"Car radar, 5G honeycomb, IoT applications, electronic systems are increasing to the use of radio frequency sources. All of these RF sources need to monitor and control radio frequency power levels, while do not cause loss of transmission lines and loads In addition, some applications require high power transmitter output, so designers need to monitor output signals, rather than direct connection sensitive instruments to avoid damage from high signal levels.
There are also many other challenges: how to determine the characteristics of the radio frequency load (such as antenna) within a wide frequency range; how to monitor load changes and standing wave ratios during the radio state, to prevent large reflection power and amplifier damage, etc. .
Simply access the orientation coupler to the transmission line, these requirements and challenges can be solved. This method can accurately monitor the radio frequency energy stream in the line while reducing the level of power levels. During the sampling process, the interference of the directional coupler to the main line signal is extremely small. In addition, the forward and reflected power can be separated, allowing monitoring of echo loss or standing wave ratios to provide load variation feedback at the time of broadcast.
This article discusses the operation of the directional coupler, introduces three topologies and related products launched by Anaren, M / A-COM and Analog Devices. Then, this paper describes the typical product characteristics and demonstrates effective use.
What is a directional coupler?
The orientation coupler is a transmission line between the measuring device, an access signal generator, a vector network analyzer, and a transmitter, and the transmitter, for measuring the radio frequency power (forward component) from the radio source to the load. And the power (reflection component) of reflecting the frequency source from the load. If the forward and reflective components are measured, the total power, the echo loss and the standing wave ratio are performed.
The schematic symbol of the three port (left) and four-port directional coupling (right). (Source: Digi-Key Electronics) Normally, the power connection coupler's input port is connected, and the load is connected or transmitted. The coupled port output is a forward signal after the attenuation. Decagism, such as the three-port equipment schematic shown. In the three-port device, the isolation port has been terminated internally; in the four-port device, the port output is proportional to the reflection signal. The arrow in the schematic symbol represents the component path. For example, in the four-port configuration, the input port points to the coupling port indicating that it receives a forward component, and the output port is connected to the isolation port, and the latter is used to read the reflected signal. The port number is not standardized, which is different from the manufacturer. However, the port naming of each vendor is relatively unified.
The coupler is a symmetric device, and each port connection is interchangeable. For the three port devices, the reverse input and output ports make the port 3 become the isolation port. In the four-port device, the reverse input and output ports will interchange the coupling and isolation ports.
The output of the coupler is a radio frequency signal. The output of the coupling and isolation ports typically connect the peak or RMS detector, which can generate baseband signals associated with forward and reflected power levels. The directional coupler is combined with the associated detector constitutes a reflectometer.
In some cases, two orientation couplers back-to-back connections can form a dual directional coupler to minimize leakage between the coupling port and the isolation port.
Directional coupling device
Directional couplers have several key features, including bandwidth, rated input power, insertion loss, frequency flatness, coupling coefficient, direction, isolation, and residual voltage station ratio (VSWR).
Bandwidth: The bandwidth of the coupler represents the frequency range, with Hz. Within this frequency range, the coupler can operate within the specification range.
Rated input power: For continuous wave (CW) and pulse input signals, coupler has maximum rated input power, in watt. This value indicates the maximum power that the device can process without reducing performance or physical corruption.
Insertion loss: The power loss caused by the device access primary transmission path is units in decibels (DB).
Frequency flatness: The frequency flatness refers to the amplitude response variation of the main transmission path of the device specific bandwidth, which is a function of the input signal frequency change, in units of DB.
Coupling coefficient: The coupling coefficient is the ratio of the input power and the coupled port output power when all ports of coupleders are correctly terminated, in units of db. This is one of the main characteristics of the directional coupler. The output of the coupled port is proportional to the power level of the straight path (from input to output), and the proportional coefficient is a known value. The coupled port outputs other instruments that can connect to the oscilloscope without the risk of the instrument overload.
Isolation: When all ports are terminated correctly, the power ratio of the input port and the isolation port is in units of DB.
Direction: When all ports are properly terminated, the power ratio of the coupled port and the isolation port is in db. For three-port coupler, two power measurements are typically performed: once in normal forward end connections, the other is carried out in the case where the input and output ports are reversed. This specification is used to measure the degree of separation of forward and reflective components; usually, the larger directionality, the better the interposer. The directionality cannot be directly measured, and can only be calculated by the measured value of the isolation and the reverse isolation.
Residual VSWR: The standing wave ratio measured when all ports were properly terminated. This value is used to measure the inherent impedance matching of the coupler.
Directional coupler topology
Directional coupler design can be implemented in a number of ways, three of the most common topologies are RF transformers, electrical resistance bridges, and coupled transmission lines, respectively. Two RF transformers are used based on the topology of the radio frequency transformer (Fig. 2). The transformer T1 is used to detect the main line current between the input and load. Another transformer T2 is used to detect the pair of ground voltages of the main line. The coupling coefficient depends on the transformer turns ratio N.
The directional coupler topology based on the radio frequency transformer uses two radio frequency transformers to detect the forward and reflection components on the main line. (Source: Digi-Key Electronics) By combining the induced voltage of each transformer on the coupling line, the result is added to the theoretical operation analysis (Fig. 3). VIN is a forward voltage, VL is a reflected voltage.
