The CRT waveform analyzer project

[sahko123]

I'm working on a CRT based spectrum analyzer made from an old CRT monitor. No matter how long I searched I can't find a way to control the CRT tube to produce a single colour beam onto the CRT as I'm not sure what voltages to apply where. If anyone out there has any info on CRT tubes and how to control them the help would be much apreciated.

[singapol]

Try searching for info here:

http://diagramas.diagramasde.com/otros/Oscilloscopes_FifthEdition_Ian_Hickman.pdf

http://www.tubebooks.org/

[GK]

https://www.eevblog.com/forum/projects/home-brew-analog-computer-system/msg909256/#msg909256

[sahko123]

Thanks! This help is much appreciated and the documents provided will be much help!

[Ian.M]

The classic way to do this for any application that does *NOT* require an X:Y plot was to keep the monitor intact and turn it on its side so the frame scan could be used for the frequency scan.   One could tap into the timebases for the ramp waveforms, but it was usually easier to simply build Miller integrator ramp generators, and use a pair of astables for line and frame sync pulse generation, that also reset the integrators.  The frame ramp provides the control voltage for the L.O. VCO to scan the spectrum, and the rectified and low-pass filtered detector output goes to a comparator, where its compared with the line ramp, and the comparator output drives a resistor network to generate the actual video signal.  If you want a line graph rather than a bar graph, capacitively couple it!   Its worth adding a couple of cursors - more comparators with a pot on one input, driving a different RGB pin to the spectrum signal (or a different gray scale level via the resistor network), and for the frequency cursor, you need a couple of flipflops as a 2 bit shift register after the comparator so it's clocked through one scan line at a time by the Hsync pulse, and XOR the Q outputs together to drive the RGB pin.  Then all you need is an accurate readout of the pot wiper voltages to be able to take measurements off the screen.

[GK]

Oooh. A toy audio analyser I did a few years ago. Uses a bank of discrete filters multiplexed on each horizontal scan rather than a synchronized ramp generator for sweeping the LO as in a superhetrodyne swept-frequency analyser, but the basic display concept is the same. Also the screen is not rotated 90 degrees, but the horizontal scan rate might be quite fine for an RF spectrum analyser if the minimum IF (resolution) bandwidth is wide enough.

Some simple logic implements the graticule. The vertical sensitivity is 3dB per division, the logarithmic response is achieved by using the exponential charge on a capacitor synced/reset at the vertical scan rate as the reference potential for the comparator. But you wouldn't do that for a swept-frequency analyser - you'd use a linear ramp and use a logarithmic IF detector (not a crude diode detector) such as an AD8307 (or an AD8306 if your fixed IF frequency is 5MHz or more). Before those AD parts old FM IF subsystems like the NE605 were useful as log detectors using the RSSI detector output. That part has evolved to the current SA615, which actually has quite good RSSI log conformance and dynamic range, but I'm not sure it'd be worthwhile going that route save for pure experimentation.

Schematics attached at bottom of post.

[sahko123]

There's the thing I don't want to make it digital I want to use mainly analog components. Though I am planning a modular design to be able to implement a storage function with play back. So I could display the waveform or the spectrum.
 What would be the benefits of electrostatic over electromagnetic deflection?

[GK]

Well if you want true linear X-Y manipulation of the beam position with a CRT having electromagnetic deflection you need big and beefy (current sensing-feedback) power amplifiers to drive the deflection coils. With this approach there will be severe frequency response limitations but for the given application they are manageable.

A pair of (high-voltage) deflection plate driver amplifiers for a CRT with electrostatic deflection is much less bother and less limited but you won't find a CRT with electrostatic deflection that is as large as a typical (obsolete) television set or computer monitor screen. You'll be restricted to an old analogue oscilloscope CRT.
 

[sahko123]

Yeah there's the thing I'm only 15 and an oscilloscope CRT would be hard to acquire for me especially here in Ireland. But high capable power supplies are a favourite of mine to design but if I consider digital capabilities the bandwidth wouldn't be as big a problem but the design would be more difficult to create if I do. I also want to be able to view the wave form itself as well as a spectrum

[CJay]

Yeah there's the thing I'm only 15 and an oscilloscope CRT would be hard to acquire for me especially here in Ireland. But high capable power supplies are a favourite of mine to design but if I consider digital capabilities the bandwidth wouldn't be as big a problem but the design would be more difficult to create if I do. I also want to be able to view the wave form itself as well as a spectrum

Stick with a ready made monitor if you can, let it deal with the sweep, grid, focus and EHT requirements because they're nasty to get right, you'll end up chasing around all sorts of issues, the drive for deflection coils for instance is particularly unpleasant as it's rarely if ever linear and almost always a compromise.

CRT voltages start at 6.3V (usually) and work their way up to and over 35KV which has to be well regulated or you get weird pulsing effects on screen.

Frankly, they're a pain in the butt to drive.

If you *really* can't find a cheap 'scope on eBay to gut or even just use as is then you're not trying hard enough.

If you're absolutely desperate to make a colour CRT work as a display then I would suggest finding a computer monitor or television service manual to study.

[sahko123]

The problem is if I had the funds to buy an oscilloscope off of eBay I would have a while ago and this thread wouldn't have been needed since I could have used the scope in x,y mode to do what I want to do. But I don't have the financial backing of my parents to by me a scope since they think its not needed for my type of projects.

[CJay]

The problem is if I had the funds to buy an oscilloscope off of eBay I would have a while ago and this thread wouldn't have been needed since I could have used the scope in x,y mode to do what I want to do. But I don't have the financial backing of my parents to by me a scope since they think its not needed for my type of projects.

Patience, I've seen them go through for £20 quite a few times, you'll spend a lot more than that trying to get your CRT working.

If you avoid the 'names' like HP, Tektronix, LeCroy, Fluke etc. you can pick up some real bargains

[sahko123]

Well if I find the right scope for a low price I could try and buy it but it would be difficult as alot of them are auctions. Once I found a nice goldstar scope for not more then 19£ so I bid 24£ but within an hour it was 90£ and rising and I lost the auction. I have been looking for a scope that includes a probe or two which is also not common but they are cheap so it won't be hard to buy a cheap no name probe.

[GK]

Yeah there's the thing I'm only 15 and an oscilloscope CRT would be hard to acquire for me especially here in Ireland. But high capable power supplies are a favourite of mine to design but if I consider digital capabilities the bandwidth wouldn't be as big a problem but the design would be more difficult to create if I do. I also want to be able to view the wave form itself as well as a spectrum

Oh. Unfortunately you can't make an analogue oscilloscope for general waveform viewing with a television CRT that is useful for frequencies beyond kilohertz. And to build such a contraption takes considerable effort and requires parts that cost $; follow the link I posted in my first post.
Probably not what you want to hear, but I think you'd best heed the advice you've been given.