Larry Benko wrote:
Roy,

It is amazing how a simple concept such as impedance can be made obscure
but most current probes are spec'd this way by the manufacturers and
test procedures for DO-160E (FAA aircraft testing) call out probes the
same way. By saying that a current probe has a transfer impedance of
0dB-ohm means 0dB relative to 1 ohm, but R = V/I so 0dB-ohm means that
for 1A of current thru the primary of the probe produces 1V across a 50
ohm load. Similarly a -20db-ohm transfer means that 1A produces .1V
across a 50 ohm load. For most of us, this means a 1 turn primary and a
50 turn secondary which yields a theoretical primary impedance or 0.02
ohms. One of the probes I have says the primary impedance is less than
.1 ohm.

Larry Benko, W0QE

Thanks for the explanation. My concern is with the insertion impedance,
which at 0.02 or even 0.1 ohm, is certainly adequately low for this
device -- as long as it's properly terminated.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Larry Benko wrote:

Roy,

It is amazing how a simple concept such as impedance can be made
obscure but most current probes are spec'd this way by the
manufacturers and test procedures for DO-160E (FAA aircraft testing)
call out probes the same way. By saying that a current probe has a
transfer impedance of 0dB-ohm means 0dB relative to 1 ohm, but R = V/I
so 0dB-ohm means that for 1A of current thru the primary of the probe
produces 1V across a 50 ohm load. Similarly a -20db-ohm transfer
means that 1A produces .1V across a 50 ohm load. For most of us, this
means a 1 turn primary and a 50 turn secondary which yields a
theoretical primary impedance or 0.02 ohms. One of the probes I have
says the primary impedance is less than .1 ohm.

Larry Benko, W0QE


Thanks for the explanation. My concern is with the insertion impedance,
which at 0.02 or even 0.1 ohm, is certainly adequately low for this
device -- as long as it's properly terminated.

Roy Lewallen, W7EL

That's why these probes which are about the size of a small donut cost
over $1000. Since they are used to qualify EMI emissions from other
equipment they are priced like test equipment and certified to be
accurate. Generally the frequency response if terminated properly is
flat to within a fraction of a dB. Occasionally they show up on eBay
and usually go for less than $100. With no parts to wear out they never
go bad unless they have been used to test equipment for EMI
susceptability where you transmit RF into them, sometimes at high power
levels.

Larry, W0QE

Larry Benko wrote:
Roy Lewallen wrote:
Larry Benko wrote:

Roy,

It is amazing how a simple concept such as impedance can be made
obscure but most current probes are spec'd this way by the
manufacturers and test procedures for DO-160E (FAA aircraft testing)
call out probes the same way. By saying that a current probe has a
transfer impedance of 0dB-ohm means 0dB relative to 1 ohm, but R =
V/I so 0dB-ohm means that for 1A of current thru the primary of the
probe produces 1V across a 50 ohm load. Similarly a -20db-ohm
transfer means that 1A produces .1V across a 50 ohm load. For most
of us, this means a 1 turn primary and a 50 turn secondary which
yields a theoretical primary impedance or 0.02 ohms. One of the
probes I have says the primary impedance is less than .1 ohm.

Larry Benko, W0QE
Thanks for the explanation. My concern is with the insertion
impedance, which at 0.02 or even 0.1 ohm, is certainly adequately low
for this device -- as long as it's properly terminated.

Thanks, Larry. "dB-ohm" was a new one on me, too.

That's why these probes which are about the size of a small donut cost
over $1000. Since they are used to qualify EMI emissions from other
equipment they are priced like test equipment and certified to be
accurate. Generally the frequency response if terminated properly is
flat to within a fraction of a dB. Occasionally they show up on eBay
and usually go for less than $100. With no parts to wear out they
never go bad unless they have been used to test equipment for EMI
susceptability where you transmit RF into them, sometimes at high power levels.

They also come in larger sizes than donuts. A friend who works in
radiation protection uses them to measure RF currents in the wrists and
ankles of workers, eg operators of machines for RF welding of plastics.

But all these things have to be tested first... which is how he found
*himself* with one of these big toroids clamped around his ankle,
standing outdoors on the chicken-wire groundplane of an HF monopole, in
bare feet, in November. All in the name of "Occupational Health"... but
definitely not his own.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek