Type 519 -- Why 125 ohms?


Tom Lee
 

Pretty much everyone on this list has the analysis chops to do it. It's just that everyone has better things to do with their time. So many Netflix series to binge-watch! :)

And your intuition is spot on about those other factors mattering. The numbers you cite come from assuming a coaxial line with air dielectric.  The answers will be different for other geometries, and the introduction of dielectrics makes the answers also depend on frequency. But that's too complicated, so the industry settled on values from the air-dielectric coax calculation. You have to standardize on something.

If anyone gives a rat's patootie about the derivation, please contact me off list and I can send you a couple of pages.

-- Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 5/13/2021 13:33, Jim Ford wrote:
I thought it was about 77 ohms.  75 ohm TV coax being very close.  Maximum power handling supposedly occurs for about 30 ohms, and 50 ohms is a compromise between lowest loss and highest power handling.  I've always thought that there would be variations In both  with dielectric and conductor materials and geometry (e.g. coax, microstrip, coplanar waveguide, etc).  Don't have the analysis chops to calculate it myself....             Jim Ford Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: Göran Krusell <mc1648pp@gmail.com> Date: 5/13/21 12:55 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Type 519 -- Why 125 ohms? My memory tells me that the Germans in the 1930s (?) made calculations to figure out if there would be a certain characteristic impedance that would give you the lowest possible cable losses and found 60 ohm as the result. Or am I out in the blue?Göran



Jim Ford
 

I thought it was about 77 ohms.  75 ohm TV coax being very close.  Maximum power handling supposedly occurs for about 30 ohms, and 50 ohms is a compromise between lowest loss and highest power handling.  I've always thought that there would be variations In both  with dielectric and conductor materials and geometry (e.g. coax, microstrip, coplanar waveguide, etc).  Don't have the analysis chops to calculate it myself....             Jim Ford Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: Göran Krusell <mc1648pp@gmail.com> Date: 5/13/21 12:55 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Type 519 -- Why 125 ohms? My memory tells me that the Germans in the 1930s (?) made calculations to figure out if there would be a certain characteristic impedance that would give you the lowest possible cable losses and found 60 ohm as the result. Or am I out in the blue?Göran


Tom Lee
 

(Sorry if this is a duplicate -- my email crashed just after I hit 'send' earlier, and the post had not shown up upon restart)

Close -- the minimum-loss impedance for coax is about 77 ohms, which is why the video industry is based around 75 ohms. The impedance for maximum power handling (again, for coax) is around 30 ohms. The mean is about 50 ohms, which is why general-purpose rf plumbing is based on that value.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 5/13/2021 12:55, Göran Krusell wrote:
My memory tells me that the Germans in the 1930s (?) made calculations to figure out if there would be a certain characteristic impedance that would give you the lowest possible cable losses and found 60 ohm as the result. Or am I out in the blue?
Göran




Göran Krusell
 

My memory tells me that the Germans in the 1930s (?) made calculations to figure out if there would be a certain characteristic impedance that would give you the lowest possible cable losses and found 60 ohm as the result. Or am I out in the blue?
Göran


Tom Lee
 

Correction: I misread my own table. The 475's differential termination is 2x184, so 368 ohms. I'm extra curious to see where that structure's impedance falls apart now.

Thanks, Bob.

-- Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 5/13/2021 11:15, Tom Lee wrote:
Hi Bob,

The schematics I have for the 475 seem to show a differential termination of 184 ohms for the vertical deflection. But in any case, I don't think that it's good out to 1GHz. That said, I would be very interested in seeing the TDR results to see where the impedance starts to look screwy, if by some chance you and your colleagues happen to have taken a photo. TDR'ing CRT deflection structures is one of the many things I have on my never-to-be-completed to-do list.

Higher impedances are of course possible (by increasing the L/C ratio), but there is a complicated tradeoff among bandwidth, deflection sensitivity and physical size. Higher impedances are nice for sensitivity, but it's hard to get that in a small size without trading off bandwidth.

The following values are differential impedances for the vertical deflection lines of a few scopes:

475, 475A 184 ohms (from service manual)
7904, 7904A 385 ohms (from John Addis's article, Fast Vertical Amplifiers); the 7704 is (or should be) the same
7104 200 ohms (also from John)

-- Cheers,
Tom


Tom Lee
 

Hi Bob,

The schematics I have for the 475 seem to show a differential termination of 184 ohms for the vertical deflection. But in any case, I don't think that it's good out to 1GHz. That said, I would be very interested in seeing the TDR results to see where the impedance starts to look screwy, if by some chance you and your colleagues happen to have taken a photo. TDR'ing CRT deflection structures is one of the many things I have on my never-to-be-completed to-do list.

