General > General Technical Chat

Why weren't electrostatic deflection CRT's more popular for TVs and monitors?

<< < (3/5) > >>

TimFox:

--- Quote from: ELS122 on August 26, 2023, 09:49:26 pm ---
--- Quote from: TimFox on August 26, 2023, 09:28:01 pm ---Magnetic deflection is used in other applications, such as electron microscopes and particle accelerators.
I believe there is no real frequency limit, so long as you can drive the inductance of the coil.
Of course, RS-170 (US monochrome TV) uses 60 Hz vertical and 15,750 Hz horizontal for 525 line interlaced.
1080p HDTV with color CRTs needs 64.8 kHz horizontal for 60 Hz vertical.
Since rasters are produced with two fixed-frequency periodic waveforms, "bandwidth" applies to the "video" signal, modulating the beam intensity, done with a grid in the usual vacuum-tube manner.

--- End quote ---

Well at some point the inductance will require you to drive it at such high voltages that it isn't practical anymore.

The video signal is actually driven into the cathode, a common grid setup.
Screen grid voltage set for baseline brightness and matching, then the cathode gets fed the video signal, and the cathode voltage is offset for brightness adjustment.

--- End quote ---

You can design a lower inductance deflection coil, requiring more current for the same B field, but keeping the voltage down.
Yet another quantitative question.
In very high energy electron accelerators, magnetic deflection is always used, since electrostatic deflection would require excessive voltage.
Yes, normally the video signal is connected to the cathode, but that modulates the grid-cathode voltage against a fixed grid-1 voltage.
The circuit shown in my RCA RC-24 has the brightness control driven by the supply of the grid-2 of the horizontal-output tube, and then coupled to the cathode to set the DC level there.
Better TV systems included a proper "DC restorer" (misnomer) circuit instead of just C-R coupling, to keep the black level constant against changes in the brightness range of the video.

ELS122:

--- Quote from: TimFox on August 26, 2023, 10:10:37 pm ---
--- Quote from: ELS122 on August 26, 2023, 09:49:26 pm ---
--- Quote from: TimFox on August 26, 2023, 09:28:01 pm ---Magnetic deflection is used in other applications, such as electron microscopes and particle accelerators.
I believe there is no real frequency limit, so long as you can drive the inductance of the coil.
Of course, RS-170 (US monochrome TV) uses 60 Hz vertical and 15,750 Hz horizontal for 525 line interlaced.
1080p HDTV with color CRTs needs 64.8 kHz horizontal for 60 Hz vertical.
Since rasters are produced with two fixed-frequency periodic waveforms, "bandwidth" applies to the "video" signal, modulating the beam intensity, done with a grid in the usual vacuum-tube manner.

--- End quote ---

Well at some point the inductance will require you to drive it at such high voltages that it isn't practical anymore.

The video signal is actually driven into the cathode, a common grid setup.
Screen grid voltage set for baseline brightness and matching, then the cathode gets fed the video signal, and the cathode voltage is offset for brightness adjustment.

--- End quote ---
Yes, normally the video signal is connected to the cathode, but that modulates the grid-cathode voltage against a fixed grid-1 voltage.

--- End quote ---
But common grid will have different characteristics than common cathode because the cathode-screen voltage varies.

TimFox:
Yes.  The effect depends on the g2:g1 mu factor in a tetrode.
Also, the polarity of cathode drive vs. grid drive is opposite in current changing.

TimFox:
Back to the question about required voltage to drive magnetic deflection at high rates.
Magnetic deflection "yokes" are multi-turn coils with a shape designed for the desired geometry of the B field induced therein.
The number of turns N (at least one) is the choice of the designer.
Now, for a given B field, the current I is proportional to 1/N, since B = aN x I , where a is some constant.
For a given waveform, the derivative dI/dt is proportional to I, and therefore also proportional to 1/N.
However, the inductance L for a given geometry is proportional to N2.
Therefore, the required drive voltage V = L dI/dt is proportional to N2 x (1/N) = N.
We see that a given B field, under these constraints, requires a voltage drive proportional to the number of turns, and we can trade off drive voltage for drive current by changing the number of turns within a given winding geometry.

David Hess:
I respectively mostly disagree with every reason given above.  The reason is because of the voltage difference between the electron gun(s) and electrostatic deflection plates.

In an oscilloscope CRT, the electron gun operates at about -2000 volts so that the deflection plates can operate at 0 volts and the deflection amplifiers can be direct coupled.  This requires the z-axis amplifier to be coupled into the grid with a DC restorer circuit because it has to drive the grid at a potential of about -2000 volts, which would be unacceptable with a television CRT, although it could be done.

Alternatively if the oscilloscope CRT operated the electron gun at zero volts, which would considerably simply the z-axis circuits, then the deflection plates would be at 2000 volts which is completely unacceptable, although some very early designs did this.

With a television CRT, magnetic deflection provides acceptable performance and it allows the electron guns to operate at zero volts for easy coupling to the red, green, and blue drivers without DC restoring circuits.  I suspect magnetic deflection was more effective in this case simply because the pre-deflection acceleration voltage is lower than with an oscilloscope CRT, however I did not find any service manuals showing exactly what was going on and the online references I found did not discuss the most common type of CRT.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod