cable termination question

[narwhaler]

What exactly does it mean to have a 50 ohm terminator? Is there actually a 50 ohm resistor in the terminator connecting the center conductor to the shield, i.e. if I do a DC measurement from center to shield will I get 50 ohms? Or does it refer to the fact that the connector/terminator is matched to the 50 ohm cable?

THanks!

 

[albbg]

It means you have a 50 ohm resistor placed between center conductor and shield.
Often it is just a resistor, so you can measure it by means of a multimeter.
But sometimes it could also be a 50 ohm resistor in series with a DC block capacitor, in order not to have DC current flowing through it (in case of non-zero average signals). Using this solution the 50 ohm matching will be effective over a certain range of frequency depending from the capacitor. In this case it's not possible to use a simple multimeter to check the termination.

 

[narwhaler]

Thank you for the reply albbg. So if I have a cable with a 50 ohm terminator, connected to a board with a 50 ohm jack, and then a 50 ohm resistor to ground at the amplifier input on the board, isn't that cable then seeing a load of three 50 ohm resistors in parallel, or 50/3 = 16.67 ohms?

 

[albbg]

What you describe seems to me a strange connection. Are all the 50 ohm loads connected at the same end of the cable ? If it is so then yes the resistors will be all in parallel. However could you post a sketch with the actual connections you have in mind ?

 

[narwhaler]

Thank you again for your help albbg. For an example, the attached image is a circuit from the TI THS4034 datasheet showing the schematic of the evaluation board. In the parts list, J1 is a 50 ohm SMA connector. R11 is a 49.9 ohm resistor to ground terminating the input in 50 ohm. And assuming that the terminator on the cable connecting to the board is also 50 ohms, that is 3 50 ohm resistors in parallel to ground at the end of the cable/input to the amplifier.

THS4034 Datasheet link (see pages 16-17 for evaluation board):
https://www.ti.com/lit/ds/symlink/ths4304.pdf

 

[albbg]

No, your understanding is not correct.

The aim of impedance matching is to maximize the transfer of power from the source to the load. This is accomplished when the load impedance equals the complex conjugate of the source impedance. In case of real impedances when the load resistance equals the source resistance. For examples when you have a generator having 50 ohm output resistance (series to the generator) and a load of 50 ohms (from source resistance to load).
But when you need to transfer such a power through a trasmission line you have to take into account that also trasmission lines have their characteristic impedance (typically real: 50 ohm, 75 ohm, 32 ohm,...). To maximize the power transfer this last have to matches with source and load impedances. The impedance of the cable doesn't have to be consided as in parallel with the load impedance; it just a "transfer impedance". Mimatches of one of these three values will cause reflections that leads in many effects you can learn on a book.
The connector impedance has the same meaning of that of the cable: the connector behave like a transmission line.
So, in your example, you will have a generator with 50 ohm output impedance connected to a 50 ohm cable to a 50 ohm load. Everything is 50 ohm. Of course I'm neglecting al other effects present in the real world.

 

[narwhaler]

So what you are saying is that the connectors have a characteristic impedance of 50 ohms, i.e. matched to the cable, not an actual 50 ohm connection from center conductor to shield?

 

[albbg]

Yes, correct.

 

[narwhaler]

Thank you again for your help. That was my understanding of the situation coming in. What got me a bit confused was something I read in an LTC application note:

http://cds.linear.com/docs/en/application-note/an47fa.pdf

The note is on general techniques for wideband (~100 MHz) amplifiers. If you look at the end of page 7 to the first few paragraphs of page 8, and then again on page 15, it seems to me that he's talking about BNC cable connectors as though they actually have a 50 ohm resistor built in for termination. He's talking about connecting them to a board, so he's not referring to a 50 ohm termination cap. Am I reading this wrong? Thank you again for all your help.