Characteristic impedance in other than conductor...

[Externet]

Hi.
Does a device inserted in a transmission line presents a characteristic impedance as a coaxial cable does ?

As a RF attenuator, a combiner, a splitter ...?

Just images...

If a transmission line has a characteristic impedance of say 75 ohm; and gets inserted a device as above claimed also as 75 ohm, at what frequency is 75 ohm ? Having some difficulty in expressing this question properly.
Perhaps other wording... a device with specifications of being 75 ohms impedance... at what frequency is it 75 ohms ? Specifications also mention 5 to 1000 MHz !

 

[barry]

I depends on the device, doesn't it?

A capacitor has an impedance of 75 ohms, but only at one frequency; nobody is going to sell it to you as a "75 ohm capacitor".

For a device like a splitter the manufacturer will specify the operating range. Why, look at that, right there on your picture it shows 5-1000MHz!! If you look at the data sheet they probably specify return loss (which is related to impedance mismatch) over the operating range. 75 ohms is nominal, but not absolute. Read the data sheet.

 

[crutschow]

Does a device inserted in a transmission line presents a characteristic impedance as a coaxial cable does ?

As a RF attenuator, a combiner, a splitter ...?

Yes, all do within there operating frequency range as barry stated.
But their impedance is usually determined by internal resistors, not by the LC distributed impedance as a coaxial cable does.

 

[volker@muehlhaus]

Does a device inserted in a transmission line presents a characteristic impedance as a coaxial cable does ?
As a RF attenuator, a combiner, a splitter ...?

Yes, but only if the outputs are terminated into that system impedance.

If you measure the attenuator or splitter *WITHOUT* the external terminations, it will not show the system impedance.

 

[biff44]

over a LIMITED bandwidth, yes.

 

[amihomo]

Any device in a transmission line should have the same impedance as the characteristic impedance of the line (Z0). otherwise it causes mismatch and reflected waves.
All devices have a limited bandwidth in which they are matched to the line Z0, refer to the device datasheet to find its operating bandwidth.

 

[volker@muehlhaus]

The attenuator will work wideband down to DC.

But as mentioned above: to measure the (correct) input impedance, you will need to terminate the output into 50 Ohm.

 

[Externet]

Any device in a transmission line should have the same impedance as the characteristic impedance of the line (Z0). otherwise it causes mismatch and reflected waves.
All devices have a limited bandwidth in which they are matched to the line Z0, refer to the device datasheet to find its operating bandwidth.

Thanks.
That 75 ohm rated splitter with its 75 ohm threaded F ports connected to 75 ohm F terminals inserted on a 75 ohm transmission line; It shows 5-1000 MHz stamped. If we choose to believe that bandwidth stamped figure is also in its datasheet specification and true; how is a device impedance value being function of a frequency be 75 ohm at the entire huge bandwidth ? Perhaps if I had that concept clearer I would be able to formulate the questions better...

Same on a television tuner module. In the past, TVs specifications showed antenna input being "75 ohm". Now, TVs specify "75 ohm antenna F connector"
Is it because the tuner is NOT and cannot present 75 ohm to all of its 170 to 800MHz tuning span ?

Modern TV tuners and their FET front ends present a much, much higher impedance to the antenna they are connected to, is that correct ? (which may be good, no problem with that)
Am still looking to find any TV tuner manufacturer that plots input impedance versus frequency in the specifications.

 

[volker@muehlhaus]

Very wideband power splitters are often transformer-based designs, have a look here at Mini Circuits splitters for 50 Ohm with bandwidth 5 MHz to 2 GHz etc.

RF Power Splitters/Dividers, RF Power Combiners

2-Way, 3-way, 4-way, 6-way, 8-way, 10-way, 12-way, 16-way and up to 24-way models for 50 Ohm and 75 Ohm systems from DC to 67 GHz!

www.minicircuits.com

Input impedance into low noise amplifiers is not always the specified system impedance, if the LNA is optimized for noise figure.

Regarding "75 ohm" vs. "75 ohm antenna F connector", I think should only give a hint on the connector type (F instead of IEC) because both types are common. It doesn't mean that input impedance is questionable.

What is the reason/background/issue for your question?

 

[FvM]

Even high performance measurement instruments don't specify more than maximal VSWR over frequency range. If you want to narrow down the input impedance, you are required to connect an additional attenuator in front of the instrument.

I doubt that it's of much practical relevance to know the input impedance of consumer devices like TV tuner exactly.

 

[BigBoss]

CAN TV Tuners are dead and they are not used anymore due to cost.Instead, RFIC ALL Standard Tuners are common.Input Reflection Coefficient is around 8-10 dB and this is acceptable for TV Receivers.So they don't have to present Perfect Matching.You can think that other equipments have pretty poor Specifications because of very wide-band.Designing an LNA/Amplifier for TV Broadcasting Purpose is one of the most difficult task of RF-Microwave Engineering. ( I designed many..)Therefore some Series and Parallel Feedback techniques are used in order to obtain wanted Flatness and Input Reflection Coefficient over a wide-band.
For Instance

https://cdn.macom.com/datasheets/MAAM-010373.pdf