50 ohm termination using rg 59 (75ohm) cable

Thread Starter

Mrdouble

Joined Aug 13, 2012
105
20171123_173742.jpg
Im in the process of calibrating a previously dead tektronix 564 oscilloscope (half tube half transistor).
The manual says "Connect the calibrator output to both inputs using a bnc 'T' connector and a 50ohm cable"

I have a heck of a time understanding impedance matching so I want to run this by you guys

The calibrator is on board the oscilloscope and I have a rg 59 cable going from that to a bnc T which is plugged into channel 2 and the other side to another rg 59 which goes to another T. One side of that T is plugged into channel 1 and the other is terminated with a 50 ohm bnc termination resistor.

Does the 50 ohm resistor pull the otherwise 75 ohm cable down to 50?
 
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nsaspook

Joined Aug 27, 2009
6,261
The mismatch will cause some timing signal reflections but it's unlikely to be a real problem with that old scope calibration.
 
When you tallking 50 and 75 ohm cables, you are talking about "transmission lines".

When impeadances don't match "reflections" occur.

The 50 ohm terminator on the 1M input is forcing that end to be 50 ohms.

With a greater than 1X scope probe, the purpose of compensation is to make the cable purely resistive.

This scope says 1M || 47 pf. Well, you can never have exactly 47 pf, nor exactly 1 M ohm. The cable of the scope probe has some capacitance per foot. The idea is to have the scope probe to be purely resistive by adjusting the compensation capacitor on the probe. What you have is an impedance divider.

The compensation is adjusted so that there is no overshoot or ringing of the square wave calibrator and the displayed trace is indeed square.

The difference of 75 and 50 may not matter in this case. Keep the cables as short as possible.

The impedance of a cable is a mechanical property. The dielectric and spacing matter.

You can't fix it at the ends. There does exist 50 and 75 ohm BNC connectors, but nearly all "BNC" connectors and cables are 50 ohms.

I do have a 75 ohm F to 50 ohm BNC adapter.

See here: http://cablesondemandblog.com/wordpress1/2014/03/06/whats-the-difference-between-50-ohm-and-75-ohm-coaxial-cable/
 
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Thread Starter

Mrdouble

Joined Aug 13, 2012
105
Ok, so just get rid of 50 ohm termination and let the BNCs handle the 50 ohms?
 

Reloadron

Joined Jan 15, 2015
4,500
Yes, you do not use a 50 ohm termination with the calibrator out. I assume you already calibrated or checked the calibrator out in accordance with the scopes manual. Tektronix calls for using a 50 Ohm impedance cable but using a 75 ohm cable won't effect much for what you are doing. Placing a 50 Ohm feed through termination on the calibrator output will attenuate the signal by about 5:1 meaning with the calibrator set for .5 volts you will see a .1 volt signal. When checking the vertical gain of the pre amplifier plugin a 50 ohm termination is not used and the impedance difference between a 50 ohm and 75 ohm cable with the 1 kHz test signal from the calibrator out is not likely to make any difference.

Really nice classic scope.

Ron
 

Thread Starter

Mrdouble

Joined Aug 13, 2012
105
Actually, no, I have not calibrated the calibrator. Was hoping it was good enough to get me in ball park. The calibrator relies on the amp and time modules being calibrated. Since that circuit is the simplest I did what I could. I'll probably bring g out my oscope
 

Thread Starter

Mrdouble

Joined Aug 13, 2012
105
Ok, here's what is causing confusion

Screenshot_20171124-144109.jpg

so, are they indicating that you "can" use a 50 ohm termination if you are so inclined?
 

nsaspook

Joined Aug 27, 2009
6,261
The calibrator can be used for things other than the direct scope calibration. If you connect that calibrator, set to the .5 volt switch position to an external circuit with a load resistance of 50 ohms and use the scope to measure that voltage with a normal scope probe, the measured voltage with a correctly calibrated scope vertical channel should be 0.1 volts.
 

Reloadron

Joined Jan 15, 2015
4,500
Uh Oh, I had a bad back there, I read the manual and still screwed it up.
"When checking the vertical gain of the pre amplifier plugin a 50 ohm termination is not used and the impedance difference between a 50 ohm and 75 ohm cable with the 1 kHz test signal from the calibrator out is not likely to make any difference".
That should have read Line Frequency test signal. I was actually looking at the manual when I typed that too! :(
Anyway, the difference between a 50 and 75 ohm impedance cable won't matter for what you are looking at. NSA spook covered things nicely.

Ron
 
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