Take a look at:http://sosnick.uchicago.edu/BNC_50_75.html
And then look at the BNCs on your equipment.
Unless all of your cables and equipment is new, as in since
~2000, I bet you have a lot of 50 Ohm BNCs in your facility.
It is real simple: plastic "inserts" in the business end mean
you have a 50 Ohm BNC. And for analog/NTSC it doesn't
matter.

Terry

On 13 May 2005 09:41:26 -0700, wrote:

Take a look at:http://sosnick.uchicago.edu/BNC_50_75.html
And then look at the BNCs on your equipment.
Unless all of your cables and equipment is new, as in since
~2000, I bet you have a lot of 50 Ohm BNCs in your facility.
It is real simple: plastic "inserts" in the business end mean
you have a 50 Ohm BNC. And for analog/NTSC it doesn't
matter.

Terry

Pacific Radio has always specialized in 75 Ohm BNCs. They are THE
place to go for that sort of thing in the Television Capitol of the
World. And they had them well before the year 2000.

http://www.computermodules.com/broad...aster-fd.shtml

The worst thing that ever happened to TV is stupid coaxial ethernet.
Before then, 50 Ohm BNC jumpers were very rare. I don't know anything
about Trompeter patch bays. We always use ADC.

All I know is that talking with my friends who work in
TV around the central Kentucky area are laughing at the
pennies saved versus the dollars that will now have to be
spent on upgrades. Kind of a mini Y2K event where penny
wise pound foollis was the logic. Although to be fair from what
I have read in the amphenol catalog, there was no measureable
difference between 50 and 75 ohm in the NTSC universe. Kind
of like a retired cobal programer who was shocked to find stuff he
had written in the early 70' was still mission critical software that
hadn't
been upgraded because "it worked". He made a bundle off that mess.
Non of the recent Computer Science grads couldmake heads nor
tails of 30 year old code.

My TV friends have "funny" stories about when a 50 ohm termination
got into the 75 ohms video system. Pulls synch and video down, and
once you have seen it you are foreverer "learned", but that first time
is
a real dozy!

Terry

On 13 May 2005 11:46:19 -0700, wrote:

All I know is that talking with my friends who work in
TV around the central Kentucky area are laughing at the
pennies saved versus the dollars that will now have to be
spent on upgrades. Kind of a mini Y2K event where penny
wise pound foollis was the logic. Although to be fair from what
I have read in the amphenol catalog, there was no measureable
difference between 50 and 75 ohm in the NTSC universe. Kind
of like a retired cobal programer who was shocked to find stuff he
had written in the early 70' was still mission critical software that
hadn't
been upgraded because "it worked". He made a bundle off that mess.
Non of the recent Computer Science grads couldmake heads nor
tails of 30 year old code.

My TV friends have "funny" stories about when a 50 ohm termination
got into the 75 ohms video system. Pulls synch and video down, and
once you have seen it you are foreverer "learned", but that first time
is
a real dozy!

Terry

The only terminations I ever use are the ones built into the
equipment.

wrote

All I know is that talking with my friends who work in
TV around the central Kentucky area...

Terry


Hey Terry, I didn't know you guys had electricity in central Kentucky. You
really have tv's too?

;-)

Jack

In article om,
wrote:

Take a look at:http://sosnick.uchicago.edu/BNC_50_75.html
And then look at the BNCs on your equipment.
Unless all of your cables and equipment is new, as in since
~2000, I bet you have a lot of 50 Ohm BNCs in your facility.
It is real simple: plastic "inserts" in the business end mean
you have a 50 Ohm BNC. And for analog/NTSC it doesn't
matter.

I didn't know they made 75 ohm BNC connectors.

I have an assortment of microwave connectors and the easiest way to tell
the difference between them is to look at the air gap from center to
outer conductor.

--
Telamon
Ventura, California

In article .com,
wrote:

I have a Trompeter PDF tittled "Getting Reday for HDTV". Up unitl the
last few years it was common practice to use 50 Ohm male and female
BNCs in 75 Ohm systems. With a maximum frequency of under 5MHz the
mismatch had ":no" effect. Trompeter arns that older 75 Ohm patch
bays will not pass DTV signals, which can reach up to 1GHz, with
"serious distortion".

I have tried to find a link at Trompoeter, but haven't succeded yet.

These pages shows the difference between 50 and 75 Ohm BNC
connectors. http://sosnick.uchicago.edu/BNC_50_75.html
http://www.levitonvoicedata.com/supp...ote_detail.asp
?tnID=17 5

Almost all of the "older" professional video gear used 50 Ohm BNCs
this

includes Grass VallyD2, BetaCam(BetaCam digital uses real 75 Ohm
BNC), U-matic and Ampex quad and 1".

A local TV station had a major nightmare with their ~20 year old
Trompeter video patch bay. On pass through wouldn't kill the DTV
signal, put a 2nd pass and the video died.

A chief engineer explained that 50 Ohm connectors where "much less
expensive then 75 Ohms" This link gives a 5:1 price differeintial:
http://www.extron.com/technology/arc...tifyingcables1

So in short, if your male BNC has plastic shell on the inside of the
mating fingers it is 50 Ohm, if the female BNC has a plastic collar
around the center pin it is 50 Ohms.

For HF connections I think it is safe to say it will make no
differnce if eith4r 50 or 75 Ohm BNC connector is used. If only
HDTV/DTV were so tolerant, On the good side for us is the fact that
older, but perfectly

good NTSC "75" Ohnm video patch bays will become available on the
surplus market. I was given an older Trompeter 75 Ohm video patch bay
because of the 50 Ohm BNCs. Good deal for me!

