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Peak Wattmeter Board Information
Peak Wattmeter Board Information, Page 3

Stability, Damping, and Bandwidth Issues

Stability
Most peak detector circuits have elements operating at high gain. Stability cannot be overlooked when addressing accuracy and tracking problems. The unstable circuit may exhibit non-linear operation, and inaccurate readings at certain levels of operation. In order to stabilize some circuits, the compensation circuit may effect bandwidth and the ability to track low duty cycle pulses.

Damping
In order to “catch” short peaks and high frequencies, the peak detector circuit must be quite fast. Observing the operation of many peak detector circuits, we can see over damped and under damped performance. In the case of under damping, the peak detector will overshoot by some amount, which leads to readings that may display higher than the actual correct value.

In over damped circuits we often see the leading edge of the peak is “ignored” due to the circuit damping (slow response time). This type of problem leads to low readings and loss of accuracy at higher modulation frequencies.
 
Circuit Topology
When selecting components for a peak detector circuit, all of the above (and more) need to be considered. Other factors include the need for battery power operation, the requirement for powering the unit with either single or bi-polar power supplies, and of course, over all cost.

Many operational amplifier IC chips do not have the combination of high slew rate and low quiescent current to function well in a battery operated circuit. Others may have a requirement for dual voltage operation. (leading to the cumbersome and expensive requirement of two batteries).

Since in most cases the signal output from the power sampling element is very low and is referenced to ground, the amplifier must function with common mode voltages at or below ground. Many circuits will not operate properly this close to ground, and this forces the requirement of dual power supplies, and sensitivity to radiated RF disturbances.

In any case, the circuit must be optimized and tested to insure that it will function over time, and across the require temperature range. It must be stable, and have the required bandwidth to detect and display peaks in the frequency range that is required for the application. Cost is always a consideration, but as can be expected, the most expensive units are not always the best, and the least expensive units are not always the
worst. In general however, “you get what you pay for”.
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