HP 428B, Operating And Service Manual, Page: 16 / 52

Model 428B
SECTION IV
THEORY OF OPERATION
4-1. INTRODUCTION.
4-2. This section describes the overall operation of the Model 428B, the operating principle of the current probe and
the function of the different circuits of the instrument.
4-3. THEORY OF OPERATION.
4-4. The simplified block diagram of Figure 4-2 shows the basic operation of the Model 428B Clip-ON Milliammeter.
4-5. The probe clips around a wire carrying dc current and delivers a 40 kHz output signal which is proportional to the
dc current. For transducing the dc current into a 40 kHz signal, the probe requires a 20 kHz gating signal, as described
in detail under Paragraph 4-9, Current probe.
4-6. The 40 kHz output signal of the probe is amplified, detected and fed back as negative feedback current to the
probe head cancelling the effect of the measured dc current and thus reducing the 40 kHz output signal almost to zero.
The negative feedback current, being proportional to and magnetically almost equal to the dc current of the inserted
wire, is used to indicate the measured dc current.
4-7. The 20 kHz oscillator has two functions: First, it supplies a 20 kHz signal for driving the probe head, and also
provides a 40 kHz (second harmonic) signal for gating the 40 kHz Synchronous Detector.
4-8. Due- to slight unbalances, the probe head output contains a small 40 kHz signal, even with no dc current being
measured. A 40 kHz phase-shifter output cancels such residual 40 kHz signal (zero-set controls).
4-9. CURRENT PROBE.
4-10. The probe head is a specially designed second harmonic flux gate type of a magnetometer used to measure the
magnetic field around a wire carrying direct current.
4-11. The flux gate principle is easily understood by referring to the mechanical model shown in Figure 4-3.
4-12. Coil A (representing wire through probe), is energized with dc, producing a dc flux in the core. Armature is
rotating at a constant rate (F), gating the flux 2F times per second inducing a voltage of 2 F frequency in coil B. The
amplitude is determined by the dc in coil A.
Figure 4-2. Simplified Block.
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