Amphenol:http://www.amphenolrf.com/rf_made_si...hquestions.asp
says" At frequencies below 500 Mhz or so, 50 ohm connectors can be
used on 75 ohm cable (and vice versa) with acceptable performance
levels. The reason for doing this is that 50 ohm connectors are
generally less expensive due to their greater usage. Click here to
see a list of useful..."

This thread ought to be interesting......

What are you looking for? Do you want an explanation of why the
connector does not have a big effect on a 100MHz signal and yet has a
great effect on a 1 GHz signal for the same impedance mismatch?

--
Telamon
Ventura, California

Telamon wrote:

What are you looking for? Do you want an explanation of why the
connector does not have a big effect on a 100MHz signal and yet has a
great effect on a 1 GHz signal for the same impedance mismatch?


Have you ever used a TDR to look at a transmission line? A 1.5:1
impedance bump can cause problems and the more impedance bumps in the
path the more it degrades the signal.

The techs at Microdyne had both 50 and 75 ohm cables and adapters
available. I had someone tell me the video boards i had just tested and
calibrated were all bad. He was trying to use 50 Ohm cables in a 75 ohm
video test which caused the bandwidth to roll off to -3 dB at 16 Mhz
instead of 20 Mhz. It took all day to convince the old timers that they
had to use the right cables, because they had got away with the wrong
coax on the older, 5 Mhz systems for years.

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida

In article ,
"Michael A. Terrell" wrote:

Telamon wrote:

What are you looking for? Do you want an explanation of why the
connector does not have a big effect on a 100MHz signal and yet has a
great effect on a 1 GHz signal for the same impedance mismatch?


Have you ever used a TDR to look at a transmission line? A 1.5:1
impedance bump can cause problems and the more impedance bumps in the
path the more it degrades the signal.

Yes. Time domain reflectometry.

The techs at Microdyne had both 50 and 75 ohm cables and adapters
available. I had someone tell me the video boards i had just tested and
calibrated were all bad. He was trying to use 50 Ohm cables in a 75 ohm
video test which caused the bandwidth to roll off to -3 dB at 16 Mhz
instead of 20 Mhz. It took all day to convince the old timers that they
had to use the right cables, because they had got away with the wrong
coax on the older, 5 Mhz systems for years.

You need more bandwidth for digital signals because of the faster rise
and fall times. For data to be valid you need a large eye opening in
amplitude and time for margin.

There are two things of significance:
1. The amplitude of the discontinuity.
2. The length of time of the discontinuity.

#1 determines the amplitude of the reflection.
#2 determined the frequency it becomes significant.

The cable is another issue. I know you understand that the attenuation
per foot increases with higher frequency. This can occur more rapidly
with digital signals due to the faster than analog transition times. For
digital you need a path with 3.5 times the bandwidth.

You have to understand another consequence of reflections on a
transmission line and that is the amplitude of the signal varies over
the length of the line due to constructive and destructive interference
of the forward and reverse waves so it is possible to measure a bigger
swing than you set the generator to drive the line with in some places
on the line. I have seen this confound more than one person until I
explain this to them.

To test a path the best thing to do is set the generator to produce a
1/0 pattern, which is the highest frequency of the bit stream and look
at the far end with a scope. Swap the cable or connector and measure
the change in attenuation.

If the impedance of the cable is wrong you will lose signal amplitude
due to reflection along with the loss per foot number.

--
Telamon
Ventura, California

Telamon wrote (in part):

You need more bandwidth for digital signals because of the faster rise
and fall times. For data to be valid you need a large eye opening in
amplitude and time for margin.

There are two things of significance:
1. The amplitude of the discontinuity.
2. The length of time of the discontinuity.

#1 determines the amplitude of the reflection.
#2 determined the frequency it becomes significant.
--------------------------------------
I started this thread because starting now and increasingly
in the near future there will be "lots" of analog "75 Ohm" video
patch bays, and patch hairpins, and patch cables dumped as the
older ones degrade digital signals. At one local station they found
out they could go through one set normally normaled analog patch.
Inserting eithr a hairpin, short patch used to connect adjacent non
normally connected patches, or any patch cable "killed" the digital
data stream.

I was given a~25 year old 48 bay patch with hairpins and cables.
And while my current antenna system is "50 ohms", this bay passes
signals in the .1`~30MHz range with "no" attenuation. By "no", I can
dirrectly feed the attenuated output from my ancient HP test generator,
adjust it for a just audible signal, then run it through the patchbay,
and it
is still there.

Beats the heck out of my homebuilt BNC bay, and I can now seperate
my receivers from my transceivers.

I did a quick search and found Prompeter 75 ohm patch bays on Ebay
for less then $20. For anyone with a mix of receivers, converters,
filters,
and antennas, this could be a very cost effective routing solution.

And for those who think that their SW receiver antenna input,
rated at 50 ohms, is really 50 Ohms, all I can say is "not very
likely".

As to the mismatch of using a 75 Ohm patch in a 50 ohm system,
I can run 146MHz through a patch set, terminate it in a Narda 50
Ohm load, and I can not measure any increase in VSWR. This is at
low power and one of my main reasons for wanting to not use my BNC
bay is to avoid, or at least decrease the chance of feeding the output
of my IC28A, or HTX100 directly into a receiver. While I haven't
managed it, I know one friend who did.

Terry