Topband
[Top] [All Lists]

Re: Topband: Mother of all ferrite common-mode coaxial chokes

Subject: Re: Topband: Mother of all ferrite common-mode coaxial chokes
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Fri, 13 Jul 2012 13:37:08 -0700
List-post: <topband@contesting.com">mailto:topband@contesting.com>
On 7/13/2012 12:09 PM, ZR wrote:
> ** Since when is using an established procedure called not knowing how 
> to measure? 

When one does not understand (or take into account) the limitations of 
the measurement method.  The most common method of measuring Z at RF is 
an S11 measurement in a 50 ohm system. This method has the limitation 
that VERY small errors in the data, or VERY small calibration errors, 
result in HUGE errors in the result when the unknown impedance differs 
from the system impedance by more than about 5:1.  The chokes he (and I) 
were measuring have impedances on the order of 5,000 ohms, so an S11 
measurement will yield erroneous data.

Quoting from Wes Hayward on the subject of measuring high impedances:

"It is also possible to measure an L or a C attached as a load on a 
bridge attached to the VNA.   This is termed a "reflection" measurement. 
      The results are similar and remain equally difficult.     The 
severe errors of this direct method are discussed in Agilent 
Applications note 1369-6.    Much better measurements are obtained when 
one uses a scheme called RF I-V where a radio frequency source is 
applied to an unknown impedance.   Then the current through the 
impedance and the voltage across it are both measured.   The vector 
ratio of the values is calculated to obtain a better complex impedance 
value.   This method is discussed in Agilent Applications Note 1369-2. 
<SNIP> N2PK has built his own version of the Agilent RF I-V scheme and 
has obtained much better data.

Also see a brief discussion of RF-IV in 
http://sdr-kits.net/DG8SAQ/VNWA/VNWA_HELP.pdf  which is the manual for 
the software for DG8SAQ's Vector Network Analyzer, and which supports 
the N2PK RF-I-V fixture.

73, Jim K9YC

_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK

<Prev in Thread] Current Thread [Next in Thread>