RF power meter
The figure below is a complete RF power meter application circuit with a maximum operating frequency of 6 GHz. In this case, the input range of the ADL5904 linear dB RMS response detector is +15 dBm to -30 dBm, and the corresponding output voltage range is 0 to 1.8 V, which corresponds to a slope scaling factor of approximately 35 mV/dB. In this case, the detector output signal is directly connected to the input of the AD7091 12-bit precision ADC. Using 2.5 V full-scale input and 12-bit resolution, the output LSB size is 610 uV. When the slope of the detector is 35 mV/dB, the effective resolution is 57 LSB/dB, which is a very good and very high resolution.
Because the available resolution is so high, there is no practical value in adjusting the 0 to 1.8 V output signal of the detector to fit the ADC's 0 to 2.5 V input range. You can even think that in this application, a 10-bit or even an 8-bit ADC is sufficient because the available resolution is too high when using a 12-bit device.
Now, for applications like this, another device to consider is the LT5587, which integrates a 12-bit ADC on-chip. The performance of this device is very similar to the above applications, but it has an additional advantage that all functions are integrated into a single chip.
Linear V/V RF Detector
The figure below shows a linear V/V radio frequency detector from 500 MHz to 43.5 GHz. Digitize the output of the ADL6010. Because the output voltage of the ADL6010 is greater than the full-scale input voltage of the ADC, the output voltage of the detector needs to be reduced. Use a simple resistor divider to do this. By maintaining a relatively low resistance value, the need to use an op amp buffer between the two devices is avoided.
Because the ADL6010 has a linear V/V response, the measurement resolution calculation in this case is somewhat different. To clarify this point, let's take a closer look at the transfer function of the ADL6010.
The green curve shows the transfer function of ADL6010, reflecting the relationship between voltage output and dBm input. As discussed above, it can be seen that the incremental slope of V/dB decreases as the input power decreases. As shown by the orange line, the resolution is expressed in LSB/dB. Therefore, the final effect of this is that reducing the input power is equivalent to reducing the measurement resolution.
At the bottom of the range, the resolution is now reduced to a little more than 1 bit/dB. This is in sharp contrast to the previous example where the measurement resolution remained constant over the entire input range. For this application, you might argue that you need a higher resolution, higher than 12-bit ADC.
RMS response detector
If you apply a series of different signals to a log detector with a non-RMS response, you will find that each signal type produces a different response. This phenomenon can be seen on the left side of the figure below. The detector is scanned with a series of QAM modulated signals with different peak-to-average ratios. In contrast, if the same series of signals are applied to the RMS response detector, there is almost no difference in the transfer function seen. This is shown on the picture on the right
This behavior is very valuable in systems where the nature of the signal is constantly changing. A dynamic QAM transmitter whose modulation scheme changes over time is a good example. This point needs attention, and is one of the reasons for the confusion of naming. The term "logarithmic amplifier" or "logarithmic detector" generally refers to a non-RMS response linear dB output response radio frequency detector. Although the response of the RMS device on the right has characteristics similar to a logarithmic amplifier, we usually do not call it a logarithmic amplifier, but a linear dB RMS detector.
This table compares logarithmic amplifiers with RMS detectors. The first thing to note is that there is a clear functional overlap between these devices in terms of frequency range, sensitivity, and temperature stability. One significant difference is the response time. The rise and fall times of some logarithmic amplifiers are much less than 10 ns, but the response speed of RMS detectors is much slower. This is determined by the implied mean demand in the root mean square calculation.
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