Many systems need to measure radio frequency (RF) power. Some examples include communications transceivers, instrumentation, industrial controls, and radar. Sometimes, these RF-power measurements are required to assure compliance with government regulations. In other cases, they help ensure efficient system operation. Over the years, the technology that is used to detect RF-signal levels has improved dramatically. From a primitive-diode beginning, it advanced to multi-function-detector integrated circuits (ICs).

Diode rectifier circuits have been used for signal-level detection for almost a century. This type of detection can be achieved with a very simple half-wave rectifier circuit. Such a circuit includes a rectifying diode, filter capacitor, resistor, and possibly an RF choke and a second capacitor. The two simplest detector circuits are half-wave rectifiers.

Until the early part of the 20th century, solid-state detectors were comprised of a crystal like galena (lead sulfide). Galena has rectifier properties when it's contacted by a metal. An improved detector was the point-contact diode, which consisted of a uniformly doped semiconductor into which a very small, pointed metal whisker was driven. Most frequently, this whisker was made of tungsten. The rectifying junction was formed where the whisker contacted the semiconductor. The metal whisker formed the diode's anode.

Point-contact diodes are still in production today. These diodes produce a very low forward voltage, very small parasitic capacitance, and reasonably high reverse breakdown voltage. All of these characteristics are advantageous for this diode's use as a detector. The point-contact diode also is a majority carrier device. As a result, its ability to rectify high-frequency signals is good. Many radar and communications receivers have utilized point-contact diode detectors.

The point-contact diode does have two major disadvantages, however. First of all, it is quite fragile. It also is difficult to manufacture in a repeatable manner. Vibration or mechanical shock can cause the whisker to be displaced or even temporarily or permanently lose contact with the semiconductor die. Consequently, the reliability of such a device is severely compromised—especially for mobile-equipment applications.

The RF performance of the point-contact diode is affected by the following: the location on the die at which the contact is made; the pressure exerted by the whisker on the die; and the deformation of the whisker due to the force that is required to make contact. Indeed, point-contact diodes are sometimes "tuned" by the manufacturer. In other words, the manufacturer measures their detector performance and then strikes them with a hammer to adjust the contact!

The pn-junction diode solves the mechanical-fragility problem of point-contact diodes. Of course, it also introduces a number of other problems. The pn-junction diode is formed by mating a layer of p-doped semiconductor material with a layer of n-doped material. Depending on the semiconductor material that's used, the forward voltage of a pn junction is much higher than the forward voltage of a point-contact junction. For example, a Germanium (Ge) diode produces a forward voltage of approximately 400 mV, while a Silicon (Si) diode produces about 700 mV. A Gallium-Arsenide (GaAs) pn diode creates a forward voltage of about 1.2 V.

Large forward voltages limit the sensitivity of pn-junction diodes to very small signals. Compared to a point-contact diode, the junction capacitance of pn diodes also can be an order of magnitude or more larger. When it is under forward bias, the pn junction temporarily "stores" minority charge carriers in its depletion region. For the diode to rectify, this charge must either be conducted out of the depletion region or undergo recombination. In the latter case, this process may take many microseconds to complete. The diode's ability to rectify high-frequency signals will therefore be poor. For these reasons, pn-junction diodes are almost never used as RF detectors.

Now, take a look at the Schottky detector diode. This diode has many of the advantages of a point-contact diode without the mechanical fragility. It's formed by depositing a very thin, very small layer of metal on a uniformly doped semiconductor die. Because of their physical contact, the Fermi levels of the metal and the doped semiconductor are forced to be equal. The difference between the metal-work function and the semiconductor material's electron affinity determines the barrier height. It therefore defines the forward voltage of such a junction.

Silicon Schottky diodes are commercially available in four different versions. They offer forward voltages of approximately 600 mV for high barrier, 330 mV for medium barrier, 280 mV for low barrier, and 180 mV for zero-bias detectors. GaAs Schottky diodes produce a forward voltage of approximately 700 mV. Like point-contact diodes, Schottky diodes are majority-carrier devices. As a result, they can switch impedance very rapidly—in most cases well under 1 ns.

