representing connector as voltage source and 50ohm resistor

[yefj]

Hello ,In the lecture slide shown below i got the source with 50ohm resistor in series .
Its supposed to represent a connector. however when i look at the datasheet of the SMA connector i see the its only the characteristic impedance Z=E/H we dont have 50 ohm resistance in series.
Why characteristic impedance means 50 ohm in series?
Thanks.

https://www.digikey.co.il/en/products/detail/molex/0734046190/16572265?s=N4IgTCBcDaIAwHYDMAWOKBsBGAnHEAugL5A

https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/5338/SD-73404-619.pdf

 

[FvM]

Voltage source with resistor represents a generator with source impedance, not a cable or connector.

Presume you have heard about the concept of transmission line impedance, at least your other posts suggest so. An ideal connector simply continues the transmission line without causing reflections.

As you know transmission line impedance is set by the ratio of inductive series and capacitive shunt impedance per length unit, no real resistance involved for a lossless transmission line.

 

[volker@muehlhaus]

Why characteristic impedance means 50 ohm in series?

As FvM already mentioned, the voltage source + resistor represent a 50 Ohm source. The resistor is required in this model because the voltage source itself has zero source impedance, but tyical RF sources have 50 Ohm source impedance.

The picture that you show represents the RF source + coax connector by the voltage source + resistor, so it assumes an ideal connector.

 

[yefj]

impedance is the ration of voltage to current .
If i want to test source impedance and i will short circuit the generator then on some node i will have Vo/(V0/50)
I understand.
so in every PCB signal integrity system we start by representing the source by such configuration?

https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/5338/SD-73404-619.pdf

 

[FvM]

Don't understand why you posting again connector drawings. Source impedance isn't specifically related to connectors, except for the simple fact that a 50 ohm generator is equipped with a connector of matching impedance.

High frequency and fast pulse generators are designed with 50 ohm source impedance, ideal voltage source is a fiction in high frequency range, real sources have always a finite impedance. 50 ohm generator impedance has become a standard, alternative impedance values (e.g. 60 or 75 ohm) are used for special applications. Only 50 ohm impedance can be used up to high GHz frequencies.

 

[crutschow]

Ignoring their small resistance, the theoretical characteristic impedance of a line or connector is √(L/C) where L is the inductance and C is the capacitance of the connector or line.
So a 50Ω connector is configured, as determined by its dimensions and insulator dielectric constant, to have its small, intrinsic L and C values such that √(L/C) equals 50Ω.
Resistance is not involved in that.
A 75Ω connector would have its dimensions slightly changed to increase the ratio of L/C and give the higher characteristic impedance.

As noted, the characteristic impedance appears as (has dimensions of) a resistance, which is why a resistor is used to match that impedance at the signal and/or load end, but it is determined by the L/C value of the line/connector, not their stray resistance.

 

[dick_freebird]

Try some of the top connector mfrs, RF connectors are circuit
elements and SPICE models may be offered on the support
pages.