[SI-LIST] Re: AW: Just a simple question
- From: <Wolfgang.Maichen@xxxxxxxxxxxx>
- To: <ah.vinod@xxxxxxxxx>
- Date: Mon, 23 Jul 2012 09:50:40 +0000
When measuring L, did you short the far end oft the cable, i.e. connect center
conductor and shield together (at the end NOT connected to the multimeter)? You
need to do this, otherwise any value you get for L will be garbage.
For C though you must leave the end open.
Regards,
Wolfgang
-----Original Message-----
From: vinod ah [mailto:ah.vinod@xxxxxxxxx]
Sent: Monday, July 23, 2012 11:30 AM
To: Maichen Wolfgang (IFAG OP BE TTI THA 2)
Cc: Gert.Havermann@xxxxxxxxxxx; afu@xxxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: AW: Just a simple question
Hi All,
Just out of curiosity I was trying to measure the characteristic
impedance of RCA cable (video cable), which i never did before.
Ideally we should have 75 Ohms as characteristic impedance. Below is
the readings i got (by using Sqrt(L/c) method) by using standard
multimeter.
10KHz 1KHz
L 1.00E-06 3.10E-06
Henry
C 1.70E-10 1.90E-10
Farads
L/C 5.88E+03 1.63E+04
SQRT(L/C) 76.69649888 127.7332747 Ohms
But i am unable to understand why is the impedance readings different
with different frequency because characteristic impedance should have
minimal variation with respect to frequency right? At 10KHz, i am
getting close to 75 ohms but at 1KHz, the impedance is 127 Ohms......
Regards
Vinod A H
On Mon, Jul 23, 2012 at 1:44 PM, <Wolfgang.Maichen@xxxxxxxxxxxx> wrote:
> Well, at least as long as it is an ideal, homogeneous transmission line (or
> at least pretty close to ideal, homogeneous and lossless at the frequency
> range of interest), and if your "simple multimeter" can measure capacitance
> and inductance (many better units today can), then
>
> - measure total capacitance Ctot between center conductor and shield
> - measure total inductance Ltot between center conductor and shield, with
> center conductor and shield shorted together at the far end (then end NOT
> connected to the multimeter)
> - calculate the characteristic impedance as Zo = sqrt (Ltot/Ctot)
>
> Or if your "simple multimeter" can measure resistance in less than one round
> trip delay of your cable then you can simply use the Ohmmeter mode and get
> the result directly. With typical DMMs producing 2-3 reading per second, a
> cable from here a quarter the distance to the moon is just about sufficiently
> long for that.
>
> Otherwise, you will always measure the termination resistance (inifinite if
> the cable is open at the end) plus the Ohmic resistance of the cable, not the
> characteristic impedance.
>
> May I now ask what the reason for the question is?
>
> Wolfgang
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Havermann, Gert
> Sent: Monday, July 23, 2012 9:56 AM
> To: afu@xxxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] AW: Just a simple question
>
> I'd say: first verify that the statement is correct and present the data to
> proof it , or call it an unproven assumption
>
> BR
> Gert
>
>
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> -----Ursprüngliche Nachricht-----
> Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
> Auftrag von Herbert
> Gesendet: Samstag, 21. Juli 2012 16:48
> An: si-list@xxxxxxxxxxxxx
> Betreff: [SI-LIST] Just a simple question
>
> When I would say that I can measure the impedance of a coaxial cable with a
> simple multimeter, would you think that I am wrong under all circumstances or
> would you onle begin your answer with the words :
> " But only when... " ?!
>
> How free is your brain from fixed thinking pathes ?!
>
> Herbert
>
>
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