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Peak Wattmeter Board Information
Peak Wattmeter Board Information, Page 1

The information provided here is in a form to be understood by persons with a wide range of technical expertise. For more detailed information, please take the time to further investigate the subject presented.

Many older analog wattmeters were the average reading type, which did not include peak detector circuits. Simply stated, an average reading meter will not properly display short peaks, or high peak to average power signals. By average reading we mean just that. A 100 watt signal that is “ON” for 50% of the time, and “OFF for 50% of the time, would display as 50 watts. (the average power) The point is that the displayed power level would not react to short peaks. (This is an extreme and simplified example)

To measure short peaks , a circuit is required that will detect the peaks, and then sample and hold the peak level long enough for the slow analog meter to react and display the peak measurement. To accomplish this task, peak wattmeters utilize some type of R-C sample and hold circuit. The sample and hold circuit will include a discharge circuit to eventually discharge the hold function over a period of time. This allows the meter pointer to return to zero. Due to the internal circuit losses, the peak level can only be held for a finite period of time. Eventually the capacitor will begin to discharge from the peak level that the capacitor is holding if the signal is removed. Typically the discharge rate is determined to result in a reasonable length of time for the pointer to return to a lower level or to zero.

For extreme high end applications, clocked sample and hold circuits exist, that will read a short peak and set a digital variable resistor to match the peak voltage. This type of peak hold circuit can hold a peak reading indefinitely, because once the peak voltage is set by the variable resistor, the output will hold until the circuit is reset by a clock signal, or the power is turned off.

For most applications the R-C sample and hold method is sufficient, considering that the results are being displayed on a analog meter with limited accuracy as compared to a digital measurement system.
 
Important considerations
The peak detector and peak hold circuit must have sufficient frequency response to catch peaks occurring at the repetition rate and period that the user requires for the type of signal they will be measuring. For example, a very simple peak detector may not be able to fully charge the hold capacitor to the true peak voltage during a short duration peak, due to amplifier slew rate (how fast the amplifier can charge the capacitor) or how fast the hold discharge circuit discharges the hold capacitor before the reading can be stabilized on the meter pointer.

For most applications some amount of error can be tolerated, and the user must be aware of the possibility of inaccuracy in their measurements due to low peak detector and peak hold response time. A good peak detector and peak hold circuit must be able to faithfully display short duration and low duty cycle peaks, to give the user accurate results. Even voice modulated SSB (single side band) measurements require peak circuit bandwidth much higher than the typical 2 khz communication bandwidth. Remember that the RF output of a SSB transmitter is varying at a complex audio rate of the modulating signal, and that voice content can have very low duty cycle, and high repetition rates.
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