The first thing that boggled my mind is that the face plate says the output is 52 ohms ?! Yes 52 not 50 ! So you need 52ohms coaxial cables and plug that into a 52 ohm load/scope, if that even exists ?! I guess there must exist 52/50 ohms adapters ?! Still, why not make the oput 50ohms to begin with ?! how pervert is it to make a 52 ohms output ?!
WHY ?! ![Scared :scared:](https://www.eevblog.com/forum/Smileys/default/scared.gif.pagespeed.ce.oZptHNcbsD.gif)
Is it a " thing " to be 52 ohms in the test and measurements arena maybe, at that time anyway ? What's so special about 52 ohms that you can't do with traditional 50 ohms ? I am genuinely curious, I am sure I will learn something...
Back in the day, the "official " impedance for coax of that type was 52 ohms.
Over time, it became "normalised" to 50 ohms, which is amusing, as 52 ohms was, itself, a "normalisation" of 51.5 ohms.
Marconi, in the UK, still used 51.5 ohms as their standard.
Oh, OK.... so manufacturers took their time in the early days, to converge all to 50ohms...
Since we are alwyas told that teh world / signal integrity will collapse if the impedance is not match throughout... output, in put, cables, connectors.... I guess back there was also, just might, partially.... some incentive for manufacturers to have their own particular value of " 50 " ohms, to motivate customers to buy not one piece of gear from them.... but all of it, and charge through the roof for it. Kinda the Apple ecosystem with proprietary everything.
So basically non-50ohms stuff is normally only Vintage gear then... so it should be kept in mind if one is being vry critical about signal shape/integrity.... that maybe the distortion of his signal is nothing to do with the vintage signal generator say, nor the scope he is looking at it with.. but simply a slight impedance mismatch ! IOW.... guy needs to know what he is doing, shocking ! ![Grin ;D](https://www.eevblog.com/forum/Smileys/default/xgrin.gif.pagespeed.ic.QVVz6XIT20.png)
The figure of 50
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif)
is entirely arbitrary. If you want to
maximize the
safe power handling of an air spaced coaxial cable it wants an impedance of around 30
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif)
. If you want to
minimise the
attenuation of an air spaced coaxial cable you want a characteristic impedance of 77
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif)
. However, there is also the issue that you need to match impedance between cables, connectors and so on to maximise power transfer efficiency and minimise reflections. So if you don't want to have impedance matching networks or transformers everywhere you need a compromise 'standard' impedance - 50
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif)
sits happily half way between maximum power handling (30
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif)
) and minimum attenuation (77
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif)
)
and is quite close to the minimum attenuation characteristic impedance for cables with non-air dielectrics (\$\epsilon_r\$ ~ 2.25). So that's how we got 50
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif)
for general purpose equipment and 75
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif)
for RF stuff (the latter minimising losses from attenuation, which matters if you're talking about an antenna down cable). It's easy to see that on something like a big transmitter you'd use 30
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif)
hardline between the transmitter and its antenna (and matching network) to minimise the physical size of that hardline.
A few percent either side of 50
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif)
doesn't make a big difference in how effective a compromise it is, and a small mismatch between a 50
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif)
cable and a 51
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif)
connector is not going to make a whole heap of difference either in terms of power transfer efficiency or reflections.
If one encounters a situation where it's justified either by reflected power levels or by the required precision then you can either put matching transformers or get really fussy about connector/cable impedance.