Likely impedence of this scenario?

------------------------

** You are joking ??

** A few pF of capactance - eg:

Fromm plates X1 to X2 = 6pF

** Doubt it.

Gotta know the frequencies involved first.

.... Phil

Resistive: infinite. Capacitive: "x" pF.

--
 Thanks, 
    - Win

Thanks, Win.

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Phil gave a capacitance value, 6pF. That's a reasonable typical value. The big deal in the old days, but not too old, when high-resolution CRT video monitors were all that rage, like my $2k Sony monitor, was: maximum current through video output transistors, and minimum output capacitance: High Ic for low-value collector resistors, with low Ccb, to get fast response. Alibaba has parts from that era, that I use to make fast high-voltage amplifiers, see AMP-70A: Not an easy combination. E.g., the KSC2682 was rated 180V and 8 watts, but Cob was only 3.2pF. KSC3503: 300V, 7W, 2.6pF and FOM = Pd/Cob = 2.46. There were npn/pnp pairs, for EF-output stages, able to handle even higher plate capacitances.

If you want all the info in a firehose:

--
 Thanks, 
    - Win

On a sunny day (Mon, 7 Oct 2019 00:02:09 -0000 (UTC)) it happened Cursitor Doom wrote in :

Depends on 'speed'.

Basically it is a capacitor (few pF), but some scopes had a construction of deflection plates that looked more like a transmission line. Look up the CRT and get the datasheet. At 300 MHz everything matters including the leads to the defection plates, some Teks were fine calibrated by bending those a bit.

So for HF it is a complex impedance.

For DC AFAIK no current flows UNLESS you put on such a high voltage that the electron beam hits the plates,

1.2 M is a much too high value look up time constant 100 pF at 1 M is about 10e-10 * 10e6 = 10e-4 or abour 10 kHz........

Not even audio... :-)

50 Ohm and a BFY90 is better.

BFY90 ?

--
 Thanks, 
    - Win

used to be the VHF NPN in Europe for quality applications. Including metal case with shield connection.

regards, Gerhard

Yes, I should have stated I'm carrying out these tests at the very lowest sweep speeds, so slow it's effectively DC. Sorry for that omission.

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Yes, I know, how does that work for wideband HV video?

--
 Thanks, 
    - Win

infinite z then :) Sort of.

NT

Well, quite.

I think the 1.2Meg resistor should take care of it; just really only need something to very lightly load the outputs to *something* real at least.

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On a sunny day (Wed, 9 Oct 2019 23:30:33 -0000 (UTC)) it happened Cursitor Doom wrote in :

remember that for a scope horizontal scan the waveform is a sawtooth, the flyback part of that sawtooth is usually very fast. Then again it is the deflection plates capacitance + the capacitance of your driver stage that matters.

You also need trace suppression (cut off CRT beam) during that flyback, else you get a curved extra trace.

For very slow things I would not bother with a CRT, just use a micro and some LCD..

China now sells scopes with a micro for < 40$ I think...

But for playing to get understanding CRTs are cool, you could use magnetic deflection too :-) My first scope was a TV CRT :-) HV with a car ignition coil on the output of an audio amp, magnetic deflection...

Personal accelerator :-)

That's a good, informative webpage, Jan. You, Miles, and a few others inspired me to "grow" my own like minded website. A while ago a single channel JYE DSO was purchased by me. Peers advised me to purchase it directly from JYE. It seems that in the interim since my purchase, JYE evolved its entry level DSO into

2-Channels:

Thank you, 73,

--
Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
She set out one day In a relative way And returned on the previous night.

80 is a lotta bucks for claimed 200kHz, real 80kHz. You could get a vintage real 2 channel scope for that.

NT

snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@googlegroups.com:

Better off getting a USB scope and using your laptop or PC for the display.

It does have an X/Y mode, though. Maybe it could be used to make a handheld curve tracer.

It turns out that they still sell this /single channel/ DSO for $39:

Thank you, 73,

--
Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
She set out one day In a relative way And returned on the previous night.

I had looked at that. You need at least two inputs and X/Y mode for a curve tracer.

There are several variants of the DSO138 things out now. They have their uses but bandwidth is firmly in the court jester ballpark. There's one claiming 20MHz that can manage about 2MHz iirc.

NT