Because a Schottky diode's junctions can be made very small, the junction capacitance can be correspondingly small. These two factors make Schottky diodes good candidates for use at the higher microwave and lower to moderate millimeter-wave frequencies. Note that Schottky diodes are extremely sensitive to electrostatic discharge (ESD). They also are easily damaged.

SOLID-STATE THERMOMETER
The performance of all of these diodes is sensitive to variations in temperature. Indeed, the pn junction is used as the temperature sensor in many electronic thermostats. Consequently, the output voltage of a diode detector isn't just a function of the input-signal amplitude. It also is a function of its junction temperature. This characteristic spawns the need to temperature-compensate diode-detector circuits. To accomplish such compensation with only a limited degree of effectiveness, add another diode. For a more effective approach, add another diode—used solely as a thermometer—as well as a differential amplifier. A practical diode-detector circuit isn't as simple as it initially appears to be.

The diode-detector transfer function can be divided into two distinct regions, which are known as "square law" and "linear" (FIG. 1). The square-law region is operative for very small input-signal levels. In this region, the detected output voltage is proportional to the square of the input-signal voltage. For larger input signals, the detector varies linearly with the input-signal voltage.

This phenomenon occurs with all of the diode types that are discussed here. One distinction does exist, though. For each of the junction types, the input-signal level at which this transition occurs is different. This transition from square-law to linear-transfer function does not occur abruptly as the input-signal magnitude approaches the transition region. Rather, it occurs gradually, so that there is little chance for a potential error in determining the input-signal magnitude.

LOGARITHMIC AMPLIFIERS
For signals up to 8 GHz, the IC-demodulating logarithmic-amplifier (log-amp) detector offers many advantages over diode detectors. Well-designed log amps can be much better than diode detectors when it comes to input dynamic range, input sensitivity, and temperature stability. The demodulating log amp consists of a series of cascaded linear-amplifier cells. The gain of each of these amplifier cells is typically the same: between 6 and 12 dB depending on the design goals for the log amp. Envelope detectors are connected to the output of each gain stage and at the input of the first gain stage.

The total voltage gain of a precision log amp, such as the AD8306, can be as high as 120 dB (a factor of one million). Even with no input signal applied to the first amplifier stage, the output stage is near compression. Such compression results from the cascaded amplification of internally generated noise. As the amplitude of the input signal is increased, each of the gain stages goes into compression. This compression starts at the output stage and progresses toward the input stage.

The detectors that are connected to the outputs of these gain stages produce currents that are proportional to the signal voltages at these points. The sum of the output currents is logarithmically related to the input signal's magnitude. The detected output signal has a linear-in-dB variation with respect to the input-signal voltage. The log-amp detector's linear-in-dB response offers two important advantages over square-law detection:

Due to their logarithmic relationship, very large changes in input-signal voltage can be represented in relatively small changes in detector output voltages.

In terms of dB, the sensitivity of the log-amp detector is well-defined and constant over the log amp's entire rated input-signal range.

In addition to these advantages, the log amp is typically comprised of several hundred transistors. As a result, the use of a few additional transistors to perform temperature compensation adds virtually no extra cost. Yet it greatly simplifies the task of the circuit designer who uses the log amp.

As the crest factor of the input signal to a log amp increases, the output voltage of a log amp will be shifted (transfer function will shift vertically; mV/dB slope is unchanged) in response to the changing voltage peaks of the input signal. Table 1 shows the correction factor for various input signals. An error in the interpretation of the log amp output can occur if the crest factor of the signal is unknown, as is the case for a multi-carrier W-CDMA base station transmitter with constantly varying call loading and carrier powers. Crest factor affects the performance of diode detectors in the same way, since these detectors are also not rms-responding.