Display driver for PPI RADAR picture tube - very low frequency sweep - Page 1
Forum section: electronics / projects

author: Dongulus
author_rank: Regular Contributor
timestamp: May 25, 2019, 01:30:11 am
content: Within the last year I have accumulated several picture tubes (CRTs) with the hopes of building a system to power and drive these tubes (Using Eric's design fromTubeTimeas a starting point). One of the tubes I snagged is of the P7 phosphor variety which may have been designed for use as a RADAR display. The box it shipped in has a stamp with the US Army and Navy and a label which is dated in spring/summer of 1945. While it doesn't look to have been used, it's exciting to have a piece of tech built for the war effort and I can't wait to get it displaying.While the effort on the CRT driver is crawling forward and I can't help but put the cart before the horse and think ahead to designing a circuit to drive the X/Y deflect to produce a PPI style RADAR sweep. The angle to sweep will be determined by two oscillators tuned to the sweep rate with one delayed 90 degrees from the other. I am thinking of, potentially variable, sweeps in the range of 1 Hz - 0.1 Hz for a good display. The difficulty with this goal is that I get the feeling that many oscillators designs are completely impractical at frequencies this low.Instead of going down the likely futile path of attempting to tune oscillators for these low frequencies, I have figured that it may be more practical to create 2 oscillators with a very precise frequency difference and produce a low frequency sine wave by using beats. I have attached a proposed block diagram. Basically I'm thinking I can precisely tune two oscillators in the 10's of kHz range with a slight frequency difference corresponding to the sweep frequency, f_rot. Mixing the two frequencies together will result in 2 frequency components at 2f_osc + f_rot & f_rot and a LPF will isolate the f_rot component. The mixing would probably be accomplished using Gilbert cells. The resulting f_rot & f_rot /_90deg will provide the X and Y component of the current sweep angle and multiplying this with a ramp function would produce a line along the sweep angle.I understand that this could be easily accomplished using a microcontroller and a DAC, but that's boring and I would rather do it in analog if possible.I'm curious if anyone has any insights about how to best design a stable, tunable oscillator in the kHz range or perhaps other ways of generating low frequency sine waves.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 25, 2019, 03:42:22 am
content: The very first RADAR displays were plain old oscilloscope style.-----if you wanted to determine the direction of an echo, there was a "slave" selsyn motor driving a compass like dial which showed the direction your antenna was pointing.This was used in conjunction with the CRT display to manually track the echo.All the PPI tubes I have seen used electromagnetic deflection to create the rotating display.The "one time" common (in surplus stores) 5BP1, 5BP7 & VCR97 were for the earlier type of "non-PPI" displays.I have never worked on RADAR personally, but I spent a bit of time "hanging out" with my brother when he was servicing PPI RADAR on ships, back in the late 1950s, (no Occupational Health & Safety in those days!).From memory, when the tube was removed, the very large deflection yoke was revealedThis was mechanically rotated around the tube, in sync with the antenna, giving the rotating display.

author: Dongulus
author_rank: Regular Contributor
timestamp: May 25, 2019, 04:19:50 am
content: I have done enough research to have an idea of the various RADAR display types. I know that in the earliest implementations the display only showed a delayed burst of RF energy along the X axis from which range could be determined (called "A-scope"). I would like to dive in a little deep trace the evolution of RADAR displays and what years they we're adopted. Most likely the PPI display was not in adoption when the tube I own was manufactured. However I would be remissed if I didn't attempt to produce a PPI on this tube because it looks so cool.I have seen a footage of an old RADAR display with the side panels removed to show a motor that rotates a magnetic deflection yoke. It's a clever solution with the technology of the time.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 25, 2019, 04:49:51 am
content: Quote from: Dongulus on May 25, 2019, 04:19:50 amI have a vague memory of seeing a diagram of a "somewhere in between" type display, which produced a circular display, a bit like a lissajous pattern with equal frequency on the X & Y plates.The "blip" was shown as narrow "bulge" of that line whenever the echo occurred.I might still have the book somewhere.

author: CaptDon
author_rank: Super Contributor
timestamp: December 18, 2020, 05:51:25 pm
content: Are you still looking for some simple ways to do the PPI display??Is your surplus tube magnetic deflection (usually a very smalldiameter neck 3/4") or electrostatic deflection like an oscilloscope(neck diameter closer to 1.25")? I have pioneered a simple wayusing sine/cosine data values driving D to A converters whichthen drive deflection amplifiers. I can hook my PPI generatordirectly to any oscilloscope or vector X/Y display. You can alsodrive an eprom from the DAC address lines to create video targetsto drive the CRT grid.

author: james_s
author_rank: Super Contributor
timestamp: December 18, 2020, 07:51:47 pm
content: I have a similar radar tube from that era, it's in the original carton packed in natural straw, pretty cool. I've lit it up once to play around with it and draw on the screen using a magnet held near the neck. Mine is a magnetic deflection tube, originally it would have had a yoke that mechanically rotated around the neck connected to a selsyn to synchronize the rotation to the rotating antenna. Is your tube magnetic or electrostatic deflection?You may already be well aware of this, but be very, very careful handling these old CRTs, especially larger ones. They are made of quite thin glass and lack any sort of implosion protection. There is a scary amount of force pushing in on the walls and if it breaks it will go off like a bomb spraying glass everywhere.

