why BNC can't operate at high f

[xxiang1106]

why bnc is not good for high frequency

Hi,

I notice some paper say BNC only can operated below 1G (or several G). I am wondering where the number 1G come from? if operated at high f, what problem will induce.
thx!

 

[Rame]

Hi,

The reason because of the bandwidth........Anyways more information on this matter are welcome..


Regards

Rame

 

[egemini]

The contact resistance of BNC is not negligible at frequencies above a Gigahertz, due to which the losses will be high. So it is not prefered at high frequencies

 

[xxargs]

The contact resistance of BNC is not negligible at frequencies above a Gigahertz, due to which the losses will be high. So it is not prefered at high frequencies

BNC is not good and precision connectors ie, using sliced screen-ring-connectors with connect only part of radius (depend of bending or stress), not possible to tigthen in firm state and different and varius contruction of center tap and socket in simple non uniform contruction in various cheap materials and manufacture (see in cheap 10-base 2 network cards connector...)

If you try using BNC with VNA (Vector Network Analyzer) you see why BNC is so bad connectors on higher frequency... is give very varying SWR (return loss) and is more or less impossible to measure with aqurate value depending of BNC:s sensivity of tension and bendig in case moving around measure cable.

Connectors for high frequency i very high precision manufactured mechanical equipment in 1/100 of mm precision with not sliced screen ring connectors and very precision turned center tap and socket in expensive materials (often silver or gold-plated) with high surface finish for low passive intermodulation - remember high frequency travels only on surface on conductor in part of um deep - so good connect on surface edge on connectors is importent design rule for high frequency.

Example of good connectors is SMA, N-connectors and for huge power 7/16-connectors (big area for low passive intermodulation, used for all mobile and GSM base station equipment) - for high precision measure in VNA-envirmoment using APC (7mm) connectors.

     Very importent point on center tap for good no cracking surface connection
          |
          \/
___________________________________
         /|__________________
         -----------------\  \
         -----------------/  /                                 center tap
_________\|________________________



not sliced
screen-ring
____________________________________
__________________//
__________________/

                   /\
                   |
               also important point for good  no cracking surface connection toward center conduct

(principial for N-connectors)

Fingers or slice screen-ring can make unwanted (and varying) stop or band rejection for real high frequency (often around 2 GHz and up) ) - Also not proper tigthen connectors (7/16 always tigthen with wrench, not only by hand).

I see many time big and strange problem in RF-system and big installation, after waste many trouble shoting hours in the last step find one not proper tighten or bad mounted connector - fix this and all problem solved...

Even sligty missing mounted RF-connectors in system can give so much reflection (bad SWR) at other active component (as amplifier or LNA) feeds back reflective power and begin to intermodulate over acceptable limit and give very strange and hard to find failure - you not want couple of always be loose BNC-connectors in this situation!!!

If in situation build RF-system based on BNC-connector, you make project to very difficult to make succefully End, depend on every touch on cables or connector (or equipment box) with BNC - reasign whole systems patterns of reflective power between equipment... Situation i simular as squezing a ballon, tighting i one part and bould out in other place, you fix this also, bould out on third place or/and first place again and you can spend time for ever hunting depend of wrong choice of RF-connectors. Never, never use BNC in RF-system!!! IMHO.


/Xxargs