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 Post subject: Millivac 900-2100 Open Circuit BNC Termination
PostPosted: Sep Wed 11, 2019 2:32 am 
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Could someone explain what this attachment is used for? There is a 50ohm one for the probe I have and this one pictured.

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 Post subject: Re: Millivac 900-2100 Open Circuit BNC Termination
PostPosted: Sep Wed 11, 2019 11:59 am 
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It's a precision terminator, or at least looks to be one. Cannot read the text from the photo resolution

Here's a pretty decent description of why you would need to use one:

https://www.iitk.ac.in/ibc/BNC_T.pdf

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 Post subject: Re: Millivac 900-2100 Open Circuit BNC Termination
PostPosted: Sep Wed 11, 2019 12:02 pm 
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It's a device used for transmission line measurements (return loss, VSWR, reflection coefficient, impedance). When a transmission line is terminated in a perfect open circuit, 100% of the incident wave is reflected back to the source. However, there is no current in an open circuit so the reflected wave is in phase with the incident wave. The voltage developed at that point depends on the length of the line and the wavelength and magnitude of the applied RF or AC voltage.

That termination is probably capacitively coupled to the meter probe so the open circuit voltage can be measured. This is a complicated thing to make, as the termination must not create any reflections of its own to avoid errors in the measurements.

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 Post subject: Re: Millivac 900-2100 Open Circuit BNC Termination
PostPosted: Sep Wed 11, 2019 7:27 pm 
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The reason I ask, the RF millivoltmeter comes with a 900-2050A RF 50 Ohm BNC Termination, a 900-2002A
High Impedance Tip w/Ground Clip and the 900-2100 open circuit termination. I tested the unit today with the 50ohm termination with my HP 8640B signal generator and it is pretty spot on. To use the RF meter with a 75ohm unit like a Sencore SG-165, how would you hook it up with BNC connections? I assume the high impedance tip is a 75ohm connector?

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 Post subject: Re: Millivac 900-2100 Open Circuit BNC Termination
PostPosted: Sep Thu 12, 2019 12:14 am 
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That bnc unit replaces the one on the end of the probe that has the 50 ohm tetmination resistance so that you can use an external termination for higher power, or of different impedance (like 75 ohms or 300, or whatever).
http://www.millivacinstruments.com/

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 Post subject: Re: Millivac 900-2100 Open Circuit BNC Termination
PostPosted: Sep Thu 12, 2019 7:05 pm 
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My apologies, still a little confused. So the 50ohm attachment is for impedance matching 50ohm units. The attachment with the clip grabber is for 75ohm impedance matching and the middle one is for impedance matching 75 and 300ohm?

Attachment:
IMG_0664.JPG
IMG_0664.JPG [ 147.31 KiB | Viewed 62 times ]

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 Post subject: Re: Millivac 900-2100 Open Circuit BNC Termination
PostPosted: Sep Thu 12, 2019 7:51 pm 
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The 50 ohm attachment has a built-in 50 ohm termination resistance.
The open-circuit termination doesn't have any built-in termination resistance (effectively an infinite impedance, thus the sideways-8 notation).
The high impedance tip with ground clip also doesn't have any built-in termination resistance.

tbone8 wrote:
To use the RF meter with a 75ohm unit like a Sencore SG-165, how would you hook it up with BNC connections?

To match a 75 ohm transmission line, you'd use a BNC T connector with the open-circuit termination on one side of the T and a 75 ohm BNC terminator on the other side. There would be some slight mismatch due to the 50-ohm BNC T connector, but it shouldn't present a practical problem at the frequencies produced by the SG-165.

tbone8 wrote:
I assume the high impedance tip is a 75ohm connector?

Nope, it's a standard 50 ohm BNC.

Which Millivac RF voltmeter did you get?

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 Post subject: Re: Millivac 900-2100 Open Circuit BNC Termination
PostPosted: Sep Thu 12, 2019 11:51 pm 
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stevebyan wrote:
The 50 ohm attachment has a built-in 50 ohm termination resistance.
The open-circuit termination doesn't have any built-in termination resistance (effectively an infinite impedance, thus the sideways-8 notation).
The high impedance tip with ground clip also doesn't have any built-in termination resistance.

tbone8 wrote:
To use the RF meter with a 75ohm unit like a Sencore SG-165, how would you hook it up with BNC connections?

To match a 75 ohm transmission line, you'd use a BNC T connector with the open-circuit termination on one side of the T and a 75 ohm BNC terminator on the other side. There would be some slight mismatch due to the 50-ohm BNC T connector, but it shouldn't present a practical problem at the frequencies produced by the SG-165.

tbone8 wrote:
I assume the high impedance tip is a 75ohm connector?

Nope, it's a standard 50 ohm BNC.

Which Millivac RF voltmeter did you get?


Ok, it's a 50ohm meter then... :D I assumed it would be 75ohm, my bad. It's a 823B, real nice shape from the Airforce. Been trying to find a manual for it, none to be found. Millivac wants $85 for a copy :shock: .

So, something like this with a BNC tee will impedance match everything up then?

https://www.mouser.com/ProductDetail/AIM-Cambridge-Cinch-Connectivity-Solutions/27-9007?qs=sGAEpiMZZMvlX3nhDDO4AAx%2Fo14ICUl1kRqzNfVRztM%3D

Thanks again Steve!

Also got these attachments with it.

Attachment:
IMG_0665.JPG
IMG_0665.JPG [ 151.23 KiB | Viewed 49 times ]

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 Post subject: Re: Millivac 900-2100 Open Circuit BNC Termination
PostPosted: Sep Fri 13, 2019 12:30 am 
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tbone8 wrote:
Ok, it's a 50ohm meter then... :D I assumed it would be 75ohm, my bad. It's a 823B, real nice shape from the Airforce. Been trying to find a manual for it, none to be found. Millivac wants $85 for a copy :shock: .

So, something like this with a BNC tee will impedance match everything up then?

Yep, that's what I meant by a 75 ohm terminator.

The MV-823B looks like a nice meter, though it's hard to find much information on it. Too bad Millivac is so ungenerous about their manual.

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 Post subject: Re: Millivac 900-2100 Open Circuit BNC Termination
PostPosted: Sep Fri 13, 2019 12:42 am 
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stevebyan wrote:
tbone8 wrote:
Ok, it's a 50ohm meter then... :D I assumed it would be 75ohm, my bad. It's a 823B, real nice shape from the Airforce. Been trying to find a manual for it, none to be found. Millivac wants $85 for a copy :shock: .

So, something like this with a BNC tee will impedance match everything up then?

Yep, that's what I meant by a 75 ohm terminator.

The MV-823B looks like a nice meter, though it's hard to find much information on it. Too bad Millivac is so ungenerous about their manual.


Ya, I got an Airforce buddy looking for a copy but that is looking like a dead end. :(

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