Question about coaxial cables & screened twisted pair ca

Dear all,

My signal is 10MHz.
Normally we use coaxial cables for the high frequency transmission.
However, my friend told me I can use screened twisted pair cables.
In my opinion, screened twisted pair cables won't give me good quality signal.
I am wondering if it's right??

Hi,
in my opinion is the only relevant question: what is transport distance?
Second: Whats for you "good quality"?
K.

Re: Question about coaxial cables & screened twisted pai

Hi,

first of all a twisted pair is a balance transmission line, that means the current is carried in a given direction in one of the two wires an in the opposite direction in the other one; a coaxial cable is, instead, a unbalanced transmission line, that mean the signal propagates inside it as a wave. Also impedance of twisted pair and coaxial cable (usually 50 ohms) are in general much different.
So if you have an unbalanced generator (e.g.: signal generator, oscillator, etc) you can choose a good coax cable. If you generator is balanced (e.g. a dipole antenna) you can use a twisted pair paying attention into matching the source and the line impedance. However it is possible to adapt a balanced generator to an umbalanced line (or viceversa) using a device called BalUn (Balanced-to-Umbalanced).
Twisted pair are suggested when you have to operate in a field where magnetic field interferer are present. As a matter of fact a twisted pair is composed by a series of single-turn coil each having opposite direction then ideally canelling the magnetic field. The screen of the twisted pair is used to mitigate disturbances given by electric fields.

Regards
Alberto

I think we can talk over transmission lines only if the ends are with Z0 loaded...

Twisted pairs are more sensitive to mechanical montage loads/bendings, vibrations_stresses_ and have HIGHER dampings on the same frequency_OK for 10MHz its not very relevant... How long is these cable pls?
You can buy coaxial cables too with strongly other impedances as the usual 50/75 Ohms! Their are to find so between ca. 30 & good 100Ohms too, but most diff pairs arent 100 Ohm lines-more 117 or 120 ohms..!

In newest times (over teen years)you can order diff microstrip cables; i.e. from Raychem, Belden & Th&B_
Their are very precise as impedance value, dont change some thing with mechanical stresses, but you must pay it with much money...
K.

Re: Question about coaxial cables & screened twisted pai

you can send signals along a twisted pair, but as previously stated you need to:
1) go from balanced (coaxial connector) to unbalanced twisted pair
and
2) go from either 50 or 75 ohm coaxial impedance to the impedance of the twisted pair. Cat5e cable is 100 ohm impedance.

If you want to improve the emi rejection, you can use shielded twisted pair, which has a metal or foil shield over the twisted pair. You have to connect that shield to something on both ends, like chassis ground.

Re: Question about coaxial cables & screened twisted pai

Hi Karesz, the length of the cable will be around 2 meters.

Hi biff44, I don't understand what did you mean "balanced" & "unbalanced". For the "balanced", does it mean that for example, I have two signals and they have a 180-degree phase difference??

Kind Regards,

Oliver

Re: Question about coaxial cables & screened twisted pai

For the "balanced", does it mean that for example, I have two signals and they have a 180-degree phase difference??

Click to expand...

Yes you need a differential driver, either a transformer or an active circuit. Also a differential receiver is needed to benefit from
the better common mode interference supression of differential or "balanced" signaling. If you don't have it, you better stay
with single-ended coaxial cables, if they are generally working in your application.

Re: Question about coaxial cables & screened twisted pai

I know normally we use BNC/SMA/SMB/SMC connectors for the coaxial cables.
So, what kind of connectors are used on the screened twisted pair cable???

Re: Question about coaxial cables & screened twisted pai

why don't you tell us exactly what your "signal" is, and what type of connector is there right now.

Re: Question about coaxial cables & screened twisted pai

I will use differential signals (Two signals, 180-degree-phase difference).
I just have BNC connectors.
However, I think I shouldn't use BNC connectors for the screened twisted pair cables.

Hi,
Take pls a two pole "Lemo 00", has same place/footprint as a BNC, but BNC is so & so not a high quality conn...
You see, its not optimal, but if you dont like Lemos; use two times a SMA/B...
For twisted pairs is a press-connector(flat kabel) so & so better to take, but dont forget, you can press it in all 50 cm only!:-(_ newly has Farnell (or Distrelec?) a 25 cm version too)
FvM has had abs. right; you need diff TX & RX in your system(i.e. RS485/Tiny Logic/ECLinPs...), but their are in small SO8/SOT packages to buy, also needs incl. bypass C`s ca. 1cm2 of place.
K.

Re: Question about coaxial cables & screened twisted pai

Yes, LEMO connectors, it's a good idea.
I didn't think about it.

For the length of the cable, your suggestion is 50 cm. Did you mean the longest one I can get from shops is 50cm??

Oliver,
You did missunderstand me; I told: twistedpair cable has in all 50 cm (quantized distances) a short part (~5cm long) for pressing the (Flatcable) connectors. (& a Farnell verison in all 25 cm too)
You becoms it on rolls, maybe in 100m of length!
K.

Re: Question about coaxial cables & screened twisted pai

Sorry about that, Karesz.
I got it now.

Not a problem!
K.