What advantages can a tiny diode bring to the detector?

Figure 1. Diode-based Schottky RF detector.

Figure 2 shows the transfer function for this circuit. The input power is scaled in dB and the output voltage is shown on a logarithmic vertical scale. Looking at the 25°C transfer function, there are two distinct operating regions on the curve. The region known as the linear region is from the top of the input range (approximately 15 dBm) to approximately 0 dBm. The term “linear region” is derived from the fact that the output voltage in this region is roughly proportional to the input voltage .

Figure 2. Diode-based Schottky RF detector transfer function.

Below 0 dBm is the so-called square-law region. In this region, the output voltage is roughly proportional to the square of the input voltage. This results in a high slope of the curve.

Figure 2 also shows the output voltage vs. input power transfer function of the circuit at –40°C and +85°C. It shows that the power levels below 0 dBm have large deviations. This makes the device unstable in applications with slightly larger temperature variations.

There are some techniques that can mitigate temperature drift to some extent. These techniques include introducing a second reference diode as part of the circuit or as a separate circuit with a separate output. The temperature drift of the reference diode is matched to the primary diode. Through a subtraction process (either in the analog or digital domain, depending on the circuit structure), the drift can be eliminated to some extent.

Figure 3 shows the transfer function of the ADL6010, an integrated diode-based Schottky detector with a number of innovative features, at 25 GHz. As part of the signal processing, the input signal passes through a circuit that performs a square root operation only when the signal is below a certain power level. The transition point is intentionally set to be equal to the power level at which the diode transitions from the square-law region to the linear region. As a result, the square-law effect of the diode is canceled, and the transfer function in the two regions is not as obvious as in Figure 1.

Figure 3. Output voltage vs. input power and linearity error of an integrated Schottky diode detector at 25 GHz.

Figure 3 also shows the transfer function plots for various temperatures from –55°C to +125°C. In addition, the change in the transfer function with temperature is plotted. The error at each temperature is plotted in dB using the linear regression of the 25°C transfer function as a benchmark. Due to the integrated temperature compensation circuitry and square-law cancellation circuitry, it can be seen that the error due to linearity and temperature drift is approximately ±0.5 dB over most of the input range.

Although RF and microwave detectors are sometimes used in analog power control loops, it is more common to build a digital power control loop, as shown in Figure 4. In these applications, the output of the power detector is digitized using an analog-to-digital converter. In the digital domain, the code from the ADC is used to calculate the power level. Once the power level is known, the system responds by adjusting the transmitted power as needed.

Figure 4. Typical digitally controlled RF power control loop.

Although the response time of this loop depends only to a small extent on the detector response time, the ADC sampling rate and the speed of the power control algorithm have a much greater impact.

The loop’s ability to measure and accurately set the RF power level is affected by a number of factors, including the transfer function of the RF detector and the ADC resolution. To better understand this, let’s take a closer look at the detector’s response. Figure 5 compares the response of the ADL6010 diode-based detector at 20 GHz to the HMC1094 microwave log amp. A log amp has a linear-in-dB transfer function, where a 1 dB change in input power always results in the same voltage change at the output (approximately a linear input range of –50 dBm to 0 dBm). In contrast, a diode-based detector such as the ADL6010 has an exponential transfer function with a dB scale on the horizontal axis and a linear vertical axis for output voltage.

Figure 5. Linear dB comparison.

Since the transfer function of an analog-to-digital converter is in bits/voltage, this means that the system resolution expressed in dB/bit will continue to degrade as the input power decreases. The plot in Figure 5 also shows the bits/dB resolution that can be achieved by driving a 12-bit ADC with an ADL6010 at a full-scale voltage of 5 V (the plot is scaled on a logarithmic secondary axis for ease of viewing). At the low end of the device’s power range, at about –25 dBm, the incremental slope is approximately 2 bits per dB, resulting in a resolution of approximately 0.5 dB/bit. This means that a 12-bit ADC is sufficient to accurately resolve the ADL6010 output over its full-scale range.

As the RF input power increases, the incremental slope in bits/dB rises steadily to a maximum of approximately 300 bits/dB at a maximum input power of 15 dBm. This is valuable for RF power control applications where precision performance is critical when the system is at maximum power. A very typical application scenario is when RF detectors are used to measure and control high power amplifiers (HPAs). In applications where power often needs to be controlled to prevent the HPA from overheating, high-resolution power measurements at maximum power are extremely valuable.

In contrast, the HMC1094 log amp transfer function in Figure 5 also shows that its slope remains constant over the linear operating range. This means that to achieve resolutions much lower than 1 dB, a lower resolution ADC (10-bit or even 8-bit) is sufficient.

In the application circuit shown in Figure 6, the ADL6010 interfaces with the AD7091, a 12-bit precision ADC that can sample at up to 1 MSPS. The ADC has an internal 2.5 V reference that sets the full-scale input voltage. Since the maximum voltage that the ADL6010 detector can reach is approximately 4.25 V, a simple resistor divider can be used to adjust this voltage downward so that it never exceeds 2.5 V. This adjustment can be implemented without an op amp buffer. The dB/bit resolution that can be achieved at the lower end of the input power range is similar to the previous example (i.e., approximately 0.5 dB/bit).

Figure 6. Integrated microwave power detector interfaced to a precision ADC.

Integrated RF and microwave detectors offer many advantages over discrete implementations: