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AD8309ARU-REEL7 Folha de dados(PDF) 7 Page - Analog Devices |
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AD8309ARU-REEL7 Folha de dados(HTML) 7 Page - Analog Devices |
7 / 20 page REV. B AD8309 –7– THEORY OF OPERATION The AD8309 is an advanced IF signal processing IC, intended for use in high performance receivers, combining two key func- tions. First, it provides a large voltage gain combined with pro- gressive compression, through which an IF signal of high dynamic range is converted into a square-wave (that is, hard limited) output, from which frequency and phase information modulated on this input can be recovered by subsequent signal processing. For this purpose, the noise level referred to the input must be very low, since it determines the detection threshold for the receiver. Further, it is often important that the group delay in this ampli- fier be essentially independent of the signal level, to minimize the risk of amplitude-to-phase conversion. Finally, it is also desir- able that the amplitude of the limited output be well defined and temperature stable. In the AD8309, this amplitude can be con- trolled by the user, or even completely shut off, providing greater flexibility. The second function is to provide a demodulated (baseband) output proportional to the decibel value of the signal input, which may be used to measure the signal strength. This output, which typically runs from a value close to the ground level to a few volts above ground, is called the Received Signal Strength Indication, or RSSI. The provision of this function requires the use of a logarithmic amplifier (log amp). For this output to be suitable for measuring signal strength, it is important that its scaling attributes are well controlled. These are the logarithmic slope, specified in mV/dB, and the intercept, often specified as an equivalent power level at the amplifier input, although a log amp is inherently a voltage- responding device. (See further discussion, below). Also important is the law conformance, that is, how well the RSSI approximates an ideal function. Many low quality log amps provide only an approximate solution, resulting in large errors in law conformance and scaling. All Analog Devices log amps are designed with close attention to matters affecting accuracy of the overall function. In the AD8309, these two basic signal-processing functions are combined to provide the necessary voltage gain with progressive compression and hard limiting, and the determination of the logarithmic magnitude of the input (RSSI). This combination is called a log limiting amplifier. A good grasp of how this product works will avoid many pitfalls in their application. Log-Amp Fundamentals The essential purpose of a logarithmic amplifier is to reduce a signal of wide dynamic range to its decibel equivalent. It is thus primarily a measurement device. The logarithmic representation leads to situations that may be confusing or even paradoxical. For example, a voltage offset added to the RSSI output of a log amp is equivalent to a gain increase ahead of its input. When all the variables expressed as voltages, then, regardless of the particular structure, the output can be expressed as VOUT = VY log (VIN /VX) (1) where VY is the “slope voltage.” VIN is the input voltage, and VX is the “intercept voltage.” The logarithm is usually to base-10, which is appropriate to a decibel-calibrated device, in which case VY is also the “volts-per-decade.” It will be apparent from (1) that a log amp requires two references, here VX and VY, that determine the scaling of the circuit. The absolute accuracy of a log amp cannot be any better than the accuracy of its scaling references. Note that (1) is mathematically incomplete in rep- resenting the behavior of a demodulating log amp such as the AD8309, where VIN has an alternating sign. However, the basic principles are unaffected. Figure 19 shows the input/output relationship of an ideal log amp, conforming to Equation (1). The horizontal scale is loga- rithmic, and spans a very wide dynamic range, shown here as over 120 dB, that is, six decades of voltage or twelve decades of input-referred power. The output passes through zero (the “log-intercept”) at the unique value VIN = VX and becomes negative for inputs below the intercept. In the ideal case, the straight line describing VOUT for all values of VIN would con- tinue indefinitely in both directions. The dotted line shows that the effect of adding an offset voltage VSHIFT to the output is to lower the effective intercept voltage VX. VOUT 5VY 4VY 3VY 2VY VY –2VY VOUT = 0 LOG VIN VSHIFT LOWER INTERCEPT VIN = 10–2VX –40dBc VIN = 102VX +40dBc VIN = 104VX +80dBc VIN = VX 0dBc Figure 19. Ideal Log Amp Function Exactly the same modification could be achieved raising the gain (or signal level) ahead of the log amp by the factor VSHIFT/VY. For example, if VY is 400 mV/decade (that is, 20 mV/dB, as for the AD8309), an offset of 120 mV added to the output will appear to lower the intercept by two tenths of a decade, or 6 dB. Adding an offset to the output is thus indistinguishable from applying an input level that is 6 dB higher. The log amp function described by (1) differs from that of a linear amplifier in that the incremental gain DVOUT/DVIN is a very strong function of the instantaneous value of VIN, as is apparent by calculating the derivative. For the case where the logarithmic base is e, it is easy to show that ∆ ∆ V V V V OUT IN Y IN = (2) That is, the incremental gain of a log amp is inversely propor- tional to the instantaneous value of the input voltage. This re- mains true for any logarithmic base. A “perfect” log amp would be required to have infinite gain under classical “small-signal” (zero-amplitude) conditions. This demonstrates that, whatever means might be used to implement a log amp, accurate HF response under small signal conditions (that is, at the lower end of the full dynamic range) demands the provision of a very high gain-bandwidth product. A wideband log amp must therefore use many cascaded gain cells each of low gain but high bandwidth. For the AD8309, the gain-bandwidth (–10 dB) product is 52,500 GHz. |
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