Nothing Average About Power-Sensor Averaging: What You Need to Know
by Orwill Hawkins, LadyBug Technologies LLC
“What should I set my power sensor’s averaging to?”
This is one of the most common questions posed by users of power sensors. It is asked by new users as well as the more advanced, and often by engineers creating test systems with programmatic remote control.
Here we will explain averaging, what is occurring in the sensor, and how to best determine the requirements for your specific needs and situation.
What is “Averaging?”
What is “averaging?” Modern power sensors use a detector circuit coupled to analog-to-digital (A/D) converters to measure signal power. The sample conversion occurs at a very rapid pace, often over a million times per second on any input signal, including CW signals. This rapid conversion of the signal makes it possible to measure very short variations in power level, including noise, by high-sensitivity sensors such as LadyBug’s LB480A. Regardless of the detection system employed, most modern power sensors utilize a fast analog to digital (A/D) converter system in order to deliver the greatest possible flexibility.
To manage the measurements and create a stable result, individual digital converted values are averaged together as selected by the user. This is what we call “averaging.” In some instances, averaging is referred to as integration or integration time. For our purposes we will use the term “averaging.” Averaging may be referred to in units of time or the number of samples. We generally refer to averages in terms of the number of samples.
The samples may already have been pre-averaged by analog or digital methods. For example, in a diode detector circuit, the diodes will drive a capacitor, and this capacitor will establish the initial averaging. In many test system environments where measurement speed is very important, users may choose set averaging as low as possible, particularly if the signal is at a relatively high level.
A user can set a LadyBug LB479A or LB480A to continuously collect over 2,000 pre-averaged measurements per second. Pre-averaging in these sensors amounts to 124 samples for each of the 2,000 measurements. In this case, the downside can be measurement variation due to noise or other signal-related factors on low level signals. Conversely, a power sensor user replacing a traditional thermal sensor and desiring a very stable measurement on a CW signal may choose to set averaging to a very long period of time. Longer averaging periods are often used to average out noise on low level signals.
The Effects of Noise
Even though your settings may be different, whether you are looking at a CW signal or a modulated signal, averaging works in the same way. A good example for the use of averaging is noise reduction.
Noise is always present in any electrical system. Further, all components generate electrical noise in some form or another. A Google search for “resistor noise” will generate voluminous results and may help put noise sources into perspective. Noise reduction in passive components is a science in itself. As a result of noise power, with no user signal present, there can be measurable power that may interfere with a low-level signal when applied. Figure 2 depicts a power sensor input system with internal noise sources indicated.
Today’s top-shelf power sensors generally have a very wide dynamic range and may be capable of measuring signal levels down to the sensor’s noise floor. To make low-level measurements, all noise sources must be mitigated. Averaging is one tool used to do this. Because noise is random in nature, averaging the noise samples and processing the information can eventually result in a stable, meaningful measurement on extremely low-level signals.
Noise can be averaged to a single value; however changing conditions may cause this stable value to shift. For example, if the temperature is changed, the level of noise generated by electronic components will also change. With LadyBug sensors, this is largely managed for you already, via a patented No-Zero system. These patents allow for management of zero offset and eliminate the need to zero the sensor prior to use. Further, LadyBug sensors account for thermal change, making it unnecessary to stop measuring in order to zero. This is a significant advantage; many sensors with built-in zeroing circuitry must stop making measurements to perform the zero function. This can often take an additional 10 – 20 seconds.
Averaging Power from Modulated Signals
Signal variation is an important consideration when setting averaging for a stable, accurate average power measurement. Signal power can vary for various reasons, both intentional and unintentional. Today, there are many modulation schemes that must be tested in manufacturing, in the laboratory, and in the field. With sufficient averaging, any modulated signal can be smoothed into a stable value. Known factors such as pulse repetition rate can be applied to the stable average power to acquire the desired information.
In the example shown, a pulse stream is depicted with a repetition rate of 10ms and a pulse width of 1ms. The signal shown in Figure 3 with an averaging time of 5ms would result in an inaccurate average power measurement. The power meter will display erroneous data as the 5ms averaged measurements are reported.
In this case, averaging of 100 times the 10ms repetition rate (1 second of averaging) would result in a fairly small error. If higher accuracy is desired, averaging should be increased further. Averaging of several seconds may be needed. LadyBug sensors can also be set to employ extended averaging to optimize power measurements. Extended averaging uses exponential averaging along with normal averaging and can be used to stabilize the measurement and provide fast averaged measurement times.
Time and Number of Averages
The LadyBug LB479A sensor continuously collects data at over 2,000 measurements per second. Each measurement is considered a single average. It is easy to convert from the number of averages to averaging time using the formula below:
Averaging Time = Number of averages/2,000 or
Number of Averages = Averaging time * 2,000
Therefore, setting averages to 2,000 will result in about 1 second of averaging on a LadyBug sensor. In reality, each of the 2,000 measurements per second is pre-averaged by the sensor’s analog front end, digital processing, and any additional digital filters the user may engage.
Noise Reduction: Visual Results
In the case of a pulse-profiling sensor with trace measurement capability such as the LB480A or LB680A, the entire repetitive signal can be averaged. This is a great way to visualize the signal and results of averaging for noise reduction.
The following example was accomplished with an LB680A, 20 GHz Pulse Profiling Power Sensor. A 1.9 GHz signal modulated with a 1us pulse repeated every 11us, and 50db of attenuation was applied to the sensor, and then various averaging scenarios were implemented.
With this low-level pulsed signal, the results of averaging can easily be seen as the noise floor becomes cleaner and cleaner as the number of averaged measurements is increased. See Figures 4, 5, and 6.
In Figure 4, the attenuated pulsed signal with no averaging applied is buried in the noise, and the pulses are nearly indiscernible.
In Figure 5, using the same signal, the sensor is set to apply 20 averages and the signal begins to appear. Finally, in Figure 6, the sensor is set to average the low-level repetitive signal 100 times.
As can be seen, the signal can now be examined with sufficient detail to gain important information. LadyBug’s LB480A can automatically provide statistical and triggered pulse information on the signal. The LB479A sensor can measure statistical peak & pulse power and duty cycle on a low-level signal such as this one, without triggering.
Summary
There are several primary considerations needed to determine the amount of averaging required for making RF power measurements. Some of these are noise and power level, signal type and stability, required accuracy, and required measurement speed. While these can be quantified fairly easily, the interaction between some or all of them often requires user judgment.
LadyBug offers a broad selection of sensors that include very flexible methods to average CW and modulated signals. LadyBug’s LB5900 product line also includes selectable analog filters along with the already present digital filters that may be employed to pre-average incoming signals.
Using these methods and a LadyBug sensor, fast, reliable, accurate power measurements can be made on any signal between 9 kHz and 40 GHz.
About the Author
Orwill Hawkins serves as Vice-President of Marketing at LadyBug Technologies, Santa Rosa, Calif. He has over three decades of experience in management, marketing, engineering, and manufacturing.
Orwill has extensive hands-on design and manufacturing experience in the RF, analog, and digital fields. Among the many products he has designed and marketed are a self-contained RF field-disturbance burglar alarm system; a sailboat speedometer; and various robotic servo systems. Additional inventions include a prototype oscilloscope and a CNC cutting system. He can be reached at orwill_hawkins@ladybug-tech.com.
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