[SI-LIST] Re: NTSC/PAL interface issue
- From: "Lyndell Asbenson" <Lyndell.Asbenson@xxxxxxxxxxx>
- To: <telegrapher9@xxxxxxxxx>, <aramareddy.s@xxxxxxx>
- Date: Fri, 23 Jan 2015 17:21:32 -0800
This is very cool thanks Lyndell Lee Asbenson
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Dave Cuthbert
Sent: Friday, January 23, 2015 3:56 PM
To: aramareddy.s@xxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: NTSC/PAL interface issue
There are two issues: signal amplitude and frequency response. I dont' see
that the impedance mismatch affects differential gain or differential phase.
Adding a 150 ohm shunt at the board edge improves frequency response but
attenuates the signal by 2 dB. I would not recommend this. As to the
frequency response, if the mismatch degradation is (much) less than the
frequency response error of the encoder/decoder ICs, it is not a problem.
For the case of a 20" cable of 75 ohms and a 5" trace of 50 ohms (2.6 ns and
0.87 ns respectively) the signal rolls of by 0.003 dB at 10 MHz.
For the case of a 20" cable of 50 ohms and a 5" trace of 50 ohms (2.6 ns and
0.87 ns respectively) the signal rolls of by 0.037 dB at 10 MHz.
Dave Cuthbert
On Fri, Jan 23, 2015 at 4:31 PM, Dave Cuthbert <telegrapher9@xxxxxxxxx>
wrote:
> To clarify, the 20" cable is 75 ohms and the 5" trace is 50 ohms?
>
> Dave Cuthbert
>
> On Fri, Jan 23, 2015 at 7:07 AM, Aramareddy Sreekanth reddy <
> aramareddy.s@xxxxxxx> wrote:
>
>> Hi,
>> NTSC/PAL standard recommends 75ohm transmission line.
>> But i have used 50ohms trace on my board and designed the board.
>>
>> Topology is shown below:
>>
>> NTSC/PAL encoder--->20inch cable(50ohm cable)---->onboard
>> 5inches----> NTSC/PAL decoder.
>>
>>
>> What could be the impact?
>> Have anybody faced this problem?
>> Its very urgent for me, could you please reply.
>>
>>
>> thanks
>>
>>
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