal performance of the double passive repeater. The net result is a received signal of -58 dBm, which is plenty acceptable for a 72-channel voice carrier system. This is all done at a significantly lower price than the construction of a full radio site on the ridge [1]. The combination of relocating radio equipment to a more convenient location and simply saving money leads to one of the iconic applications of passive repeaters, the "periscope" or "flyswatter" antenna. Microwave antennas of the 1960s were still quite large and heavy, and most were pressurized. You needed a sturdy tower to support one, and then a way to get up the tower for regular maintenance. This lead to most AT&T microwave sites using short, squat square towers, often with surprisingly convenient staircases to access the antenna decks. In areas where a very tall tower was needed, it might just not be practical to build one strong enough. You could often dodge the problem by putting the site up a hill, but that wasn't always possible, and besides, good hilltop sites that weren't already taken became harder to find. When Western Union built out their microwave network, they widely adopted the flyswatter antenna as an optimization. Here's how it works: the actual microwave antenna is installed directly on the roof of the equipment building facing up. Only short waveguides are needed, weight isn't an issue, and technicians can conveniently service the antenna without even fall protection. Then, at the top of a tall guyed lattice tower similar to an AM mast, a passive repeater is installed at a 45 degree angle to the ground, redirecting the signal from the rooftop antenna to the horizontal. The passive repeater is much lighter than the antenna, allowing for a thinner tower, and will rarely if ever need service. Western Union often employed two side-by-side lattice towers with a "crossbar" between them at the top for convenient mounting of reflectors each direction, and similar towers were used in some other installations such as the FAA's radar data links. Some of these towers are still in use, although generally with modern lightweight drum antennas replacing the reflectors. Passive microwave repeaters experienced their peak popularity during the 1960s and 1970s, as the technology became mature and communications infrastructure proliferated. Microflect manufactured thousands of units from there new, larger warehouse, across the street from their old hangar on McNary Field. Microflect's customer list grew to just about every entity in the Bell System, from Long Lines to Western Electric to nearly all of the BOCs. The list includes GTE, dozens of smaller independent telephone companies, most of the nation's major railroads, electrical utilities from the original Montana Power to the Tennessee Valley Authority. Microflect repeaters were used by ITT Arctic Services and RCA Alascom in the far north, and overseas by oil companies and telecoms on islands and in mountainous northern Europe. In Hawaii, a single passive repeater dodged a mountain to connect Lanai City telephones to the Hawaii Telephone Company network at Tantalus on Oahu---nearly 70 miles in one jump. In Nevada, six passive repeaters joined two active sites to connect six substations to the Sierra Pacific Power Company's control center in Reno. Jamaica's first high-capacity telephone network involved 11 passive repeaters, one as large as 40'x60'. The Rocky Mountains are still dotted with passive repeaters, structures that are sometimes hard to spot but seem to loom over the forest once noticed. In Seligman, AZ, a sun-faded passive repeater looks over the cemetery. BC Telephone installed passive repeaters to phase out active sites that were inaccessible for maintenance during the winter. Passive repeaters were, it turns out, quite common---and yet they are little known today. First, it cannot be ignored that passive repeaters are most common in areas where communications infrastructure was built post-1960 through difficult terrain. In North America, this means mostly the West [2], far away from the Eastern cities where we think of telephone history being concentrated. Second, the days of passive repeaters were relatively short. After widespread adoption in the '60s, fiber optics began to cut into microwave networks during the '80s and rendered microwave long-distance links largely obsolete by the late '90s. Considerable improvements in cable-laying equipment, not to mention the lighter and more durable cables, made fiber optics easier to install in difficult terrain than coaxial had ever been. Besides, during the 1990s, more widespread electrical infrastructure, miniaturization of radio equipment, and practical photovoltaic solar systems all combined to make active repeaters easier to install. Today, active repeater systems installed