HP手册上讲的很清楚啊
This circuit configurationcannot be used at frequencies higher than 100 kHz because of the performance limits of the operational amplifier.
Low frequency instruments, below 100 kHz, employ a simple operational amplifier to configure the
null detector and the equivalent of OSC2 as shown in Figure 2-5 (b). This circuit configuration
cannot be used at frequencies higher than 100 kHz because of the performance limits of the operational
amplifier. The instruments that cover frequencies above 100 kHz have an auto balancing
bridge circuit consisting of a null detector, 0°/90° phase detectors, and a vector modulator as shown
in Figure 2-5 (c). When an unbalance current is detected with the null detector, the phase detectors
in the next stage separate the current into 0° and 90° vector components. The phase detector output
signals go through loop filters (integrators) and are applied to the vector modulator to drive the
0°/90° component signals. The 0°/90° component signals are compounded and the resultant
signal is fed back through range resistor (Rr) to cancel the current flowing through the DUT. Even if
the balancing control loop has phase errors, the unbalance current component, due to the phase
errors, is also detected and fed back to cancel the error in the range resistor current. Consequently,
the unbalance current converges to exactly zero, ensuring Ix = Ir over a broad frequency range up to
110 MHz.
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