[SI-LIST] Re: AW: Just a simple question
- From: hws <afu@xxxxxxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 23 Jul 2012 18:17:35 +0100
Hello Vinod,
I do not know, how your multimeter measures the inductance, but one
thing is clear at 1kHz you have a tenth of the inductive resistance than
at 10kHz, which can mean, that at 1 kHz just the tolerances are playing
with you ?!
If you can, try with a longer cable.
Herbert
Am 23-07-2012 10:30, schrieb vinod ah:
> 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
>>
>>
>> ----------------------------------------
>> Absender ist HARTING Electronics GmbH& Co. KG; Sitz der Gesellschaft:
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>> persönlich haftende Gesellschafterin: HARTING Electronics Management GmbH;
>> Sitz der Komplementär-GmbH: Espelkamp; Registergericht der
>> Komplementär-GmbH: Bad Oeynhausen; Register-Nr. der Komplementär-GmbH: HRB
>> 8808; Geschäftsführer: Edgar-Peter Duening, Torsten Ratzmann, Dr. Alexander
>> Rost
>>
>> -----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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