[time-nuts] BNC Question

[shalimr9 at gmail.com]

As pointed out earlier (by Bruce and others), there is a vast quantity of 75 ohm BNC connectors which mate perfectly with 50 ohm BNC sockets (save for impedance mismatch). I have a set of 10 cables bought off the *bay with such connectors. These cables are 75 ohm, so there will be a mismatch in a 50 ohm system no matter what the connector is.

I am not sure what would make those less *true* that anything else. I am sure the mismatch of the connectors themselves, within the frequency range they are intended for will be well below the level where most would care.

If you try to use BNC for precision RF measurements, be prepared to be surprised.

Unless the argument is about what they "should" be, what the standard says is irrelevant when you have a box of parts and wonder if you can used them.

Don't assume anything, just check before you plug.

Didier KO4BB
 
Sent from my BlackBerry Wireless thingy while I do other things...

-----Original Message-----
From: Oz-in-DFW <lists at ozindfw.net>
Sender: time-nuts-bounces at febo.com
Date: Sat, 16 Apr 2011 13:23:30 
To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: Re: [time-nuts] BNC Question

No Stan, you are correct.

**True** 75 ohm BNCs (and Ns and TNCs as well) use a substantially
smaller center pin and no insulation support around the female contacts.
Mating a 50 ohm spec male part (plug) will invariably damage a 75 ohm
female (receptacle.) 

Amphenol made a **proprietary** series that maintained specs to 4 or so
Ghz when mated in the proper system impedance and would reliably
interate without damage. This was not a full spec part, but a
proprietary variant that was a market response to the damage probelms. 
They were widely used in broadcast video, and rarely used in RF
applications.

If need be we can pull the mil drawings as a group and compare them.   I
learned this lesson by proxy in the days I worked for a cable TV
manufacturer and a tech made the mistake for a second time and got his
pay docked the repair cost from HP.  Fortunately, the parts cost was
small.  Unfortunately he got to pay for a new cal on the instrument,
about 2 weeks pay for him *before* withholding.

Rich, N1OZ

On 4/9/2011 9:28 PM, Stan, W1LE wrote:
> Sorri, I must have been thinking type N connectors.
>
> Stan, W1LE
>
>
> On 4/9/2011 8:18 PM, Brucekareen at aol.com wrote:
>> Interestingly, 50 and 75-ohm BNC connectors have the same pin/shell
>> dimensions and properly couple together.  For more information go to
>> _http://www.cliftonlaboratories.com/august_2007.htm_
>> (http://www.cliftonlaboratories.com/august_2007.htm)  and  scroll
>> down to the 02 August 2007 entry.  There is a
>> link to the Amphenol  site for specifics about the connectors.
>>
>> I am planning to use an Extron video distribution amplifier for standard
>> frequency distribution.  The 75-ohm connectors will be OK.  These 
>> amplifiers
>> feed multiple 75-ohm outputs from a "zero-impedance" source by 
>> inserting a
>> 75-ohm resistor in each leg.  Some folks using these for  standard
>> frequency distribution change the resistors to 50-ohms.
>>
>> Bruce, KG6OJI
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