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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 25th Mar 2006, 3:16 pm   #1
jim_beacon
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Default RADAR scan conversion

continuing from a thread in the TV section:

Quote:
Originally Posted by jim_beacon
We had some valve standards convertsers at work, for turning polar radar displays into raster scan for the addition of secondary radar labels. They used a special converter tube, which was effectively a camera and picture tube in one envelope. The writing beam wrote onto a charged plate, which was then read by a second electron beam.

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That's cheating! It's just a souped up version of pointing a camera at a monitor. Doing polar to rectangular scan conversion in electronics is a lot harder than just changing the number of lines.
The later scan converters were moved from the control centre (previously, radar video was sent over valve based microwavelinks), to the radar site.

Two pieces of equipment were installed, a plot extractor and a plot combiner. The plot extractor turns the normal picture into a number of cells, known by range and bearing, and marks whether they have a return in them or not - note, the cells are all equal size in the range direction, but will get progressivly bigger in the arc as you move away from the radar head. As you know were each cell is, when you read from it, you get its angular co=ordinates froma look up table, and a simple "1" or "0" shows whether a target is present (in practice, an average of a group of cells is taken to declare a target - a single cell is probably just noise. This gives this type of plot extractor its name "sliding window plot extractor"). This means that your baseband video, with several MHz of bandwidth (normally around 5 to 10MHz), is replaced by a data stream that is easily transported by a 2400 Baud modem (there is enough spare bandwidth on the modem to include control and monitoring data).

The plot combiner takes the extracted plot from the extractor, and adds the data from the secondary radar - this alters the target data to give primary only, secondary only or combined targets (the databit becomes three bits), and attachs the 4 bit octal codes for identification and altitude to the target. The whole lot still easily goes down a 2400 Baud modem.....

The early plot extractors were mostly hard wired logic, (ours were made by Stansaab), with lots of PROMs for look-up data the dual plot extractor and dual combiner was around 6 x 6ft, 19" cabinets. The next generation were built in software, using mini-computers - there are still a lot of Ferranti Argus 700 computers out there!

The latest generation are micro-processor based, and the plot extractor / combiner is now part of the radar itself - the software interface allows for modifaction of tables and maps (used to blank out permanent echoes and the like) with ease.

Jim.
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Old 16th May 2006, 3:01 pm   #2
Kat Manton
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Default Re: RADAR scan conversion

Hi,

Maybe drifting off-topic but I've been pondering how to replicate a polar display on a 'scope. I think I've figured I need sine and cosine generators fed to X and Y plates via a circuit which ramps the amplitude of each with a sawtooth (to get a scan originating in the centre, radiating out to a rotating point on a circle, and feed the signal to be displayed into the Z input.

This was one of the ideas I had knocking around for an entertaining way of displaying signal strength for radio DF, though in this case the signal input would be used to alter the scan amplitude draw a pretty pattern

Actually, another point - what exactly are the tubes used in polar displays?

Regards, Kat
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Old 18th May 2006, 6:09 pm   #3
JHGibson
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Default Re: RADAR scan conversion

Hi Kat,
You can find the kind of drive circuit you need by examining the CRT circuits of the conversions of surplus radar indicators to TV sets which were published in PW in the late 40's.

They used rather ordinary high mutual conduction sharp cut off RF pentodes which could sustain a plate voltage of some 250 volts, RC coupled to the deflection plates. Valves often used were the EF50 and SP61, however sockets for these are rather hard to find. 6AC7 and 6SH7 may be more suitable as they are octals.

If you can locate a library with a full set of "The Massachusetts Institute of Technology Radiation Laboratory Series", you will find every circuit you need and then some. Including deflection circuits and the sine/cosine generators.All circuits use radio valves.
John.
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