author: Dongulus
author_rank: Regular Contributor
timestamp: December 18, 2020, 09:02:34 pm
content: I haven't touched this project in a while as I unfortunately haven't had the bandwidth.I did design a circuit and tested it in Spice a while ago, though never tried it out on a breadboard. The CRT tubes I'm trying to drive are the type with electrostatic deflection. I wanted to do all of the circuitry in analog since I have a digital design background and wanted to get some practice in the analog domain.Essentially what is needed to produce a PPI signal is to create the following signals.X_def = f_sweep(t) * cos(wT*t)Y_def = f_sweep(t) * sin(wT*t)The challenge was to create the two separate periodic signals at such a low frequency and with a 90 degree phase offset between them. The method I went with was to produce triangle waves then convert them to sine waves. The picture attached is the circuit I designed. My sweep generator circuit has a signal which is high when the sweep is active. The falling edge of that signal triggers a 555 timer to produce an adjustable length pulse. That pulse is then used to charge/discharge a high value capacitor. Integrating pulses of current produces a rising/falling linear function which is stable for the relatively lengthy periods of time needed for the PPI sweep. The cap charges until it reaches a positive threshold set by a comparator, the output polarity of the comparator flips and the cap discharges with each pulse until the negative threshold is reached. The circuit on the right converts linear functions to a sine-like shape.A second sinusoid generator would be a duplicate of this one with one difference. When the "cosine" output generator reaches the comparator maximum threshold the "sine" generator's cap will be discharged through a MOSFET. Since the relationship between a sine and cosine is a zero crossing for one at the maximum of the other, this will ensure the correct phase relationship between the two generators.Finally the X/Y signals are just the sweep generator output multiplied by the "cosine" and "sine" signals. I plan to perform the multiplication using a gilbert cell. I actually have some LM1496 balanced mod/demod chips to try this out with.

author: james_s
author_rank: Super Contributor
timestamp: December 19, 2020, 02:17:55 am
content: Maybe you could find a service manual for an actual radar display? There were a lot of different models produced, I'd have thought a schematic for one of them would be floating around somewhere. I'd be curious to see how a real one works too.

author: CaptDon
author_rank: Super Contributor
timestamp: December 19, 2020, 02:31:03 am
content: Very nice that you are still playing with the project. I had a 5CP7 as my displaybut it was an old unrefined first generation CRT and it developed a weird problem.One of the gun mount welds broke loose and a 'centered' trace was way off center!!I am now using a 5ADP7 which works well although I wish it was an aluminized 'A'version with more light output. It is the same length and pinout as the 5CP7 so itwas plug and play. Many of the old tubes used the same base pinout!! Mine iscurrently in a home built PPI signal strength display and I have a 64k pixel memoryto retain the display until the next antenna rotation update. The unit input comesfrom a 3 element VHF yagi rotating on slip rings. There are 512 'spokes' on thePPI wheel and each spoke has 128 pixels. The encoder on the shaft is 1024 butI divide by 2 to get 512 spoke triggers and there is a true 0 degree index pulseto reset my count chain and keep them true. It is always a zero-degree-up display.Cheers mates!!!

author: james_s
author_rank: Super Contributor
timestamp: December 19, 2020, 02:33:55 am
content: Many "modern" CRTs use the same base pinout too. I have swapped random TV tubes into 80s arcade monitors many times. These days it has become much harder to find donor tubes though as the TVs themselves have become vintage.

author: CaptDon
author_rank: Super Contributor
timestamp: December 19, 2020, 02:42:14 am
content: 99% of the older PPI units used a rotating yoke and a P7 phosphor tube.I even have a pleasure boat radar from Decca with this exact arrangement.All of the newer stuff has a raster scan display with 'address conversion' toget a linear rotational input converted to proper formatting for a conventionalraster scan video display, some even obeying the original NTSC format!!

author: james_s
author_rank: Super Contributor
timestamp: December 30, 2020, 01:20:44 am
content: Quote from: CaptDon on December 19, 2020, 02:42:14 amThat's neat, if you ever have a reason to open the thing up it would be cool to see a picture of the rotating yoke mechanism in that system.

author: CaptDon
author_rank: Super Contributor
timestamp: December 31, 2020, 02:54:50 pm
content: The yoke doesn't have a lot of wire or very much inductance. I wantedto slow the sweep rate (the time taken to go from screen center to screenedge) so I could use the display for acoustic echo work but with the smallinductance the current rises to a destructive level when the sweep is slowed.In fact even on the 20 mile range the yoke drivers get hot. The brushesmake a screeching sound on the slip rings, probably needs a bit of contactlube. It has some oddball number of steps per revolution also, somethinglike 424 or 454, I forget. Probably has to do with the gear ratio of the antennadrive motor, like 424:1 to get the torque to spin the antenna. My home builtsystem is 810:1 on the drive motor but I use a rotary 1024 step encoder onthe antenna shaft and divide by 2 to get 512 azimuth pulses per revolutionand an index pulse at 0 degrees to prevent counter pulse slipping from noise.Most military/commercial radars use 4096 azimuth pulses per revolution butI don't need that much resolution, not when the antenna beamwidth is 3 degrees!!!