Analog Angle Blog

An ‘Analog Mystery’ to Investigate

We're so used to the line-by-line, frame-by-frame raster scan on our TV — whether on a now-obsolete CRT or up-to-date flat screen — that we may forget that displays were not always driven this way. The traditional analog oscilloscope was not a raster-scan device; the amplified signal of interest controlled the vertical-axis deflection, while a time base or second signal of interest drove the horizontal axis. Although not raster scan, it was a straightforward X-Y drive setup. So screen drive seems pretty straightforward, and you can go with raster scan or X-Y deflection.

That's why I was puzzled after watching an old WWII movie, with a destroyer in pursuit of a submarine. The destroyer had radar (yes, it was in use towards the end of the war, on land, ships, and even some aircraft), and the display was a round CRT with the classic “sweep” of a bright line emanating from the screen center, rotating around 360⁰ in sync with the antenna (Figure 1). Any reflected signal blip was seen as a bright spot due to the CRT's phosphor persistence as the sweep line went around — all very intuitive.

Figure 1

A typical CRT-imaged radar sweep used as early as the end of WWII

A typical CRT-imaged radar sweep used as early as the end of WWII

A little later, though, I started to think about this screen display, and I wondered: What kind of analog-drive signal waveforms were used to generate that circular sweep (and they had to be analog, of course)? Were they related to the classic Lissajous figures used “back in the day” for frequency calibration? Were they sine/cosine signals, and if so, how were they used?

Also, did the CRT of the day use magnetic or electrostatic deflection, and how were the deflection coils or plates arranged? How was the sweep of the screen synchronized to the outside rotating antenna? (Some sort of synchro/resolver, I assume.) Finally, when we see this kind of traditional image displayed on a modern system, such as the TV weather folks like to show us, I assume it is actually a rasterized image of a circular-antenna or phased-array antenna sweep — but is it?

I call these questions “analog mysteries,” but they are really questions with knowable and discoverable answers, once we do the historical research. Unlike a real mystery, which may not have an answer we can find or understand (such as “what is gravity?”), this one requires delving into something that was real and worked but used a technique that now is obsolete.

It's sort of like the story of those huge columns of the classic Greek and Roman temples. Researchers are fairly confident they know how the column sections were fabricated and how their precise placement was plotted, but no one knows how they moved, lifted, and carefully positioned column sections, each weighing at least 10 tons and more, on top of each other. It was a combination of human and animal muscle power plus some sort of “machinery,” but that's about all that is known.

When I have time, I'll dig into the design of early radars and hopefully find some clear answers. Those folks were pretty clever at accomplishing some amazing things with what they had available, although by our standards it was all so very crude.

Are there any similar “How did they do that?” questions you have ever wondered about? One of my other ones is understanding how trackers of space vehicles, ranging from orbiting satellites to deep-space probes, worked propagation-related return delays and relativistic effects into their track-analysis calculations.

Have you found answers to your own analog mysteries?

23 comments on “An ‘Analog Mystery’ to Investigate

  1. Victor Lorenzo
    August 19, 2014

    Many years ago I had access to maintenance documentaion for one now obsolete ultrasound equipment that used a combination of rasterized and not rasterized imaging techniques. During the scanning (sensor moving in one direction) they updated part of the display using a modified polar representation and, at some intervals when the sensor was stopping and reversing direction, they used rasterized display control for updating texts, markers and alike.

    It was very precise and clever.


  2. Bill_Jaffa
    August 19, 2014

    One of the other mysteries on my list is to understand how Internet message routing actually works. For example, if I send a text message to Joe, does it get-broadcast to all zillion users out there, so Joe's phone gets it? Of course not. So does the sender's node look around the whole Internet, to see where Joe's phone was last known to be, and it gets directed there? Doesn't make sense, either. Does Joe's phone periodically identify itself to the system, saying “here I am”? I assume that's done—but then how and where does the sender find this locator information?

    So: how does my text to Joe go to where he is, without clogging up the system?

  3. RedDerek
    August 20, 2014

    After spending years as a display applications engineer and my time working as missile countermeasure developer (years ago folks), I believe I could enlighten a bit on the method a radar screen scans.

    1. Having a phosphor display to provide the “blip” is a control of the persist, like the old analog scopes.

    2. There has to be some type of time lag to deal with images far to near.

    3. The distance out is based on how far out the polar radius where the “blip” is placed and the intensity of the return, calibrated, is the brightness.

