The popularization of various wireless connection technologies such as WiFi, 4G, and Bluetooth has led to a blowout growth of various terminal equipment, including the Internet of Things, wearables and other emerging industries based on wireless connection technologies have grown rapidly, and various wireless signal chain solutions have emerged Promote the continuous development of this upsurge. In the wireless signal chain, it has been a long time since someone mentioned a key component - the detector. A recent technical lecture by ADI experts shared the "original" and important components in this wireless design, giving the author the opportunity Reorganize this important but a little unfamiliar product technology.
Detection is also called amplitude demodulation, and its function is to restore the original modulated signal from the modulated high-frequency oscillation. From the spectrum point of view, detection is to move the sideband signal in the amplitude modulation wave from near the carrier frequency to near the zero frequency without distortion. With the widespread application of RFID, radar, and the Internet of Things, radio frequency transmission is ubiquitous, and detectors are used more and more. The RF detector has a sensitivity and stability much higher than the traditional diode detector, and gradually occupies the market of the radio frequency industry.
In these typical applications, the detector is critical
Let's first look at the various applications of RF detectors. In test and measurement applications, RF power detectors are used for precision measurements of RF power and as part of input protection circuits in spectrum and network analyzers. In communications and medical applications, RF detectors are used to monitor and control transmit power and antenna return loss.
An emerging application is RF-based material analysis, where the detector is like a miniature network analyzer that analyzes the magnitude and phase of the signal reflected by the material and uses algorithms to determine material properties, such as moisture content. There are also many applications where RF detectors are used to measure pulsed power, such as electronic payment systems, radar and electronic warfare.
RF detector is a miniature RF power meter
Now, for those unfamiliar with the operation of RF detectors, their function is very simple and best viewed in the time domain. Imagine an RF detector driven by a signal whose input level varies with time, as shown on the left. As the input level increases, the detector's DC output level also increases. Now, this baseline response is common to all RF power detectors, although the exact relationship between input level and output level will vary by device and function.
Understand the differences in performance characteristics to select the correct detector for the application
Now about the different types of RF detectors. The most common type is the logarithmic amplifier, which provides a DC output proportional to the logarithmic value of the input signal. A log amp has a detection range of 40 dB to 100 dB and a relatively flat response time.
The RMS detector performs a full root mean square calculation. RMS detectors can have a linear V/V or linear dB output response. Gain and phase detectors are special log amplifiers that calculate the magnitude and phase difference between two input signals. These devices have two RS inputs.
SDLVA stands for Sequential Detection Logarithmic Video Amplifier. Architecturally, these devices do not differ from log amps, but have two special characteristics: excellent frequency flatness and fast response time. More information on these devices will be described later.
Finally there are peak and envelope detectors. These are fast-response devices that can catch and hold peaks, or follow the rapidly changing envelope of an RF pulse or QAM modulated signal.
Additional temperature compensation can generally be done if the temperature drift is consistent and repeatable from device to device. In this case, two off-chip resistors can be used to pull the thermal and cold drift back to 0 dB for an overall better temperature drift.
In wideband applications such as RF power meters, variations in output voltage and frequency become very important. Basically, the greater the variation with respect to frequency, the more frequency calibration points are required. Two types of graphs are used to show the output voltage as a function of frequency. The first plot on the left is a simple series of power sweeps, with each trace representing the transfer function at a specific frequency. We also use a second type of graph to display the frequency response. The graph on the right shows output voltage versus frequency, with each trace representing a specific RF power level. This plot is very useful for assessing how far apart the frequency calibration points need to be.
This figure shows the frequency response of the new RMS detector LTC5596, which operates up to 40 GHz. An important characteristic of this device is that its frequency response remains flat over such a wide frequency range. It can be seen that the trace remains at about 60 mV from about 200 MHz to 30 GHz. The slope of the LTC5596 is approximately 30 mV/dB. Using these two figures, we can see that this corresponds to a frequency flatness of about 2 dB; unprecedented for such a wideband device.
In the graph on the left, we plot the output voltage versus input level (dB). The blue curve is the so-called linear dB transfer function. We call it linear in dB because for every dB change in the input, the change in output voltage is constant. In contrast, the black curve in the left graph has an exponential character when you plot it as voltage output versus dBm. This is a typical response of a diode-based RF detector.
The graph on the right is plotted using the same data and shows voltage output versus voltage input. Now the blue curve has logarithmic properties. Therefore, an RF detector with a linear-in-dB characteristic is often called a logarithmic amplifier. The black curve on the right becomes a straight line, so this transfer function is often called linear V/V. Each transfer function has its own advantages and disadvantages, which we will discuss later. However, for the moment we can say that linear dB devices are log amps and tend to have excellent range and sensitivity, while linear V/V detectors tend to have lower range but excellent resolution and sensitivity at high RF power levels. precision.
Some Recommendations for the Operation of a Typical RF Detector
The basic support circuitry required to use an RF detector is ubiquitous. RF inputs are almost always AC coupled. AC coupling capacitors are sometimes internal to the chip and sometimes external. Devices with external AC coupling tend to operate at lower frequencies. RF applications generally prefer an input impedance of 50 ohms. However, the input resistance of many RF detectors is much higher than this.
Therefore, an external shunt resistor is typically placed across the device so that the effective input resistance of the circuit is 50 ohms. Some RF detectors have features that facilitate compensation for temperature drift. It usually takes the form of an externally applied voltage and is recommended to optimize temperature stability at a specific frequency.
Since the RF detector effectively converts the AC signal to a DC signal, the signal will need to be averaged at the output. In most cases, the device has a pin to which an averaging capacitor can be connected. Of course, there is an implicit tradeoff between the mean level and the response time of the output to changes in the input.
Many RF detectors also feature flexible output signal scaling. Connect the VOUT pin directly to the VSET pin to set the nominal slope. But the output can be easily amplified by connecting these pins through a resistor divider.
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