[SI-LIST] Re: Running analog video on micro-coax
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- To: <ldieguez@xxxxxxxxxx>, <joel@xxxxxxxxxx>, "SI-List" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 1 Jun 2006 17:51:52 -0700
Leonard
The driver is loaded into a 75 Ohm trace. The output of the trace sees a 25
Ohm resistor in series with a 50 Ohm Zo. So the output of the 75 Ohm trace
see 25 + 50 Ohms or 75 Ohms. If properly designed there should be minimal
reflections from this point. This system works if the end of the 50 Ohm
coax is well terminated and you have no reflection coming back. If the end
of the 50 Ohm coax is not well terminated and are getting reflections that
need to be taken care of at the driver's end then you will need to design a
matching network at the 50/75 Ohm junction. But with 100 MHz signals you
should be able to get a good 50 Ohm termination, i.e. no reflections from
the destination that need to be properly terminated at the driver's end.
That is why I suggested going to a matching network from suppliers like Mini
Circuits if Joel needed better performance.
The simple resistor, if suited to Joel's needs is the lowest cost system.
Next up the line would be the matching network. Adding a buffer is the most
expensive. From a private email Joel stated that he could not tolerate the
insertion loss of a passive network so it is likely he will have to use a
buffer.
You are correct in that looking back from the load's end of the line the 50
Ohm coax will see a 100 Ohm load but this is not be a problem if there is no
energy coming from the load back to this point.
Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx
www.teraspeed.com
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Leonard Dieguez
Sent: Thursday, June 01, 2006 5:33 PM
To: tom@xxxxxxxxxxxxx; joel@xxxxxxxxxx; SI-List
Subject: [SI-LIST] Re: Running analog video on micro-coax
Tom,
That does not make sense if you have a driver that is 75 ohm and put a
25 ohm resister in series with it you will get a source impedance of 100
ohms.=20
I would think that receive terminating to 75 ohms the original 75 ohm
signal, then re-driving the signal with a driver set up to drive 50 ohms
to a 50 ohm receiver is the way to go.=20
Any thoughts?
Leonard.=20
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Tom Dagostino
Sent: Thursday, June 01, 2006 4:26 PM
To: joel@xxxxxxxxxx; SI-List
Subject: [SI-LIST] Re: Running analog video on micro-coax
Joel
You could try a simple impedance matching network. The simplest would
be a
25 Ohm resister in series between your 75 Ohm driver and the 50 Ohm
cable.
You also might look at what Mini Circuits or other vendors have that
will
meet your requirements in surface mount networks.
Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx
www.teraspeed.com
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Joel Brown
Sent: Thursday, June 01, 2006 2:13 PM
To: SI-List
Subject: [SI-LIST] Running analog video on micro-coax
We have an application which requires running 75 ohm analog video over
coax for a distance of 75 Inches.
Conventional VGA cables are much to large and bulky because we need to
run the cable through hinges and cable retractors.
I have seen some micro-coax products from several companies which use
very small diameter cable.
However all of the micro coax products are 50 ohm and seem to be
targeted for digital signals such as LVDS.
One approach I am considering is to use a buffer amplifier to receive
the 75 ohm signal and then redrive it on the 50 Ohm cable.
The only drawback is possible signal distortion from the buffer
amplifier. The signal bandwidth is 100 MHz.
Any thoughts or ideas would be appreciated.
Thanks - Joel
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