Figure 3: Analysis of the coupler based on the transformer by analyzing the voltage of the two transformers on the coupling line. (Source: Digi-Key Electronics) Above the figure, in order to calculate the coupling port voltage (Vf ') and the isolation port voltage (VR') on the coupling line, the access current detects the transformer, but the voltage detecting transformer is removed. Similarly, the current detection transformer is removed in the following figure, and VF "and VR" can be calculated in the port access voltage to detect the transformer. The coupling port voltage Vf can be obtained by VF 'and VF "":
The isolation port voltage is equal to the negative number of the reflected voltage to the transformer turns ratio. The negative sign indicates that the reflected voltage is 180 ° in the forward voltage.
Such directional couplers are in a wide range of frequencies, such as MACP-011045 bandwidth of M / A-COM from 5 to 1225 MHz. This coupling coefficient based on the transformer is 23 dB, the rated power is 10 W. The isolation depends on the frequency and the frequency range is from 30 MHz or more, and the corresponding isolation range is 45 dB to 27 dB. The device adopts a surface mount package with a size of 6.35 mm x 7.11 mm x 4.1 mm, so it is compatible with most wireless applications.
The coupler based on the coupling transmission line is composed of a coaxial cable or a printed circuit transmission line. This mechanism is tightly arranged in two or more transmission lines (lengths of lengths of wavelengths), thereby leaking a small amount of controlled signal power from the main line to one or more coupling lines
An example of a dual directional coupler using a coupling transmission line. The length of the transmission line is usually 1/4 of the design of the center wavelength of the frequency band. (Source: DIGI-Key Electronics) Enter the connection port 1, most of the power transfer to the load of the connection port 2. A small amount of power is coupled to a secondary wire of the connecting ports 3 and 4. Port 3 is a coupled port. The power level of this port accounts for the percentage value of the input power. The coupling coefficient can be used to describe the coupling port power, depending on the geometric arrangement of the coupling line. Reflected power is coupled to port 4 (isolated port).
Anaren's 11302-20 is a typical coupling transmission line oriented coupler with a frequency range of from 190 to 400 MHz, which can process up to 100 W. The nominal coupling coefficient of the device is 20 dB, and the insertion loss is 0.3 dB. The package is surface mount form with a size of 16.51 x 12.19 x 3.58 mm, which can be used to monitor the power level of the medium power transmitter and VSWR measurements. Such couplers are related to the frequency range, the lower the operating frequency, the longer the length. Therefore, it is often used in UHF and high frequency applications, and the corresponding equipment is small.
The last directional coupler topology is the directional bridge, and the circuit is related to the classic Whellbia. The ADL5920 RMS and VSWR detectors of Analog Devices use this topology (Figure 5).
ANALOG DEVICES ADL5920 RMS and two-way bridges used in VSWR detectors simplified schematic. In the case where all ports are correctly terminated, the analysis derivation is 33 dB, and as shown in the figure) The ADL 5920 uses the resistance bridge to separate the forward and reflected voltages on the transmission line. As shown, in the case where all ports are properly terminated, the theoretical directionality of the low frequency device can be calculated. The directionality is obtained is 33 dB. In the bridge, VREV and VFWD output signals are transmitted to the RMS cascade detector (dynamic range is 60 dB). The detector outputs linearly read, in DB. The third output voltage from the normal output and the reflection output is proportional to the echo loss, and in units of dB. The coupling frequency range based on the bridge is 9 kHz to 7 GHz. When the matching load is 50?, The rated power is 33 dBm (2 W). When the frequency range is 10 MHz to 7 GHz, the corresponding insertion loss ranges from 0.9 dB to 2 dB. The device adopts 5 x 5 mm surface mount package with a thickness of 0.75 mm.
Analog Devices launched an ADL5920-Evalz evaluation board for ADL5920. This fully equipped evaluation board needs to connect to 5 V, 200 mA power. Input, output, and main output are connected by a 2.92 mm connector. The following schematic shows a typical connection required for ADL5920 (Figure 6). The evaluation board is an ideal tool for easy trial ADL5920.
The ADL5920-EALZ evaluation board schematic shows the typical connections required by the ADL5920 two-way RMS and VSWR detectors of Analog Dec. (Source: analog devices) The frequency range provided by the directional coupler implemented by the resistance bridge is widely adjacent DC (DC). Based on the bandwidth limit of the coupler bandwidth of the transformer and the transmission line, the rated power is greater.
Any type in the above apparatus can extract the input power sample for signal monitoring circuit. The resulting sample, frequency, and adjustment can be determined by means of a traditional instrument such as an oscilloscope or a spectrum analyzer. The data can also be integrated into the feedback loop to adjust the output to remain within the desired range.
The load state can be represented by a voltage stationer ratio (VSWR). The load VSWR of the output port can be calculated using the output (ie, forward voltage and reflected voltage) using the coupled port and the isolated port.
Summarize
For radio frequency system designers, the directional coupler is a quite useful measuring device. It not only provides an amplitude proportional view of RF power levels, but also separates forward and reflected signal components, which helps load characteristic analysis. As mentioned above, there are currently three common couplers topologies to provide these outputs, not only packaged, but also compatible with wireless devices. "
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