50 Ohm vs 75 Ohm: I still don't get it...

[Bicurico]

Hi,

Here is a beginners question and despite having some experience with TV, CATV, SAT equipment (reception and broadcast), as well as other test equipment, I am not embarressed to ask this question:

What does 50 Ohm or 75 Ohm on cables, input connectors, etc., really mean?

I know that for networking, it was decided to use 50 Ohm cables and termination, while for TV applications 75 Ohm seemed to be a better choice.

My questions:

1) If I measure the same signal on a TV field meter (75 Ohm) or a spectrum analyzer (50 Ohm) I get the same reading, considering the precision of the equipment used. I know that "it is not a measurable difference for these applications" - well, why are then two different values being used?
2) I have a 50/75 Ohm coupler from HP, which makes no difference if inserted or not, apart from the 2dB loss of just inserting it.
3) What does 50/75 Ohm mean? Is that the resistance of the cable? Of the terminator? What if the cable is 1m or 100m long?

Any insight and simple to understand explanation is more than welcome!

Regards,
Vitor

[danadak]

Some ref material -


http://www.rfcafe.com/references/electrical/history-of-50-ohms.htm


http://www.rfcafe.com/references/electrical/history-of-50-ohms.htm


https://www.microwaves101.com/encyclopedias/why-fifty-ohms


Generically 50 ohms means the system environment, so includes the transmission
lien/cable, terminators if used (like a 1M ohm scope terminated to 50 ohms with
a terminator), and the source and load equipment/circuits.



Regards, Dana.

[kosine]

A cable carries energy, but it comes in two forms: potential and kinetic. The potential energy is related to the voltage and is stored by the cable's capacitance. The kinetic energy is related to the current and is stored by the cable's inductance.

From Ohm's law you know that resistance is just the ratio of voltage to current, or potential to kinetic energy. In a pure resistor this energy is being lost as heat.

Impedence, however, also takes account of the reactance, which is the ratio of stored of energy.

A length of (coax) cable has the ability to store potential energy in its capacitance, and kinetic energy in its inductance. The ratio of the two is the impedence of the cable. If you double the length of the cable, you double its capacitance, but you also double its inductance. So the ratio remains the same and you get the same impedence for any length.

The physical difference between 50ohm and 75ohm cable is they have difference amounts of capacitance and inductance. It has no effect at DC, your cable will just have a (small) resistance that increases with length, as you would expect. But with an AC signal the capacitance and inductance has an effect, hence the need to measure the ratio of the two, which is the impedence.

[TimNJ]

I’m not an expert in this but I think the following is true.

50ohm, 75ohm, etc. is the "characteristic impedance" of a transmission line. You can think of it as the impedance of an infinitesimal section of the transmission line (series inductor with parallel capacitor). (I understand that’s probably not easy to imagine.)

Each inductor/capacitor snippet draws a constant current, as defined by the values of L and C and the voltage you apply. When you connect a whole bunch of these in series, you form a transmission line of some real length. When a signal propagates down the line, each of the infinitesimal sections has to be charged up. The “resistance” this signal surge “sees” each time it reaches a new section is the characteristic impedance. A constant voltage resulting in a constant current is a resistance (effectively).

You start and terminate a line with an actual resistor the same value as the characteristic impedance of the line to prevent reflections. Reflections = bad. You are effectively fooling the signal into thinking it is propagating into the next infinitesimal piece of transmission line but surprise, it’s actually the load! (But, the signal doesn’t know the difference.)

Remember, characteristic impedance isn’t an impedance in the traditional sense. It’s more like instantaneous impedance seen by the signal or “surge impedance” as it is sometimes called.

[JacquesBBB]

A very good reference for this is the appendix H of  The Art of Electronics

[lordvader88]

So if u you make a random circuit with whatever resistance + reactance, do u just have to look at the Thevein equivalent, and then add on some LCR circuit so that the new output is now 50 Ohm = resis + react. ??

[Wimberleytech]

The answer is explained by Tom Lee "The Design of CMOS Radio-Frequency Integrated Circuits" Second Edition pp229-231

Summary: 
Optimal  power handling capability of a coaxial cable with air dielectric is achieved with 30 ohms.
Minimal loss (where power handling is not important) is achieved with 77 ohms.

Engineers will be engineers:
30 ohms was 'rounded' up to 50 ohms  (see note below)
77 ohms was 'rounded' down to 75 ohms

To get the best of power handling and minimal loss together the geometric (or arithmetic) mean of 30 and 77 is about 50 ohms.

[Paul Price]

The important thing to remember is not to use 75-ohm coax in a 50-ohm circuit or vice-versa when working with fast pulses or VHF or greater CW frequencies. Also don't mix lengths of different cable impedance cables.  Doing so will result in distortion of pulses (overshoot and undershoot and ringing) and with loss of power in transmission lines due to a high SWR and a high SWR causes power to be reflected back to the source instead of being delivered to the load and this always creates transmitted power loss. In high power transmission lines, high SWR can cause transmitters and cables to overheat and fail.

[David Hess]

1) If I measure the same signal on a TV field meter (75 Ohm) or a spectrum analyzer (50 Ohm) I get the same reading, considering the precision of the equipment used. I know that "it is not a measurable difference for these applications" - well, why are then two different values being used?

The instruments are calibrated to make readings assuming a specific impedance.  For historical reasons which Wimberleytech briefly covered, 75 ohms is used in applications where the minimum loss is desired.  Some high power transmitters use 25 ohms to get the maximum power through a given sized transmission line.  50 ohms is a good compromise.  600 ohms is common in audio.  50 ohms is a little low for low power digital so a higher impedance like 92 might be used.

2) I have a 50/75 Ohm coupler from HP, which makes no difference if inserted or not, apart from the 2dB loss of just inserting it.

Are you sure it is just 2dB?  It sounds like a resistive minimum loss pad.

https://www.maximintegrated.com/en/app-notes/index.mvp/id/972

3) What does 50/75 Ohm mean? Is that the resistance of the cable? Of the terminator? What if the cable is 1m or 100m long?

It means during the propagation of the signal through the transmission line, the ratio of voltage to current is 50 or 75.  If an end termination is present, then the signal reaches the termination after about 5 nanoseconds per meter or 500 nanoseconds per 100 meters and is absorbed.  If the termination is not present, then the signal reflects and returns to the source in about twice this time or 10 nanoseconds per meter or 1000 nanoseconds per 100 meters.  It takes this long for the source to "see" the termination.