Power meters are often considered for mobile-telephone production testing, because they can quickly measure peak power, average power, and peak-to-average ratio. But an RF power meter has certain deficiencies that hinder accurate wireless handset testing. For example, an RF power meter cannot identify where in the frequency spectrum a device is transmitting its power. Because of this, a handset may pass a power test with a power meter, but transmit at the wrong frequency or suffer excessive spurious transmissions. Fortunately, frequency-domain analysis performed on spectrum analyzers and RF power analyzers can overcome these limitations. An RF power analyzer provides both power and transmission frequency information, and can also overcome false readings associated with an RF power meters limited video bandwidth.

In production, every mobile telephone is checked for carrier frequency, primary-channel power, adjacent-channel power, and alternate-channel power. Typically, such instruments as RF power meters, spectrum analyzers, and RF power analyzers are used for such testing, but the capabilities of and results from these tools can vary widely. To maximize measurement throughput and accuracy, the trade-offs of these measurement instruments should be considered.

RF power meters are used not only for measuring power levels in two-way radio systems, broadcast systems, and radar/satellite systems, but for calibrating other measurement instruments and probes. Unfortunately, an RF power meter is designed for broadband frequency coverage and cannot determine the carrier frequency associated with a power reading or whether the measured power is within the proper bandwidth. Normally, a spectrum analyzer is also needed to make these determinations.

An RF power meter is basically an untuned RF sensor or detector followed by signal-processing circuitry consisting of DC or AC-DC amplification stages and analog-to-digital-conversion (ADC) stages (Fig. 1). It is designed to measure RF power, including peak power, average power, and peak-to-average power ratio. It does not measure frequency, since all frequency information is lost before signal processing begins. While this is a drawback in some ways, no tuning is required for a measurement. If the input signal is anywhere in the frequency range of the detector, the meter will make a valid power measurement.

An RF power meter measures input signals in the time domain. An incoming carrier of constant amplitude will produce a DC output, while any amplitude variation will be reproduced in the output, as the instrument responds to the envelope of the incoming signal. Because of this, the instrument's video bandwidth is an important characteristic when peak detection measurements are required.

The video bandwidth of a test instrument, in simplest terms, is an indication of how fast it can track signal variations for peak power envelope measurement, and can be considered the range of frequencies occupied by all the modulating signals. For example, a wideband-code-division-multiple-access (WCDMA) signal consists of a 1950-MHz carrier modulated by a 3.84-MHz modulating signal; after detection, the signal that remains extends from 0 to 3.84 MHz. For an instrument to process that signal correctly, all circuits downstream of the detector must have a bandwidth greater than 3.84 MHz (and preferably 5 MHz). An instrument with a narrower bandwidth cannot follow or capture the peaks of the 3.84-MHz WCDMA signal envelope. Video bandwidth is not the frequency range over which the instrument itself can accept signals; it is an indication of how well the power meter can capture the peaks of a modulated waveform.

Time variations in the detected signal can come from several sources. The obvious source is pulse or amplitude modulation. For example, if the incoming signal is pulse-modulated at 1 MHz, the bandwidth of the circuits following the sensor/detector would have to be greater than 1 MHz to accurately measure the peak value, although average value measurement would still be accurate with a smaller video bandwidth.

How video bandwidth affects peak power measurements and time-gated average power measurements^a but not average power is illustrated in the following example. Two RF sinewave signals (in the 800-MHz range) whose frequencies differ by 1 MHz and have magnitudes equal to one are represented in Fig. 2:

Assuming the impedance is 1 for the convenience of power calculations, the power versus time (real-time power) of these two signals is represented as Eq. 1:

Substituting v₁ and v₂ by their sinewave equivalents, power versus time, P(t), is derived in Eq. 2, which indicates that P(t) consists of the power of two individual signals (v₁, v₂) and twice the amplitude product of the two signals:

Equation 3 is inferred by applying trigonometric identities^b into Eq. 2. Equivalently, P(t) consists of one DC signal and four frequency components, which are 2f₁, 2f₂, f₁ + f₂, and f₁ − f₂, respectively:

The average power is defined by and calculated from

which is the DC component of Eq. 3.

From Eqs. 3 and 4, the peak power depends on all components while the average power depends on the DC component only. If a power meter and power sensor has a 1-MHz video bandwidth, it will act as a typical low pass filter with a 3-dB rolloff point at 1 MHz. With regard to the power envelope, components 2f₁, 2f₂, f₁ + f₂ are filtered, leaving the DC offset and the |f₁ − f₂| = 1 MHz, periodic signal as seen in Eq. 3. If the video bandwidth is greater than 1 MHz, the peak value is 2. However, the |f₁ − f₂| term is attenuated in half (3 dB) due to the 3-dB rolloff at 1 MHz. This causes a 1.24-dB error for peak-to-average ratio.^c Video bandwidth affects the measurement of the peak of the power envelope but has no effect on average power measurement.

This might lead to the impression that a wider video bandwidth is always preferable to a narrower one, but that is not the case. For one thing, the greater the instrument's video bandwidth, the smaller its dynamic range and the greater the variation in linearity. For example, a video bandwidth of 5 MHz will give a dynamic range of −32 to +20 dBm, while a 300-kHz bandwidth will give a dynamic range of −42 to +20 dBm.