Rf and if digitization in Radio Receivers: Theory, Concepts, and Examples

 

34 

[34]; however, these circuits suffer from temperature variations, limited dynamic range, and slow 

rise times. Depending on the application, these circuits may or may not be adequate. A more 

powerful technique is the approximation of the logarithmic function using the summation of 

linear (or curved) lines. Typically these are implemented with differential amplifiers using 

integrated circuit technology and may be purchased as discrete components or built into the 

front-end receiver circuitry. There are three basic types of these log amplifiers: detector log 

video amplifiers, successive detection log amplifiers, and true log IF amplifiers.  

Detector log video amplifiers are suited to applications where phase and frequency information 

is not necessary. The envelope of the input signal simply is converted to a log-compressed video 

signal at the output. Total input dynamic range of these amplifiers is generally 50 dB. A typical 

application might be the demodulation and logarithmic compression of an amplitude-modulated 

signal.  

The successive detection log amplifier provides two outputs. One is the same as the detector log 

video amplifier output, described above. The other is a limited IF signal. The former provides 

amplitude information expressed in logarithmic form and the latter provides phase and frequency 

information. The limited IF signal is a copy of the input signal except that its amplitude variation 

is compressed and limited by a transfer function similar to that in Figure 14. This figure shows 

the limited IF output power as a function of the input signal power. Typically these amplifiers 

have a total input dynamic range of 80 dB. A successive detection log amplifier recently has 

been announced by Microphase, Inc. that provides a 100-dB input dynamic range.  

 

Figure 14. Limited IF output power as a function of the input signal power. 

The “true” logarithmic IF amplifier is called so because the IF output is a bipolar logarithmic 

function of the IF input signal (without any limiting). This amplifier may also have an output that 

is the same as the detector log video amplifier. Due to the dual-polarity of the output signal, 

these amplifiers function well in logarithmic IF applications. As might be expected, there is some 

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35 

deviation from the true logarithmic curve (toward a more linear function) as signals approach 

zero, but this is not generally a problem if the system noise power is set to the minimum signal 

input amplitude of the log amplifier (typically -80 dBm). These amplifiers also generally have a 

total input dynamic range of 80 dB. In addition, they inherently have low phase shifts over wide 

variations in input signal power.  

Logarithmic amplifiers have wide instantaneous dynamic range. Instantaneous dynamic range 

means that at any point in time, all signals within the dynamic range of the amplifier appear at 

the output. Take for an example, an input containing two signals, one at -5 dBm and one 

at -75 dBm. For a logarithmic amplifier with an 80-dB dynamic range, it is possible to maintain 

the integrity of both signals at the output, but in a compressed form. Instead of the log amplifier, 

consider a variable attenuator placed before a fixed-gain amplifier. With the same input 

consisting of both the -5-dBm and -75-dBm signals, assume that the fixed-gain amplifier would 

saturate if no attenuation was used. Therefore, the attenuation of the variable attenuator must be 

increased to prevent amplifier saturation. In doing so, the noise figure of the combination of the 

variable attenuator and the fixed-gain amplifier will increase. This may cause the smaller signal 

(-75 dBm) to be overpowered by the noise and go undetected. Logarithmic amplifiers are, 

therefore, good for processing multiple signals where small signals are present simultaneously 

with large signals. Logarithmic amplifiers must be used carefully in receiver systems, however. 

Being nonlinear devices, they may cause distortion of the input signals. A careful analysis of the 

effects of distortion on the desired received signals must be performed.  

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