Subject: Re: How to check for a 50 or 75 ohm connector
Date: Sun, 22 Dec 2002 16:20:32 +0000
NNTP-Posting-Date: Sun, 22 Dec 2002 21:41:40 +0000 (UTC)
User-Agent: tin/1.4.2-20000205 ("Possession") (UNIX) (Linux/2.4.19 (i686))
In sci.electronics.design DarkMatter wrote:
> On Sun, 22 Dec 2002 02:15:12 GMT, Chris Carlen
> Gave us:
> The connector does not possess the impedance. That reference is
> really about the transmission cable that gets used. Connectors can
> differ between them, but the cable is what really determines the
Well the connector is in line with the cable so if the connector is a
signifigant part of a wavelength long at the operating frequency then
it has to be viewed as a transmission line in itself and thus will have
a line impeadence in its own right.....
> You would get NOTHING from a connector alone. It would require a
> CABLE, terminated properly at both ends with no kinks, and THAT is
> what the meter would test. The connectors themselves are shaped and
> sized merely to keep someone from mismatching connection cabling.
No... It doesn't matter at low frequencies but by the time you are passing
signals with components up above 0.5Ghz (I am thinking SDV here) it becomes
measurable. The dialectric geometry is different in 75ohm vs 50ohm BNC
and this changes the charicteristic impeadence of the connector, it is
orthognal to the issue of the cable.
Consider that a bulkhead through connector with a BNC plugged into both
ends is probably about 5cm long and that 5cm (at VF=0.6) is a wavelength
at 3.6Ghz. Which means that for any frequency greater then about 360Mhz
we need to view this BNC-> through panel adaptor -> BNC as a transmission
line in its own right.....
> As far as I know, BNC is BNC is BNC, and the ONLY differences are
> the size of the rear portion of the connector for different sized
> cable. THAT would determine the proper cable for the connector, and
> *that* cable would determine the impedance. So, by default of
> application, the connectors differ. On the front, connection side of
> them, they are all the same, however. There are "hybrids" that will
> claim some super duty or used gold plated parts, but BNC nomenclature
> is the same. It has to be able to be hooked directly in to ANY BNC
> female ANYWHERE. So they are certainly ALL the same on the connectors
No. They have to be able to mate between 50R and 75R systems but that just
means the pins have to have the same geometry, it says nothing about the
dialectric which is what changes between the two connectors.
> The cabling determines the impedance. RG-59 is 75 ohm, RG-58 is 50
> ohm. Then, there is RG-6 (75ohm), RG-174, etc. ALL have BNC
> connector versions.
The cable determines the impeadence of the cable (obviously) but to avoid
discontinuities at the ends any connector used at a frequency where the
connector is a reasonable part of a wavelength in electrical length must
also be considered to be a transmission line with all that that implies.
> Sure it would. At several GHz... maybe. At Mhz frequencies the
> connector contributes little to the overall impedance of the cable
> run. That is determined practically solely on the cable run itself.
Obviously, at low frequencies the connector is short compared to the
wavelength and in this case the differences do not much matter, but we
do not all use connectors solely at low frequencies, and in any case for
a not uncommon BNC setup the threashold is only a few hundred Mhz.
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