Impedance meter integration has multiple design options for the various functional blocks in a sine-driven Z meter. In this third article in this series, some of the circuit alternatives are considered for the major Z-meter functional blocks.
The first function block of Z meters is the sine-wave oscillator. The optimal analog choice (those that dominate Z meter products) is the Wien-bridge oscillator. Shown below is a fragment of a reverse-engineered circuit diagram for a B&K 875A that includes the oscillator and its amplitude stabilization loop (Figure 1) and (most of) the bridge circuit itself (Figure 2). (To its credit, B&K did provide me a circuit diagram, but it was borderline readable. Some tracing ensued.)
A portion of the B&K 875A schematic.
The Wien-bridge oscillator.
The typical Wien-bridge oscillator is implemented as shown along with circuit derivation. This oscillator is attractive for design because only Ri needs to be varied to vary the amplitude, and it is grounded.
Referring back to Figure 1, the two frequencies of the B&K 875A meter are switched by 2/3 of U1, a 4053 CMOS analog switch. As a DPDT switch, it selects both the feedback and input resistances for the op-amp to change frequency. The amplitude is stabilized by adjusting the gain of the inverting feedback loop with JFET Q2. Its channel resistance varies with gate voltage, from op-amp U4B. The U4B input is the error quantity of the amplitude stabilizer, the input of 1.5 V from the 220kΩ divider is matched by the fed-back peak sine voltage, extracted by the U3B half-wave peak detector. Whenever the U3B input is positive, the feedback diode, D1, conducts, charging C5 to the peak voltage. U3A is a switched-gain voltage amplifier that feeds back bridge voltage vvx or vix. U3A gain selection sets VR, the sine-wave output amplitude, to two possible values.