[SI-LIST] Re: Amplitude loss ?
- From: rajneesh shukla <rajneeshs123@xxxxxxxxx>
- To: Gwen and Wolfgang <gwmaichen@xxxxxxxxxxxxxx>
- Date: Fri, 6 Jan 2012 19:01:12 +0530
Thanks a lot wolfang !!
Your explanation makes sense. I shouldn't mix up transmission line with
lumped ckt.
In your example of 20mA current driving in to cable, If I assume, total I
mA is going down through dielectric conductance so output voltage at other
end of Xmission line will be 19max50 = 0.95 Volt, which comes to insertion
loss of -0.44 db, right ?
I am sorry if i am going too much in detail but I don't wanna lose my sleep
tonight, thinking about this problem :-)
On Fri, Jan 6, 2012 at 5:45 PM, Gwen and Wolfgang
<gwmaichen@xxxxxxxxxxxxxx>wrote:
> The mistake here is that you only consider the driver impedance and thus
> assume the supplied current (and power) can be arbitrarily high regardless
> of the load. But that's not true down the transmission path: E.g. a driver
> with 1V internal swing driving a 50 Ohm cable would output only 20mA into
> the cable. Some distance further down the cable the source impedance
> driving the rest of the cable is actually 50 Ohms (the cable impedance),
> not 0 Ohms, irrespective of the driver impedance, so any shunt impedance
> will add load and reduce the signal amplitude (voltage step) continuing
> down the line. Only in the steady state (after several round trip delays,
> so the reflection have settled out, and this also assumes no further
> transitions of the driver state during that time!) the driver impedance
> will be what determines voltage and current. So your (wrong) result comes
> from mixing up lumped picture (prop delays too small to play a role) with
> the transmission line (distributed) picture (looking at time scales
> comparable to or smaller than the prop delay).
>
> Regards,
>
> Wolfgang Maichen
>
>
>
> On 1/6/2012 12:14 PM, rajneesh shukla wrote:
>
>> Thanks Steve but question remains unanswered !!
>>
>>
>> I agree there will be a loss in energy due to dielectric loss but I was
>> talking about loss in amplitude of signal (voltage) not loss in total
>> energy, Energy loss does not always translates into voltage loss, right ?
>> Receiver senses voltage( Potential Energy per unit charge) not total
>> energy.
>>
>>
>>
>> As per basic circuit theory, If I have a driver that has zero o/p
>> impedance
>> is driving a 50 Ohm load and now if I connect 100 Ohm resistance across
>> this load, Though there is energy loss due to 100 ohm resistance but still
>> my load will see the same voltage, agree ?
>>
>>
>>
>> In transmission line, If series loss i.e. conductor loss is zero then I
>> don't find the reason of any voltage drop in transmission line coz
>> dielectric loss which is shunt loss can only dissipate total energy but
>> can't change amplitude(voltage) of signal.
>>
>>
>>
>> Do you find any discrepancy in this analogy ?
>>
>>
>> On Fri, Jan 6, 2012 at 2:40 PM, steve weir<weirsi@xxxxxxxxxx> wrote:
>>
>> Follow the energy. Achieve enlightenment. Of the energy that a shunt
>>> draws, how much of that energy remains available to the signal?
>>>
>>> Steve.
>>> On 1/6/2012 12:54 AM, rajneesh shukla wrote:
>>>
>>>> Experts,
>>>> As we know for perfectly matched line insertion loss is due to ohmic&
>>>> dielctric losses and due to these losses we see loss in rise/fall time
>>>> as
>>>> well as loss in amplitude of signal.
>>>>
>>>> My question is what actually affects amplitude loss, is it ohmic loss or
>>>> dielectric loss ? My understanding says, it should be ohmic loss as
>>>> it comes in series coz dielectric one is shunt loss. If this analogy is
>>>> correct then a channel having very low ohmic loss but significant
>>>> dielectric loss will have only changes in rise time but minimal
>>>> amplitude
>>>> loss, right ??
>>>>
>>>> Rajneesh
>>>>
>>>>
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