    4. Otherwise, the radial movement is most likely related to the synchro of the pointing of the receiver.

    So, in simple terms, it is essentially an X-Y plotter, but looks at everything in radians. Only of my first challenges to understand when working with missle tracking systems.

  4. Bill_Jaffa
    August 20, 2014

    Thanks, but I still need to better understand how the rotating sweep signals were generated at the CRT and what sort of deflection-hardware setup (plates, coils, physical placement) was used around the CRT itself.

  5. green_is_now
    August 20, 2014

    Your assumed “of course” answer is more correct than wrong…wastage!

    Just because it makes no sense does not mean it is not happening!

    hard pill to swallow but true.

    not just in context, universal truth… unfortunatly…a world run on brain stems does not have to make sense…IAA

  6. green_is_now
    August 20, 2014

    Sorry, in response to the internet assumptions of traffic flow…

    the facts are some where in the middle/muddle

    1( congestion is not measured in real time, just reacted to.

    2) we must broadcast nx more // paths because of this fact.

    aka no analogistics

  7. green_is_now
    August 20, 2014

    must think in “pickardisms”

    that bothers me on many levels…

    August 21, 2014

    “must think in “pickardisms””

    Well that sounds a bit creepy to me but lets give it a try and see. 

  9. Radiotron
    August 21, 2014

    Bill, For the definitive reference on all of the early radar systems, look for the Radiation Laboratory Series of books. It is the summary of the work done at MIT's Radiation Laboratory during World War II. It is disappearing from library shelves due to age, theft, and disuse. All 28 volumes can, however, be obtained on CD's from Artech House and others. Most applicable to this topic is volume 22: “Cathode Ray Tube Displays.” It covers all of the early scan types including the Plan Position Indicator (PPI) using the radial sweep scan that you are interested in. While both electrostatic as well as electromagnetic deflection was used, one of the more interesting and mechanically complex implementations rotated a magnetic deflection coil around the slender neck of a CRT. The coil was kept in sync with the scanning antenna via a synchro system.

  10. chirshadblog
    August 21, 2014

    Any other alternative you can suggest ? 

  11. chirshadblog
    August 21, 2014

    Yes indeed there are so many MAKES NO SENSE things are happening in big time. Also we have to admit that for us to understand IT MAKES NO SENSE we have to use it and see. So the usage goes higher. Also there are people who take most of it and it works for them. 

  12. Navelpluis
    August 22, 2014

    During the war the US military had a devision called 'TYCOM' to scrape the lanscape of Gernany – not only to searching for good wine, art and other valuables, but mainly to search for technology. Tonns of goods were shipped back to the US and some devices -even today- have a “What the heck where they doing” question mark on them. Ya want an example?  Have a look at this place:

    Amazing, huh?

    Loot at these internet pages carefully, you even may found a V2 analog control system, running with small valves, all AC coupled integrators and differentiators and mixers 😉

    I wonder whenever the first Apollo documents will be released how much of this technology still can be recognized. Very interesting!

  13. zeeglen
    August 24, 2014

    IIRC on one system the CRT sweep was produced by 3 deflection coils placed 120 degrees apart around the CRT neck.  The sawtooth ramp was applied to an inductor attached to the rotating axis of the antenna drive and magnetically coupled to 3 stationary coils.  The level of ramp induced in the stationary coils at the antenna drove the 3 stationary CRT coils resulting in a corresponding sweep from center to circumference equivalent to the additive flux levels of the 3 coils.

    The range rings were produced by starting an oscillator at the “main bang” at center screen.  The resulting sine wave drove a Schmitt trigger whose edges were differentiated into a narrow pulse to paint the rings at each sweep.  Long-persistance screen phosphor continued to emit light long after the sweep had passed, and refreshed on the next antenna rotation.

    Most Hollywood radar screen representations are faked, and it can be very obvious to those who are/were radar operators.

  14. eafpres
    August 26, 2014

    Hi Bill–your thought of realization that a lot of things work and when we begin to think about them we realize we have no idea how.

    Over the last few years, the idea of using light for data tranmission (not point to point, but as an alternative to WiFi).  Due to most people not knowing really how WiFi works, they assume that LiFi will work.

    The problem is that we have RF circuits that can slice up the RF Spectrum into channels, and send many signals at the same time over channels.  For LiFi, it is not so easy–there isn't a great way to channelize light, even with a “digital” source like an LED.

  15. geek
    August 28, 2014

    “Over the last few years, the idea of using light for data tranmission (not point to point, but as an alternative to WiFi).  Due to most people not knowing really how WiFi works, they assume that LiFi will work.”