Higher impedances are of course possible (by increasing the L/C ratio), but there is a complicated tradeoff among bandwidth, deflection sensitivity and physical size. Higher impedances are nice for sensitivity, but it's hard to get that in a small size without trading off bandwidth.

The following values are differential impedances for the vertical deflection lines of a few scopes:

475, 475A 184 ohms (from service manual)
7904, 7904A 385 ohms (from John Addis's article, Fast Vertical Amplifiers); the 7704 is (or should be) the same
7104 200 ohms (also from John)

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 5/13/2021 08:51, robeughaas@gmail.com wrote:
The distributed deflection structures in CRT's used in instruments such as the 475 have impedances of over 300 ohms, as evidenced by the termination (or load) resistors in the vertical outputs. We made a measurement to confirm this using a TDR at the vintageTEK Museum.


 

The distributed deflection structures in CRT's used in instruments such as the 475 have impedances of over 300 ohms, as evidenced by the termination (or load) resistors in the vertical outputs. We made a measurement to confirm this using a TDR at the vintageTEK Museum.

--
Bob Haas


Sean Turner
 

Tom,

That explanation makes a lot of sense. I did note that the manual directs you to be a bit careful about how much power you dissipate in the termination of the deflection plate network (I think it's a 1.8 or 2W terminator)

Thanks!

Sean

On Mon, May 10, 2021 at 07:57 PM, Tom Lee wrote:


The 519 has no vertical amp (as I'm sure you know), so the input signal has to
drive the CRT's distributed deflection plates directly. The deflection
structure looks like a transmission line, basically, and has a characteristic
impedance of 125 ohms.

So, your next question would be, "Why is the deflection structure 125 ohms?"
The answer to that is that you'd actually like it to be fairly high so that
you can deliver the deflection voltage (about 10V per cm) at reasonably low
power. At 125 ohms, you're already taking quite a bit of power to deflect it.
50 ohms would be painful.

Why not much higher than 125 ohms? Because it's hard to construct -- and fit
into the CRT -- a deflection structure with a much higher impedance, since Zo
= sqrt(L/C). And even if you could pull off that trick, plumbing for, say, a
300 ohm impedance is no easy task by itself. The geometric mean of 50 and 300
just happens to be about 125 ohms (for small values of 125). That's somewhat
of a coincidence, but not entirely.

-- Cheers,
Tom


Tom Lee
 

The 519 has no vertical amp (as I'm sure you know), so the input signal has to drive the CRT's distributed deflection plates directly. The deflection structure looks like a transmission line, basically, and has a characteristic impedance of 125 ohms.

So, your next question would be, "Why is the deflection structure 125 ohms?" The answer to that is that you'd actually like it to be fairly high so that you can deliver the deflection voltage (about 10V per cm) at reasonably low power. At 125 ohms, you're already taking quite a bit of power to deflect it. 50 ohms would be painful.

Why not much higher than 125 ohms? Because it's hard to construct -- and fit into the CRT -- a deflection structure with a much higher impedance, since Zo = sqrt(L/C). And even if you could pull off that trick, plumbing for, say, a 300 ohm impedance is no easy task by itself. The geometric mean of 50 and 300 just happens to be about 125 ohms (for small values of 125). That's somewhat of a coincidence, but not entirely.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 5/10/2021 18:54, Sean Turner wrote:
Pondering the beast that is sitting in my lab got me thinking, why 125 ohm? Does anyone know of the history of why this was chosen? I haven't been able to find much about this, so insights are appreciated!

Sean




Harvey White
 

IIRC, that's the impedance, or close to it, of twisted pair non shielded cable.

Harvey

On 5/10/2021 9:54 PM, Sean Turner wrote:
Pondering the beast that is sitting in my lab got me thinking, why 125 ohm? Does anyone know of the history of why this was chosen? I haven't been able to find much about this, so insights are appreciated!

Sean





Sean Turner
 

Pondering the beast that is sitting in my lab got me thinking, why 125 ohm? Does anyone know of the history of why this was chosen? I haven't been able to find much about this, so insights are appreciated!

Sean