by helicopter with independent power supplies are not that unusual, supporting cellular service in the Mojave Desert, for example. Most passive repeaters have been obsoleted by changes in communications networks and technologies. Satellite communications offer an even more cost effective option for the most difficult installations, and there really aren't that many places left that a small active microwave site can't be installed. Moreover, little has been done to preserve the history of passive repeaters. In the wake of the 2015 Wired article on the Long Lines network, considerable enthusiasm has been directed towards former AT&T microwave stations, having been mostly preserved by their haphazard transfer to companies like American Tower. Passive repeaters, lacking even the minimal commercial potential of old AT&T sites, were mostly abandoned in place. Often being found in national forests and other resource management areas, many have been demolished for restoration. In 2019, a historic resources report was written on the Bonneville Power Administration's extensive microwave network. It was prepared to address the responsibility that federal agencies have for historical preservation under the National Historic Preservation Act and National Environmental Policy Act, policies intended to ensure that at least the government takes measures to preserve history before demolishing artifacts. The report reads: "Due to their limited features, passive repeaters are not considered historic resources, and are not evaluated as part of this study." In 1995, Valmont Industries acquired Microflect. Valmont is known mostly for their agricultural products, including center-pivot irrigation systems, but they had expanded their agricultural windmill business into a general infrastructure division that manufactured radio masts and communication towers. For a time, Valmont continued to manufacture passive repeaters as Valmont Microflect, but business seems to have dried up. Today, Valmont Structures manufactures modular telecom towers from their facility across the street from McNary Field in Salem, Oregon. A Salem local, descended from early Microflect employees, once shared a set of photos on Facebook: a beat-up hangar with a sign reading "Aircraft Repair Center," and in front of it, stacks of aluminum panel sections. Microflect workers erecting a passive repeater in front of a Douglas A-26. Rows of reflector sections beside a Shell aviation fuel station. George Kreitzberg died in 2004, James in 2017. As of 2025, Valmont no longer manufactures passive repeaters. Postscript If you are interested in the history of passive repeaters, there are a few useful tips I can give you. Nearly all passive repeaters in North America were built by Microflect, so they have a very consistent design. Locals sometimes confuse passive repeaters with old billboards or even drive-in theater screens, the clearest way to differentiate them is that passive repeaters have a face made up of aluminum modules with deep sidewalls for rigidity and flatness. Take a look at the Microflect manual for many photos. Because passive repeaters are passive, they do not require a radio license proper. However, for site-based microwave licenses, the FCC does require that passive repeaters be included in paths (i.e. a license will be for an active site but with a passive repeater as the location at the other end of the path). These sites are almost always listed with a name ending in "PR". I don't have any straight answer on whether or not any passive repeaters are still in use. It has likely become very rare but there are probably still examples. Two sources suggest that Rachel, NV still relies on a passive repeater for telephone and DSL. I have not been able to confirm that, and the tendency of these systems to be abandoned in place means that people sometimes think they are in use long after they were retired. I can find documentation of a new utility SCADA system being installed, making use of existing passive repeaters, as recently as 2017. [1] If you find these dB gain/loss calculations confusing, you are not alone. It is deceptively simple in a way that was hard for me to learn, and perhaps I will devote an article to it one day. [2] Although not exclusively, with installations in places like Vermont and Newfoundland where similar constraints applied.

Mehdi Sadaghdar, better known as ElectroBOOM, created a name for himself with shocking content on YouTube full of explosive antics. But once you get past the meme-worthy shenanigans, he is a genuinely smart guy that provides useful and accessible lessons on many electrical engineering principles. If you like your learning with a dash of over-the-top […] The post Learn how to make a 2D capacitive touch sensor with ElectroBOOM appeared first on Arduino Blog.

