Peak Wattmeter Board Information
Peak Wattmeter Board Information, Page 2
Even the best peak detector circuit will add some amount of error to the measurement system. Many of the analog meters are rated for 3-5% accuracy. You can expect an additional 2% measurement accuracy effect when using a peak detector unit. On many analog meter faces this would represent an accuracy change of one minor division.
Repetition Rate (Rep Rate)
This specification is some times listed as PPS (Pulses per second) or the frequency (In Hz) will be listed. This is most often used for digital transmitting equipment. Rep Rate is the number of times per second that the transmitting signal is “ON”. It is important to also know the duty cycle of the signal that will be measured, because even though the pulses may be repeating at a very low frequency, they may be very short pulses. (low duty cycle)
Duty cycle is the ratio of ON time and OFF time. A signal with 50% duty cycle would be on for one half of the time frame, and off for the other half. Why is duty cycle important? Let’s use an simple example: What might be recognized as a low frequency signal is 500Hz. The period of a 500Hz signal is the time it takes for one full cycle. In the case of the 500 Hz signal, that would be .002 seconds or 2 milliseconds(2ms). A square wave with 50% duty cycle would have the signal “on” for 1 ms and off for 1ms. But, this is where the importance of duty cycle comes in. If the signal has a 5% duty cycle, it means the signal is “on” for only 5% of the time period. This results in an “on” time of 1 microsecond (5% of 2 ms). This means the circuit must be able to respond to an “on time” pulse that is very short, the same as if it was a pulse from a 10,000 Hz (10Khz) 50% duty cycle signal. To further complicate the situation, the off time is now increased to 95%. So the signal is “off” for 19ms. This equates to a 50% duty cycle signal at 263 hz.
As we can see, the peak circuit must be able to react to a very short pulse, by charging up the hold capacitor to the peak in the very short time it is allowed. Another factor is the ability to hold the detected peak voltage long enough for the slow analog meter pointer to move to the correct position, before the voltage begins
to decay due to the discharge resistance.
Many digital signals have low duty cycle modulation, and will require a peak detector, and hold circuit that will handle the short “on” time, and long “off” time of a low duty cycle signal.
For voice modulation using Single Side Band, the complex audio rate of the transmitted power requires a wide measurement bandwidth for many of the same reasons that a low duty cycle digital signal does.