[SI-LIST] Re: Discussion: What is the extra attenuation if a 50ohm source interfacing a 75ohm cable which terminated with 75ohm?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 07 Jun 2012 03:04:50 -0700
No, assuming ideal components PointA acts like a simple divider: 0.6V/V
= -4.44dB versus a zero impedance voltage source directly driving the
cable. Were the cable lossless that wavefront would travel a cable of
arbitrary length. The terminator at Point B if perfectly matched, makes
the cable look like it goes on forever: In the ideal case there is no
attenuation and no reflection at PointB.
Steve.
On 6/7/2012 1:41 AM, Zhenwei Wang (zhenwwan) wrote:
> Hi experts,
> Today I have a dicussion with my colleague. The topic is What is
> the extra attenuation if a 50ohm source interfacing a 75ohm cable which
> terminated with 75ohm resistor? Just like that when using a 75ohm cable
> connected to a 50ohm output instrument.
> The scenario is:
> Source(50ohm resistor)===>PointA===75ohm cable===>PointB
>
> ||
>
> ||(75ohme resistor)
> GND
>
> My opinion: Based neglecting the multi-refection, there will be
> refection factor at pointA, which is 0.2. So 80% energy was launched
> into 75ohm cable. Then the voltage at pointB is Vin*0.8/2=Vin*0.4. But
> it should be 0.5*Vin at pointB. So the extra attenuation is
> 0.5-0.4=0.1==>-20db.
> Am I right?
>
>
> BR
> Zhenwei(Jason) Wang
> Cisco Systems (Shanghai) Video Technology Co., Ltd.
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