A long time ago, there was a small Estonian website called “Mängukoobas” (literal translation from Estonian is “game cave”). It started out as a place for people to share various links to browser games, mostly built with Flash or Shockwave. It had a decent moderation system, randomized treasure chests that could appear on any part of the website, and a lot more.1 What it also had was a basic filtering system. As a good chunk of the audience was children (myself included), there was a need to filter out all the naughty Estonian words, such as “kurat”, “türa”, “lits” and many more colorful ones. The filtering was very basic, however, and some took it to themselves to demonstrate how flawed the system was by intentionally using phrases like “politsei”, which is Estonian for “police”. It would end up being filtered to “po****ei” as it also contained the word “lits”, which translates to “slut”2. Of course, you could easily overcome the filter by using a healthy dose of period characters, leading to many cases of “po.l.i.t.sei” being used. With the ZIRP phenomenon we got a lot of companies wanting to get into the “platform” business where they bring together buyers and sellers, or service providers and clients. A lot of these platforms rely on transactions taking place only on their platform and nowhere else, so they end up doing their best to avoid the two parties from being in contact off-platform and paying out of band, as that would directly cut into their revenue. As a result, they scan private messages and public content for common patterns, such as e-mails and phone numbers, and block or filter them. As you can predict, this can backfire in a very annoying way. I was looking for a cheap mini PC on a local buy-sell website and stumbled on one decent offer. I looked at the details, was going over the CPU model, and found the following: CPU: Intel i*-**** Oh. Well, maybe it was an error, I will ask the seller for additional details with a public question. The response? Hello, the CPU model is Intel i*-****. Damn it. I never ended up buying that machine because I don’t really want to gamble with Intel CPU model numbers, and a few days later it was gone. It’s 2025, I’m nearing my mandatory mid-life crisis, and the Scunthorpe problem is alive and well. fun tangent: the site ended up being like a tiny social network, eventually incorporating things like a cheap rate.ee knock-off where children were allowed to share pictures of themselves. As you can imagine, this was a horrible, horrible idea, as it attracted the exact type of person that would be interested in that type of content. I got lucky by being so poor that I did not have a webcam or a digital camera to make any pictures with, and I remember that fondly because someone on MSN Messenger was very insistent that I take some pictures of myself. Don’t leave children with unmonitored internet access! ↩︎ “slut” is also an actual word in Swedish which translates to “final”. I think. I’m not a Swedish expert, actually. ↩︎

Help point me in the right direction.

MathJax.Hub.Config({ jax: ["input/TeX", "output/HTML-CSS"], tex2jax: { inlineMath: [ ['$', '$'], ["\\(", "\\)"] ], displayMath: [ ['$$', '$$'], ["\\[", "\\]"] ], processEscapes: true, skipTags: ['script', 'noscript', 'style', 'textarea', 'pre', 'code'] } //, //displayAlign: "left", //displayIndent: "2em" }); Introduction Inside the HP 5370A High Stability Reference Clock with an HP 10811-60111 OCXO RIFA Capacitors in the Corcom F2058 Power Entry Module? 15V Rail Issues Power Suppy Architecture Fault Isolation - It’s the Reference Frequency Buffer PCB! The Reference Frequency Buffer Board Fixing the Internal Reference Clock Fixing the External Reference Clock Future work Footnotes Introduction I bought an HP 5370A time interval counter at the Silicon Valley Electronics Flea Market for a cheap $40. The 5370A is a pretty popular device among time nuts: it has a precision of 20ps for single-shot time interval measurements, amazing for a device that was released in 1978, and even compared to contemporary time interval counters it’s still a decent performance. The 74LS chips in mine have a 1981 time code which makes the unit a whopping 44 years old. But after I plugged it in and pressed the power button, smoke and a horrible smell came out after a few minutes. I had just purchased myself hours of entertainment! Inside the HP 5370A It’s trivial to open the 5370A: remove the 4 feet in the back by removing the Philips screws inside them. remove a screw to release the top or bottom cover (Click to enlarge) Once inside, you can see an extremely modular build: the center consists of a motherboard with 10 plug-in PCBs, 4 on the left for an embedded computer that’s based on an MC6800 CPU, 6 on the right for the time acquisition. The top has plug-in PCBs as