When to use 75 Ohm input at spectrum analyzer ?

[BravoV]

All this time I've read alot of old SA datasheets mainly HP, and most have 50 Ohm input impedance.

My recently acquired SA has both 50 or 75 Ohm, just wonder when this 75 Ohm will be used ?

All my T&M equipments that can generate or receive signal are 50 Ohm only.

The explanation I got from the SA user manual are only this paragraph, and nothing else. 

Quote :

"Using Input Z [50/75] menu, selecting input impedance 50 ohm or 75 ohm. When Input Z [75], this gives the method that user can use spectrum analyzer in such environment as ignore reflection and calculate considering purely impedance matching."

TIA

[joseph nicholas]

I think they are talking about the rating of the coaxial cable you are attaching the SA too.   

[TheUnnamedNewbie]

When measuring in a 75 ohm environment. A lot of telecom infrastructure is 75 ohm. Supposed reason for this is that 75 ohm is close to the impedance of a air dielectric coaxial cable with lowest loss. I've also heard the fact that it is close to the real impedance of a half-wavelength dipole.

As a result, to get accurate readings in those environments and present the correct load impedance to the source, you need your measuring equipment to also present that 75 ohm load.

[tautech]

With the Siglent SSA3000X's both are available too by way of input impedance 'correction' in order to get accurate results. As most of the work you'll do is with 50 ohm, you'll probably never delve into 75 ohm stuff.
Just another rabbit hole for you to fall into as you'll need another full set of 75 ohm cables. connectors, adapters, attenuators and loads ! 

[BravoV]

Thanks for all replies, really appreciate it. 

Noted, when dealing with analog video transmissions type as in analog video signal, analog TV reception etc, and also stuffs on telcom industries.

Just another rabbit hole for you to fall into as you'll need another full set of 75 ohm cables. connectors, adapters, attenuators and loads ! 

Sigh ... yeah, beginning to feel the curse of the rabbit since the SA acquisition. So no toying with 75 Ohm stuffs yet, at least not now.

Especially what I called these "RF plumbing thingies" are so dizzying, like BNC/SMA/N with various connectors, converters, plugs ,attenuators ,pig tail cables, DC block and etc ... 

And I don't even have time to do the reading for the field probes yet, let alone DIY-ing it.

[TheUnnamedNewbie]

Noted, when dealing with analog video transmissions type as in analog video signal, analog TV reception etc, and also stuffs on telcom industries.

Just to be sure that you didn't mis-interpret the responses here: It is not that all video is by definition 75 ohm, or that all 75 ohm stuff is only video/telecom! It's just that these industries happened to agree on 75 ohm as their impedance. There is not something magical about the type of signal. Kinda like how most audio uses 3.5mm headphone jacks - there is no reason why you can't have other jacks for audio, nor does it mean that 3.5mm jack will be audio!

You will find this on the devices you connect to or the cables you use. They will usually state the impedance.

[BravoV]

Just to be sure that you didn't mis-interpret the responses here: It is not that all video is by definition 75 ohm, or that all 75 ohm stuff is only video/telecom! It's just that these industries happened to agree on 75 ohm as their impedance. There is not something magical about the type of signal. Kinda like how most audio uses 3.5mm headphone jacks - there is no reason why you can't have other jacks for audio, nor does it mean that 3.5mm jack will be audio!

You will find this on the devices you connect to or the cables you use. They will usually state the impedance.

Thanks for the reminder, noted. 

[Richard Crowley]

You typically do not have an arbitrary choice of what impedance to use.  The overriding factor is the SOURCE IMPEDANCE where the signal is originating from.  If you have a 50 ohm source, then you would use 50 ohm connectors and 50 ohm cable (like RG-58, etc.), and the 50 ohm input/terminating impedance of the destination (your SA).

But if you have a 75 ohm source, then you would use 75 ohm connectors and 75 ohm cable (like RG-59, etc.) and the 75 ohm input/terminating impedance of the destination. 

Note that virtually all BNC connectors are 50 ohm.  But there are 75 ohm varieties of BNC connectors. Most other types of connectors are "hard-wired" for either 50 ohm or 75 ohm.  Although for short bench-top experimentation it probably makes little difference until you get to UHF frequencies and/or high-power.

[Mechatrommer]

Noted, when dealing with analog video transmissions type as in analog video signal, analog TV reception etc, and also stuffs on telcom industries.

It's just that these industries happened to agree on 75 ohm as their impedance. There is not something magical about the type of signal. Kinda like how most audio uses 3.5mm headphone jacks - there is no reason why you can't have other jacks for audio, nor does it mean that 3.5mm jack will be audio!

there is reason why you dont use other than 50 and 75 ohm... https://www.microwaves101.com/encyclopedias/why-fifty-ohms but there is no stopping you from using other impedances just dont expect stellar performance and availability in measurement environment (you have to make your own SA or modify existing one).

[KD4PBS]

Not just analog video.  The broadcast industry uses 75 ohm coaxial cable for all signal lines, whether analog or digital, and lately, most RF lines (that aren't waveguide) exclusively. The exception being broadcast radio RF systems which is still mainly 50 ohm stuff.  That being said, even the digital audio standard for radio and TV signals (AES audio) is carried on either 110 ohm balanced lines (originally), or more typically nowadays, on 75 ohm coaxial cable.  Why coax?  Because one can assemble connectors to the ends of the coax much more quickly than preparing a balanced connector.  It makes a huge difference when building studios that can easily contain 5,000 or more point-to-point connections.
Why 50 ohms?  Back when it was developed as a standard, it was the best trade off between power handling and loss; frequency response wasn't much of an issue with the relatively low frequency transmitters of the time.  This is why radio stations still mainly use 50 ohm transmission lines.
Why 75 ohms?  Back when it was developed as a standard, it was the best trade off between loss and power handling.  Power handling capability wasn't much of an issue with video signals, and it was an easy impedance to transform to the typical 300-ohm balanced conductors used to connect to antennas.  This is why TV stations (UHF stations) have mostly switched to 75 ohm transmission line.  Even a quarter of a dB savings of transmission line loss can save thousands on the power bill over the course of a year.