RF Power Measurement Methods for Portable Systems

Designing low power circuits while achieving acceptable performance is a difficult task. Doing so in the RF bands increases the challenge dramatically. Today, nearly everything has wireless connectivity, so RF power measurement is quickly becoming a necessity. This article highlights several useful methods for accurately measuring RF signal levels to optimize the performance of these wireless systems. This article discusses optimization methods to meet the needs of a variety of different applications.

RF signals can take many forms, from single-carrier continuous wave (CW), multi-carrier continuous wave to QAM (quadrature amplitude modulation) with high crest factor waveforms. Measuring the power of these varying signals requires understanding their characteristics and the required measurement accuracy. If the signal is bursty, such as the signal in a TDD (time division duplex) system, the measurement becomes more complicated because of time domain measurement considerations. In any case, choosing the right detector type can help simplify the design task.

Measure RF power with peak detection

Take the simplest case of measuring a CW waveform. Even though the amplitude may vary, as long as the signal is within a specified time interval with a relatively constant amplitude, then for all practical purposes it can be accurately measured with a peak detector such as the Linear Technology LTC5532. This device is built with a very fast Schottky detector, with on-chip temperature compensation and a 2MHz bandwidth output buffer. The internal Schottky circuit detects the input RF signal in a peak-detection manner and performs peak-hold filtering to produce a DC output voltage proportional to the peak value of the RF input .

The LTC5532 is a very low power device, running at 500uA supply current in active mode. However, its internal Schottky circuit can detect 7GHz RF signals. A version of the device, the LTC5532EDC, is available in a 6-lead, 2mm x 2mm plastic DFN package with low parasitics and can support operation up to 12GHz and higher.

Figure 1 shows the RF input of this 12GHz detector, which is matched from 11.5GHz to 12GHz. Its input circuit can therefore be connected to the coupled output of a directional coupler or to an RF source. The detector output gain is set externally by resistors R2 and R3 (10k each) , compensating the loop around the internal amplifier (which has a non-inverting gain of 2). At 12GHz, circuit board materials can introduce circuit parasitics that can affect the input impedance match. However, acceptable performance can be achieved with standard FR-4 PC board materials. The RF input match consists of two 1.2pF capacitors, C1 and C3. The C3 capacitor also acts as a DC block, since the RF input of the device is internally DC biased. The RF input match may need to be re-optimized for each specific application layout or other operating frequency. The RF input return loss was measured to be 10dB at 12GHz. The graph in Figure 2 depicts the detector's transfer characteristics when a 12GHz RF input signal passes through the -24dBm to 8dBm (effective detection range).

Figure 1: A 12GHz RF peak detector circuit.

Figure 2: 12GHz detector characteristics.

Measuring Low Level RF Signals with a High Dynamic Range Detector

For applications that require the measurement of very low level RF signals, a high dynamic range detector with increased sensitivity is necessary. This type of function is often used in receivers that measure RSSI to provide AGC (Automatic Gain Control) feedback control. Other applications include field strength meter instruments. For this type of signal measurement, the logarithmic detector is very suitable because it measures the average power of the signal. In addition to having a large dynamic range and extremely high sensitivity, logarithmic detectors have excellent bandwidth characteristics that extend to low frequencies. Their output provides a fixed output slope in a mV/dB log-linear ratio, which is convenient for use.

An example of a high dynamic range logarithmic detector circuit is shown in Figure 3. The LT5538 is a logarithmic detector manufactured by Linear Technology that has a dynamic range of over 60dB. Although this IC can operate in the frequency range of 40MHz to 38GHz, the circuit shown is designed and properly matched from 40MHz to 2.2GHz, covering a wide frequency range including all cellular bands. This detector can resolve a signal as small as -68dBm. Its dynamic range covers nearly 70dB with ±1dB accuracy. At lower frequencies (for example, at 880MHz), its dynamic range improves to 74dB.

Figure 3: A high dynamic range log detector circuit.

Temperature drift is a problem for high accuracy instrumentation and many high performance wireless systems such as cellular base stations. Typical accuracy desired is 1/2dB or better, and this tolerance is maintained over the specified temperature extremes. The LT5538 achieves this desired accuracy over a wide dynamic range, thus minimizing the need for tedious calibration operations over temperature.

The LT5538 draws 29mA of supply current, which is required to achieve the maximum operating frequency of 4GHz. The device has a shutdown function. In sleep mode, the device consumes less than 100uA of quiescent current. The device can be turned on and start measurement in 300ns. Therefore, this detector facilitates burst mode measurements, saving power in portable applications. [page]

How to measure the true power of high crest factor signals

Modern broadband wireless data systems use complex modulation waveforms. For example, WiMAX and LTE (fourth generation, long term evolution) use multiple carriers, each of which is modulated with high-order QAM. These RF signals have peak-to-average ratios of up to 12dB and are inherently non-periodic, making accurate measurements difficult. Lookup table calibration is often attempted, with limited success for simple modulation waveforms. However, with the trend toward more complex modulation, calibration with lookup tables becomes inadequate.

A new RMS detector from Linear Technology, the LT5581, helps resolve these inaccuracies. The device features an on-chip RMS measurement circuit that makes highly accurate power measurements of high crest factor signals. It can measure signals from 10MHz up to 6GHz. It has a 40dB dynamic range at lower frequencies and 30dB at high frequencies. In addition, the device offers excellent accuracy over the entire temperature range, thus providing repeatable measurements. With all its features, the device consumes only 1.4mA of supply current. The RF input is single-ended, so no RF balun is required. Its wide bandwidth enables multi-band radios such as 3G or 4G broadband wireless data modem cards, 3G or 4G smart phones, WiMAX data modem cards, and high-performance portable radios.

The single-ended RF input is ideal for tapping directly from an RF signal source (e.g., an RF PA amplifier ). An example of this implementation is shown in Figure 4, a 5.8GHz WLAN or WiMAX transmitter PA amplifier power control circuit. The detector's RF input is connected to the PA output through a 20dB resistive attenuator (composed of a 604Ω and 75Ω voltage divider). This resistive tap eliminates the need for a directional coupler and saves cost. The 1.8pF DC blocking capacitor serves to match the detector impedance. The entire resistive tap circuit results in <0.2dB insertion loss at the PA output, which is quite moderate.

Figure 4: A 5.8GHz RMS detector implementation.

For improved coupling accuracy, the 604Ω and 75Ω resistors should be 1% tolerance components, and the 1.8pF should be 5% or better. The recommended component values ​​for the resistive tap are for reference only. In actual implementation, these values ​​may vary slightly, depending on component placement, PC board parasitics, and PA and antenna parameters. However, using a directional coupler has the benefit of providing some directionality, which is not available with a resistive tap circuit. That is, if the PA has excessive reflected power, the coupler will likely isolate that power and will have minimal effect on measurement accuracy. This is not the case with a resistive tap circuit, which may introduce small measurement errors.

Figure 5 shows the detector transfer function as the PA amplifier output is swept across the entire power range. At 5.8GHz, the detector provides 25dB dynamic range performance, which is generally sufficient for power control purposes. At lower frequencies (such as 2.1GHz or 880MHz), the LT5581's dynamic range improves to 40dB.

Figure 5: 5.8 GHz detector response.

in conclusion

Depending on the signal being measured, different RF detectors are available to provide the best solution for the measurement needs. Schottky peak detectors are well suited for fixed amplitude power measurements, as long as the dynamic range is limited. Logarithmic detectors have a large dynamic range and excellent sensitivity to measure low level signals. For high crest factor signals, RMS detectors produce the most accurate measurement results.