well, with the power supply on the left and reference clock circuitry on the right. My unit uses the well known HP 10811-60111 high-stability OCXO as 10MHz clock reference. The bottom doesn’t have plug-in PCBs. It has PCBs for trigger logic and the front panel. This kind of modular build probably adds significant cost, but it’s a dream for servicing and tracking down faults. To make things even easier, the vertical PCBs have a plastic ring or levers to pull them out of their slot! There are also plenty of generously sized test pins and some status LEDs. High Stability Reference Clock with an HP 10811-60111 OCXO Since the unit has the high stability option, I have now yet another piece of test equipment with an HP 10811-60111. OCXOs are supposed to be powered on at all time: environmental changes tend to stress them out and result in a deviation of their clock speed, which is why there’s a “24 hour warm-up” sticker on top of the case. It can indeed take a while for an OCXO to relax and settle back into its normal behavior, though 24 hours seems a bit excessive. The 5370A has a separate always-on power supply just for the oven of the OCXO to keeps the crystal at constant temperature even when the power switch on the front is not in the ON position. Luckily, the fan is powered off when the front switch is set to stand-by.1 In the image above, from top to bottom, you can see: the main power supply control PCB the HP 10811-60111 OCXO. To the right of it is the main power relay. the OCXO oven power supply the reference frequency buffer PCB These are the items that will play the biggest role during the repair. RIFA Capacitors in the Corcom F2058 Power Entry Module? Spoiler: probably not… After plugging in the 5370A the first time, magic smoke came out of it along with a pretty disgusting chemical smell, one that I already knew from some work that I did on my HP 8656A RF signal generator. I unplugged the power, opened up the case, and looked for burnt components but couldn’t find any. After a while, I decided to power the unit back on and… nothing. No smoke, no additional foul smell, but also no display. One of common failure mode of test equipment from way back when are RIFA capacitors that sit right next to the mains power input, before any kind of power switch. Their primary function is to filter out high frequency noise that’s coming from the device and reduce EMI. RIFAs have a well known tendency to crack over time and eventually catch fire. A couple of years ago, I replaced the RIFA capacitors of my HP 3457A, but a general advice is to inspect all old equipment for these gold colored capacitors. However, no such discrete capacitors could be found. But that doesn’t mean they are not there: like a lot of older HP test equipment, the 5370A uses a Corcom F2058 line power module that has capacitors embedded inside. Below is the schematic of the Corcom F2058 (HP part number 0960-0443). The capacitors are marked in red. You can also see a fuse F1, a transformer and, on the right, a selector that can be used to configure the device for 100V, 115V/120V, 220V and 230V/240V operation. (Click to enlarge) There was a bad smell lingering around the Corcom module, so I removed it to check it out. There are metal clips on the left and right side that you need to push in to get the module out. It takes a bit of wiggling, but it works out eventually. Once removed, however, the Corcom didn’t really have a strong smell at all. I couldn’t find any strong evidence online that these modules have RIFAs inside them, so for now, my conclusion is that they don’t have them and that there’s no need to replace them. Module replacement In the unlikely case that you want to replace the Corcom module, you can use this $20 AC Power Entry Module from Mouser. One reason why you might want to do this is because the new module has a built-in power switch. If you use an external 10 MHz clock reference instead of the 10811 OCXO, then there’s really no need to keep the 5370A connected to the mains all the time. There are two caveats, however: while it has the same dimensions as the Corcom F2058, the power terminals are located at the very back, not in an indented space. This is not a problem for the 5370A, which still has enough room for both, but it doesn’t work for most other HP devices that don’t have an oversized case. You can see that in the picture below: Unlike the Corcom F2058, the replacement only feeds through the line, neutral and ground that’s fed into it. You’d have to choose one configuration, 120V in my case, and wire a bunch of wires together to drive the transformer correctly. If you do this wrong, the input voltage to the power regulator will either be too low, and it wont work, or too high, and you might blow up the power regulation transistors. It’s not super complicated, but you need to know what you’re doing. 