[SI-LIST] Re: Discussion: What is the extra attenuation if a 50ohm source interfacing a 75ohm cable which terminated with 75ohm?
- From: Rohit MISHRA <rohit.mishra@xxxxxx>
- To: "zhenwwan@xxxxxxxxx" <zhenwwan@xxxxxxxxx>
- Date: Thu, 7 Jun 2012 19:02:25 +0800
Hello Jason,
I can understand your dilemma but you shouldn't treat source output impedance
as a transmission line, due to its small size, it acts as a lumped element.
When source launch a signal into transmission line (75 Ohm), It will see
transmission line as a resistive load and thus launch voltage will be decided
by voltage divider between source o/p impedance and transmission line
characteristics impedance and as Steve pointed out loss will be only -4.44dB.
If transmission line wouldn't have terminated with 75 Ohm in your case then
reflected wave would have seen o/p impedance of source as a load and reflected
coefficient for reflected wave would be 50-75/125 = 0.2 i.e. 20 % of reflected
wave energy would have re-reflected.
Hope that helps.
Rgds,
Rohit Mishra
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Zhenwei Wang (zhenwwan)
Sent: Thursday, June 07, 2012 2:36 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Discussion: What is the extra attenuation if a 50ohm
source interfacing a 75ohm cable which terminated with 75ohm?
Error in description about calculation: the attenuation =
20lg(0.5)-20lg(0.4)=(-6db)-(-7.95db)=-1.95db. Sorry for inconvenience.
BR
Zhenwei(Jason) Wang
Cisco Systems (Shanghai) Video Technology Co., Ltd.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Zhenwei Wang (zhenwwan)
Sent: 2012年6月7日 16:42
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Discussion: What is the extra attenuation if a 50ohm source
interfacing a 75ohm cable which terminated with 75ohm?
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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