    @eafpres1: From what I think, the light transmission for data has to be based on a line of sight model. The two devices (sender and receiver) have to be directly in front of each other. If that's the case then I don't think it's a very strong idea for internet transmission or even for data transmission in general. What do you think?

  16. geek
    August 28, 2014

    “Most Hollywood radar screen representations are faked, and it can be very obvious to those who are/were radar operators.”

    @zeeglen: Almost 99% of the people watching these movies would probably not be able to tell if these representations are fake. It's only people who're into this field can tell. I don't think the film producers would really mind the objection of these remaining 1%.

  17. eafpres
    August 29, 2014

    @tzubair–You are on the right track.  Some proposed schemes are for fixed location like table in a restaurant, or in an airline seat.  These can be somewhat controlled as line of sight.

    Another application suggested to me when I wrote about this in the past was for certain areas, say a surgical room for heat pacemaker implants, perhaps they don't want RF in there, but light would be OK.

    The problems of non-line of sight would likely be very bad in many practical instances.

  18. geek
    August 30, 2014

    “The problems of non-line of sight would likely be very bad in many practical instances.”

    @eafpres1: Exactly. I think it greatly limits the number of applications possible with this technology. Only short-distance data transfers are possible when it comes to using this technology. Otherwise, I doubt any other applications make use of it.

  19. Davidled
    September 3, 2014

    In the end of War World II, many German scientist and engineer immigrated to USA. They contribute to upgrade and develop all military devices. Still today, I believe that they are a big group of physics, chemistry, and engineering society in USA and other country.

  20. Navelpluis
    September 3, 2014

    DaeJ, You are absolutely right.

    This aspect sometimes hinders us in historical research now. Most of these folks already died of age, that's clear. But an aspect not many people are aware of is that lots of them told a 'nicer story' to the US government officials back then. This was simply to try to escape from prison.

    So now we find documents with names, claiming that they have 'worked' on certain projects, but heck, they didn't. We had to conclude that some German engineers -even after the war- were very smart and capable to mislead the US government 😉

  21. Davidled
    September 5, 2014

    There are two type radar system: passive radar and active radar system. I am wondering what type radar system is referred to. I image that object being displayed in the screen might have its attribute obtained through signal, for example, pulse generator. Attribute might be speed, location and etc. Display screen might use digital analog converter (DAC) connected with receiver through antenna.

  22. Bill_Jaffa
    September 7, 2014

    The system I am looking to figure out was all analog, all tubes, no DAC! It takes a whole new (actually, old) way fo thinking to figure out how they did it–and they did do it.

  23. Flagstaff Rich
    November 12, 2014

    Why stop with unknown German RADAR systems?  My father trained on SRC-584 gun-laying RADAR and later became a member of the 5250th Technical Intelligence group in the Pacific.  Before the war ended they were very interested in the Japanese Mark 5 Model B Doppler RADAR system used to detect incoming B-29 raids.

    Shortly after the end of the War the 5250th was directed to find the six known Mark 5 installations.  This they did, presumably from RDF data and aerial photos.  They asked Washington for instructions and the immediate reply was to destroy them.  By the time the order countermanding the first one was received those six had been chopped up, burned, and dumped in the ocean.  (The Colonel in charge was regular Army and he didn't mess around.)

    A seventh installation had been suspected and my father found it in a swamp.  From his description it must have been near present Misawa AFB.  The Corps of Engineers brought in a crane to remove it but it got stuck in the swamp.  When he last saw it a second, larger crane was trying to extract the first crane.  He never learned if the RADAR had been successfully extracted for return to the US per revised orders.

    Now the Mark 5 Model B seems to be completely unknown.  Even the experts at the Japanese Imperial War Museum assure me there was nothing beyond the Mark 4 RADAR system.  It was apparently a victim of the US push to “disappear” any technology sufficiently advanced that we did not want to share with our allies, the Russians.  The I-400 class submarines (submersible Aircraft Carriers) met this fate but in recent years they have been located where they were scuttled. One or possibly even two A-bomb programs also met that fate and Dad participated in collateral damage from them – destruction of the cyclotron at the U. of Tokyo.  That was one of his chief regrets from his role in WW2 but orders are orders.

    If you think the Japanese were incapable of building advanced RADAR systems, just look up the earliest reports on Magnetron development in the early 1920's.  That seminal work was done at the U. of Tokyo and the U. of Sendai.

    Back to the SCR-584, consider this.  There was one failure mode that painted the CRT with a near-perfect equilateral triangle.  I have no idea how that worked.

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