15V Rail Issues After powering the unit back up, it still didn’t work, but thanks to the 4 power rail status LEDs, it was immediately obvious that +15V power rail had issues. A close-by PCB is the reference frequency buffer PCB. It has a “10 MHz present” status LED that didn’t light up either, suggesting an issue with the 10811 OCXO, but I soon figured out that this status LED relies on the presence of the 15V rail. Power Suppy Architecture The 5370A was first released in 1978, decades before HP decided to stop including detailed schematics in their service manuals. Until Keysight, the Company Formerly Known as HP, decides to change its name again, you can download the operating and service manual here. If you need a higher quality scan, you can also purchase the manual for $10 from ArtekManuals2. The diagrams below were copied from the Keysight version. The power supply architecture is straightforward: the line transformer has 5 separate windings, 4 for the main power supply and 1 for the always-on OCXO power supply. A relay is used to disconnect the 4 unregulated DC rails from the power regulators when the front power button is in stand-by position, but the diode rectification bridge and the gigantic smoothing capacitors are located before the relay.3 For each of the 4 main power rails, a discrete linear voltage regulator is built around a power transistor, an LM307AN opamp and a smaller transistor for over-current protection, and a fuse. The 4 regulators share a 10V voltage reference. The opamps and the voltage reference are powered by a simple +16.2V power rail built out of a resistor and a Zener diode. (Click to enlarge) The power regulators for the +5V and -5.2V rails have a current sense resistor of 0.07 Ohm. The sense resistors for the +15V and -15V rails have a value of 0.4 Ohm. When the voltage across these resistors exceeds the 0.7V base-emitter potential of the bipolar transistors across them, the transistors start to conduct and pull down the base-emitter voltage of the power transistor, thus shutting them off. In the red rectanngle of the schematic above, the +15V power transistor is on the right, the current control transistor on the left, and current sense resistor R4 is right next to the +15V label. Using the values of 0.4 Ohm, 0.07 Ohm and 0.7V, we can estimate that the power regulators enter current control (and reduce the output voltage) when the current exceeds 10A for the +5/-5.2V rails and 1.5A for the +15/-15V rails. This more or less matches the value of the fuses, which are rated at 7A and 1.5A respectively. Power loss in this high current linear regulators is signficant and the heat sinks in the back become pretty hot. Some people have installed an external fan too cool it down a bit. Fault Isolation - It’s the Reference Frequency Buffer PCB! I measured a voltage of 8V instead of 15V. I would have prefered if I had measured no voltage at all, because a lower than expected voltage suggests that the power regulator is in current control instead of voltage control mode. In other words: there’s a short somewhere which results in a current that exceeds what’s expected under normal working conditions. Such a short can be located anywhere. But this is where the modular design of the 5370A shines: you can unplug all the PCBs, check the 15V rail, and if it’s fine, add back PCBs until it’s dead again. And, indeed, with all the PCBs removed, the 15V rail worked fine. I first added the CPU related PCBs, then the time acquisition PCBs, and the 15V stayed healthy. But after plugging in the reference frequency buffer PCB, the 15V LED went off and I measured 8V again. Of all the PCBs, this one is the easiest one to understand. The Reference Frequency Buffer Board The reference frequency buffer board has the following functionality: Convert the internally generated 10MHz frequency to emitter-coupled logic (ECL) signaling. The 5370A came either with the OCXO or with a lower performance crystal oscillator. These cheaper units were usually deployed in labs that already had an external reference clock network. Receive an external reference clock of 5 MHz or 10 MHz, multiply by 2 in the case of 5 MHz, and apply a 10 MHz filter. Convert to ECL as well. Select between internal and external clock to create the final reference clock. Send final reference clock out as ECL (time measurement logic), TTL (CPU) and sine wave (reference-out connector on the back panel). During PCB swapping, the front-panel display had remained off when all CPU boards were plugged in. Unlike later HP test equipment like the HP 5334A universal counter, the CPU clock of the 5370A is derived from the 10 MHz clock that comes out of this reference frequency buffer PCB4, so if this board is broken, nothing works. When we zoom down from