The most common RF transmitter measurement is RF power. Before performing any other tests, the engineer wants to know, “Is my DUT outputting power? Is it outputting power within specified parameters?” Testing RF power answers these questions and is the first indication of the system’s health.
Figure 1. The block diagram of a generic RF transmitter shows baseband information conditioned with signal processing,
and then modulated and converted to an intermediate frequency before the final conversion to RF for transmission.
RF power measurements take many forms, depending on the RF transmitter design and application. The RF output may be a simple continuous wave (CW) signal, a pulse, an analog modulated signal, or a complex digitally modulated transmission, such as an IQ or orthogonal frequency division multiplexing (OFDM) waveform. Among the types of RF power measurements, the steady state RF power of a CW tone is perhaps the simplest. Peak power, such as the overshoot of an RF pulse or IQ waveform, is the maximum value over some period of time. The power measurement can be averaged across a time period, such as across a series of RF pulses, yielding the average power. RF power can be integrated over a frequency band, as is the case for many mobile communication signals. Digitally modulated signals tend to be noise-like in nature and often have specified average and peak power values.
One accurate and cost-effective instrument for measuring RF power is the RF power meter. Because it’s a scalar instrument, incapable of measuring phase, care should be taken to minimize impedance mismatches whose reflections can add in and out of phase with the signal being measured, resulting in greater magnitude uncertainty. Power meters are broadband and not frequency selective, so any unwanted signals are averaged into the desired measurement. The dynamic range of most power meters is 50 to 70 dB. High-end power meters are capable of specialized measurements such as integrated power across a band and triggering for pulsed RF power. For these reasons, power meters are a good choice if the RF environment is well controlled.
Another instrument commonly used to measure RF power is a spectrum analyzer. With these more complex RF instruments, engineers can measure the individual spectral components across frequency. The absolute power measurement accuracy is not outstanding, typically + 0.5 to + 2.0 dB. However, the relative power accuracy (or amplitude linearity) is excellent. Therefore, calibrating at a given frequency with a known power level, from a signal generator, for example, results in corrected power measurements that are accurate across a dynamic range greater than 100 dB.
High-performance RF instruments, including vector signal analyzers and vector network analyzers, can measure magnitude and phase, offering the potential for greater error correction and measurement accuracy. In addition to the RF power measurements mentioned previously, specialty measurements can be made on pulsed and other complex RF waveforms. This includes pulse profiling, which measures the power across the envelope of an RF pulse, along with the average and peak power of the RF pulse. Complex IQ waveforms are measured in more of a noise-like fashion with measurements such as peak-to-average ratio, and complementary cumulative distribution function (CCDF).
Figure 2. RF Power Measurement of a Single CW Signal, a Broadband Signal Integrated Across a Frequency Band, and Pulse Envelope Signal Using a Vector Signal Analyzer