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.)
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.
A nifty variation on loop stabilization is used in the Elektor Z meter design of H. Kuehne. It uses no JFET. The block diagram for the amplitude stabilizer is shown in Figure 3:
The design goal is to synthesize, at the oscillator frequency, the required Ri using a multiplier with feedback. The LM13700 is used as a variable-G ZM amplifier with output resistance of
Zi ≈ 1/(GM •IY •H)
where H is the voltage feedback attenuation. A simplified circuit diagram of half of the LM13700 is shown in Figure 4. This hidden treasure in the parts catalog has a BJT diff-pair input with matching base diodes and controllable emitter current-source BJT. The diff-pair output is made single-ended with current mirrors. Diodes Q11 and Q12 are unused. The LM13700 input voltage is the Zi -loop error voltage, which is near zero, and approximate multiplication is accurate enough. It is multiplied by IY which controls Zi .
The complete amplitude control loop with oscillator is shown below in Figure 5 with a more detailed derivation of component values. The upper-right op-amp is the oscillator.
The oscillator sine-wave amplitude is extracted using a synchronous integrating peak detector. (Its derivations are in the boxed section to the right on the above notebook page.) During the positive half cycle, the zero-crossing comparator (LM13700 section B) turns on a switch that inputs the sine-wave to the integrator, along with a constant offset current, Io . The result is that the average sine-wave current into the integrator must equal the offset current out of it. The integrator output voltage is controlled by the loop to be some value of VY that sets IY and hence the transconductance gain, GM , of the LM13700 (section A).
Besides Wien-bridge oscillators, another type is the quadrature oscillators. The typical Z-meter implementation is shown in Figure 6:
It requires two op-amps but has the advantage of also generating a quadrature sine-wave (offset by 90° from the output of the first op-amp). Analog phase detectors need 0° and 90° waveforms to extract the resistive component of Zx and the quadrature oscillator provides it. Note however that changing the frequency requires varying three resistors.
The final category of sine-wave generators is digital, either by dedicated register logic (direct digital synthesis, DDS) or by microcontroller (μC) programming. DDS is used in waveform generators and is a more accurate and stable method for generation, though an analog phase-locked loop can also provide high frequency accuracy. A μC writing at regular intervals to two DACs can also output both sine and cosine waveforms for bridge excitation and as phase detector inputs.
All circuitry comprising the above oscillators and their amplitude control loops can be integrated, even the adjustment pots as digipots, if necessary. With μC calibration methods, however, few of these otherwise many adjustments found in Z meters are necessary.
- Z Meter on a Chip? Impedance Meter Range Capabilities
- Z Meter on a Chip? Impedance Meter Bridge Circuits
- An Instrument on a Chip? The Minimum-Subsystem Instrument
- An Instrument on a Chip? The Configuration Problem
- An Instrument on a Chip? Some Emerging Instruments & the China Factor
- An Instrument on a Chip? A Look Back
- Thermocouple Nodules, Cold Junctions & Integration Opportunities