block diagram to the schematic, we get this: (Click to enlarge) Leaving aside the debug process for a moment, I thought the 5 MHz/10 MHz to 10 MHz circuit was intriguing. I assumed that it worked by creating some second harmonic and filter out the base frequency, and that’s kind of how it works. There are 3 LC tanks with an inductance of 1 uH and a capacitance of 250pF, good for a natural resonance frequency of \(f = \frac{1}{2 \pi \sqrt{ L C }}\) = 10.066 MHz. The first 2 LC tanks are each part of a class C amplifier. The 3rd LC tank is an additional filter. The incoming 5 MHz or 10 MHz signal periodically inserts a bit of energy into the LC tank and nudges it to be in sync with it. This circuit deserves a blog post on its own. Fixing the Internal Reference Clock When you take a closer look at the schematic, there are 2 points that you can take advantage of: The only part on the path from the internal clock input to the various internal outputs that depends on the 15V rail is the ECL to TTL conversion circuit. And that part of the 15V rail is only connected to 3k Ohm resistor R4. Immediately after the connector, 15V first goes through an L/C/R/C circuit. In the process of debugging, I noticed the following: The arrow points to capacitor C17, which looks suspicioulsy black. I found the magic smoke generator. This was the plan off attack: Replace C17 with a new 10uF capacitor Remove resistor R16 to decouple the internal 15V rail from the external one. Disconnect the top side of R4 from the internal 15V and wire it up straight to the connector 15V rail. It’s an ugly bodge, but after these 3 fixes, I had a nice 10MHz ECL clock signal on the output clock test pin. The 5370A was alive and working fine! Fixing the External Reference Clock I usually connect my test equipment to my GT300 frequency standard, so I really wanted to fix that part of the board as well. This took way longer than it could have been… I started by replacing the burnt capacitor with a 10uF electrolytic capacitor and reinstalling R16. That didn’t go well: this time, the resistor went up in smoke. My theory is that, with shorted capacitor C17 removed, there was still another short and now the current path had to go through this resistor. Before burning up, this 10 Ohm resistor measured only 4 Ohms. I then removed the board and created a stand-alone setup to debug the board in isolation. With that burnt up R16 removed again, 15V applied to the internal 15V and a 10 MHz signal at the external input, the full circuit was working fine. I removed capacitor C16, checked it with an LCR tester and the values nicely in spec. Unable to find any real issues, I finally put in a new 10 Ohm resistor, put a new 10uF capacitor for C16 as well, plugged in the board and… now the external clock input was working fine too?! So the board is fixed now and I can use both the internal and external clock, but I still don’t why R16 burnt up after the first capacitor was replaced. Future work The HP 5370A is working very well now. Once I have another Digikey order going out, I want to add 2 capacitors to install 2 tantalum ones instead of the electrolytics that used to repair. I can’t find it back, but on the time-nuts email list, 2 easy modifications were suggested: Drill a hole through the case right above the HP 10811-60111 to have access to the frequency adjust screw. An OCXO is supposed to be immune to external temperature variations, but when you’re measuring picoseconds, a difference in ambient temperature can still have a minor impact. With this hole, you can keep the case closed while calibrating the internal oscillator. Disconnect the “10 MHz present” status LED on the reference clock buffer PCB. Apparently, this circuit creates some frequency spurs that can introduce some additional jitter on the reference clock. If you’re really hard core: Replace the entire CPU system by a modern CPU board More than 10 years ago, the HP5370 Processor Replacement Project reverse engineered the entire embedded software stack, created a PCB based on a Beagle board with new firmware. PCBs are not available anymore, but one could easily have a new one made for much cheaper than what it would have cost back then. Footnotes My HP 8656A RF signal generator has an OCXO as well. But the fan keeps running even when it’s in stand-by mode, and the default fan is very loud too! ↩ Don’t expect to be able to cut-and-paste text from the ArtekManuals scans, because they have some obnoxious rights managment that prevents this. ↩ Each smoothing capacitor has a bleeding resistor in parallel to discharge the capacitors when the power cable is unplugged. But these resistors will leak power even when the unit is switched off. Energy Star regulations clearly weren’t a thing back in 1978. ↩ The CPU runs at 1.25 MHz, the 10 MHz divided by 8. ↩