A 0.1 to 2.5 GHz Logarithmic Amplifier for RF Detection
Logarithmic amplifiers typically are used to measure widely varying signal levels at high frequencies. These signals may vary in amplitude by 90 to 100 dB at frequencies to 3500 MHz. In addition, some log amplifiers can be used for phase demodulation by utilizing the output of the log amplifier's limiter circuit. Log amplifiers also can be used as high gain compounding amplifiers in simple demodulation schemes such as the detection of amplitude-shift keyed (ASK) signals - the log amplifier output is typically sampled by an analog-to-digital converter.
In essence, the demodulating logarithmic amplifier is an RF-to-DC converter. The log amplifier's output is a DC representation that is proportional to the log of the input signal's RF envelope. The limiter output, if used, amplifies low level signals, retaining the phase and frequency modulation information but losing the amplitude information. By using both the log and limiter outputs of these devices, the input signal's amplitude and phase can be determined at a point in time.
The model AD8313 logarithmic detector/controller IC is the first of its kind to utilize a 0.1 to 2.5 GHz frequency range, offering 65 dB of dynamic range with ±1 dB accuracy, as shown in Figure 1 . When used as a logarithmic detector, the device accepts any RF signal at its input and produces a DC output that approximates the log conversion of the input signal's RF envelope. The IC is fabricated using a proprietary 25 GHz fT bipolar process and is housed in an eight-pin micro-SOIC plastic surface-mount package. The unit operates from a nominal 5 V DC supply, typically drawing 12 mA, and can operate down to 2.7 V DC.
The AD8313 IC is a complete multistage demodulation logarithmic amplifier consisting of a cascade of eight amplifier/limiter cells, each with a gain of 8 dB and -3 dB bandwidth of 3.5 GHz, providing a total midband gain of 64 dB. Figure 2 shows the device's simplified block diagram. The architecture is similar to a classic demodulating logarithmic amplifier but adds some flexibility at the output to allow for operation as a controller.
The input signal, which can be either single ended or differential, is applied to the INHI/INLO input pins and then fed to the eight cascaded stages. While the output of each stage feeds the input to the next stage, a current that is proportional to the output flows from each stage and is detected, summed and filtered to generate the log of the input signal's envelope at VOUT. VOUT has a 10 to 90 percent rise time of 50 ns and a temperature variation of -0.032 dB/°C at 1.9 GHz (-0.12 dB/°C at 900 MHz).
The first stage of the amplifier chain is designed to provide a low voltage noise-spectral density of 1.5 nV/Ö Hz. Although the device's nominal range is from -65 to 0 dBm, it can operate from smaller input power levels by adding a simple matching network at the input.
The output of the final amplifier in the gain chain constitutes an amplitude-limited version of the input signal. However, this signal is not available at an output pin due to the desire to keep the device in a small eight-pin package.
Figure 3 shows the pinout for the AD8313 IC, which has two fundamental modes of operation. In the first mode, the VOUT pin is connected to the VSET pin, as shown in Figure 4 , and the device operates as a logarithmic amplifier with its output proportional to the log of the input RF signal's envelope. The unit's controlling capabilities are realized by breaking the link between VOUT and VSET and applying a set-point voltage to VSET. Any difference between VSET and the equivalent input power to the device will drive VOUT either to the supply rail or close to ground (depending on polarity).
This mode of operation, as shown in Figure 5 , is useful in applications where the output power of an RF amplifier is to be controlled by an analog automatic gain control loop. In this mode, a set-point voltage, proportional in decibels to the desired output power, is applied to the VSET pin. A sample of output power from the power amplifier is fed to the input of the AD8313 via a directional coupler and VOUT is applied to the gain control terminal of the power amplifier.
A positive input step on VSET (indicating a demand for increased power from the amplifier) will drive VOUT toward ground. VOUT is used to control the gain of the power amplifier in an inverse fashion (that is, rising VOUT decreases gain). The loop will settle when VOUT sets the input power to a level that corresponds to the demand signal VSET.
The device's 2.5 GHz operating frequency covers all of the worldwide cellular, personal communications service and third-generation cellular telephony frequencies as well as the 900/2400 MHz industrial, scientific and medical bands, and typical IFs (950 to 2150 MHz) for satellite and radio link applications and receiver signal strength indication. The AD8313 IC also is capable of operation above 2.5 GHz, providing 40 dB of input range at 3.5 GHz for use in wireless local loop applications.
The AD8313 logarithmic amplifier is used in applications where rapid and precise tracking of widely varying signals is required. While the output power of a code-division multiple access base station can vary by up to 30 dB, the power variations in a time-division multiple access (TDMA) base station can be up to 70 dB. Third-generation cellular standards promise even more variation.
TDMA calls for continuous power ramping as each time slot is turned on and off. (The profile of the power ramp is also strictly defined.) In a TDMA system, the voltage from the set-point digital-to-analog converter would be set by the contents of a look-up table representing the required ramping profile.
In systems where digital power control is favored, the AD8313 log amplifier plays a more passive but nevertheless critical role. Operation in a log-amp mode is popular in power amplifier control applications, but also finds use in RF pulse detection applications. In a simple ASK demodulation scheme, the device can detect the presence or absence of pulses over a wide dynamic range. With its tight conformance of ±1 dB over its dynamic range, the amplifier is also useful in applications such as radar (where precise RF pulse measurement is required). Precise log conformance is also useful for power detection in the linearization loops of multichannel power amplifiers.
Compared to a diode detector, the AD8313 logarithmic amplifier provides improved linearity and temperature stability at a comparable cost (in high volume). Pricing is available upon request. Additional information is available on the Web at http://www.analog.com/logamps.