Triplett Mitigator Loop Coil – 3232 - Manual (Page 7)

SECTION 7: CHOOSING THE RIGHT FILTER
The signal output (voltage) of the Loop Coil increases in strength as the frequency of the magnetic field
increases. This is a “direct” effect which means that, for instance, if the frequency of the magnetic field
increases by 10 times, the output voltage of the coil will increase by 10 times.
With this in mind, the user can see that a magnetic field of a particular strength at 600Hz will produce an
output voltage from the Loop Coil that is 10 times stronger than the same magnetic field strength at
60Hz. This effect is inherent in all Loop Coils and does not represent a deficiency in the 3232 Loop Coil.
Because of this effect, the Loop Coil is more sensitive to higher frequencies.
To compensate for this effect, the MITIGATOR has a 20/F filter (pronounced “20 over F”), [1/F, one over
F]. The sensitivity of the 20/F [1/F] filter is just the opposite of the Loop Coil’s sensitivity. Consequently,
if the 20/F [1/F] filter is selected when the Loop Coil is being used, the “frequency response” of the
measurement will be “flat” meaning that all frequencies will be displayed with the same sensitivity.
Although this may seem to be the obvious and appropriate way to filter and display the various signals
coming from the Loop Coil, the 20/F [1/F] filter is usually not used by the typical craftsperson. The
reason for this lies in the characteristics of the magnetic field that we are attempting to measure. As
explained in the MITIGATOR Applications Manual, the magnetic field is composed of the fundamental
frequency, 60Hz [50Hz], and its harmonics. It is the general nature of these harmonics to gradually
decrease in level as their frequency becomes higher and higher. That is, it is likely that a higher fre-
quency harmonic, like 1620Hz [1350Hz] will be lower in level than a lower frequency harmonic like
540Hz [450Hz].
Remember that the 20/F [1/F] filter has a sensitivity which is opposite to the Loop Coil sensitivity. The
20/F [1/F] filter is LESS SENSITIVE at higher frequencies. Consequently, because the higher frequency
harmonics are naturally lower in level, and because the 20/F [1/F] filter makes the MITIGATOR less
sensitive at these same frequencies, if the 20/F [1/F] filter is used, these frequencies may become so low
in level that they will not be displayed on the screen of the MITIGATOR.
Is this important? Well.......Yes!
Why?
Because the user typically discovers that the magnetic field that is picked up by the Loop Coil might only
measure about 30 or 40dBrn. Why is it so weak? Because it really is of relatively low intensity!
Then how does this low intensity magnetic field cause noise on a telephone line? Because in the usual
situation, the length of exposure between the telephone cable and the power line is relatively long
(sometimes 5 to 10 miles or more). Over these long distances, the relatively weak magnetic field of the
power line will induce noise into the telephone cable. However, back to the Loop Coil output signal
level. Since the Loop Coil output level is typically low to begin with, selecting in the 20/F [1/F] filter may
further reduce the level of the higher harmonics and make them unmeasurable on the MITIGATOR. We
are interested in these harmonics because they are often indicative of problems on the power line.
Consequently, we don’t want to eliminate them from our measurements.
So, even though a 20/F [1/F] filter is the “proper” filter to use in order to examine the strengths of the
harmonics, it is often not used because the signal levels involved in the measurement are too low. In
such cases, the FLAT [NONE] filter or C-MESSAGE [PMSG] filter is used. The validity of the FLAT
[NONE] filter was just discussed. The absence of a 20/F [1/F] filter is, essentially, a FLAT [NONE] filter. In
general, the FLAT [NONE] filter does not affect the signals expected to come from the Loop Coil.
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