HP 428B, Operating And Service Manual, Page: 17 / 52

Model 428B
Figure 4-3. Magnetic Mechanical Analogy.
4-13. The Model 428B head uses this principle in a
similar way. Figure 4-4 shows the basic concept of a
saturable flux gate.
4-14. A magnetic core in saturation loses permeability
and, therefore, is comparable to a core that has been
mechanically opened (low permeability due to air gap).
4-15. Coil C saturates the core periodically with a 20 kHz
signal, driving the small cores in and out of saturation
twice per cycle or once for each peak (positive or
negative) of the input current.(See Figure 4-6) The only
function of the 20 kHz signal is to gate the dc flux in the
core of the current probe.
Figure 4-4. Basic Flux Gate.
4-16. The 428B probe head is actually analogous to the
flux gate shown in Figure 4-5. The energizing dc current
produces flux path "A". Flux path "A" is periodically
interrupted by saturation of the (transformer type) core, a
result of the two flux paths "C". Note that the current
enters L3 and L4 from the same end and that the coils are
wound in opposite directions causing opposite magnetic
polarities and the consequent circular flux path (c).
4-17. The four coils in the 428B probe head serve 3
purposes: (a) To saturate the cores, a result of the 20 kHz
current that flows between pins 1 and 2. This current is
generated by the 20 kHz oscillator-amplifier circuits. (b)
To act as a secondary, picking up a chopped signal from
the
Figure 4-5. 428B Flux Gate.
Figure 4-6. Waveforms.
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