Short Range Wireless Figure 1. ETC RSU module block diagram.
high performance, low noise buck regulators. It converts 5V to 3.3V, 2.5V, 1.8V, and 1.3V. The input 5V and the ADP5014’s four output voltages provide all the voltage rails for the FEM, AD9361, FPGA, and MCU.
Transmitter test results
All the transmitter test cases defined in the standard were tested, and the AD9361-based ETC RSU module passed the test with great margin. Several key test cases are screen captured as Figure 3 to Figure 6.
RF PLL N divider to meet the RF frequency tolerance requirement, but it depends on the crystal performance to meet the bit rate accuracy requirement, and it is not adjustable. For an AD9361-based solution, the customer can use the DCXO feature to tune the crystal frequency to meet the bit rate accuracy and RF frequency tolerance at the same time. And it is feasible to make a lookup table to compensate the crystal frequency temperature drift to guarantee the ETC RSU module meets the frequency tolerance and bit rate accuracy requirement in the whole working temperature range. Thirdly, the AD9361 implements the receiver AGC functions. It has two modes—slow AGC and fast AGC. It is totally automatic, and the customer does not need to implement any receiver gain control function in the FPGA at all. The fast AGC mode is quite useful in ETC RSU applications, and it is tested so that the gain adjustment settles down in the several beginning symbols of the uplink pilot signal. For the transmit path, first the FPGA sends the transmitter digital baseband signal to the AD9361. Inside the AD9361, the digital baseband signal is first filtered and interpolated from 10.24MSPS to 163.84MSPS. Secondly, the DACs convert the digital baseband signal to analogue baseband signal and then it is low-pass filtered. Thirdly, it is upconverted to an RF signal of 5.83GHz for Channel 1 or 5.84GHz for Channel 2. In the transmitter RF domain, the AD9361 integrates an attenuator, which can control the AD9361’s transmitter output power in a greater than 80dB range. The attenuator can be used to adjust the transmitter output power level as well as gain temperature compensation of the whole transmitter link. Then the transmitter signal is fed into the power amplifier (PA) inside the front-end module (FEM), and it is further amplified and then goes through a microstrip low-pass filter (LPF) to reject the harmonics to meet the transmitter spurious emission requirement, and finally the signal feeds to the antenna. In our ETC RSU module design, the output power at the antenna port could reach 29dBm with
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the AD9361’s transmitter attenuation set as 8dB, which means the AD9361 has enough attenuation dynamic range to compensate both high temperature gain decrease and low temperature gain increase.
For the receive path, first the RF signal from the antenna goes through the LPF and then feeds to the low noise amplifier (LNA) inside the FEM, and then it goes through a band-pass filter (BPF) to reject out-of-band interference signals. Inside the AD9361
Figure 3. Output power of 29dBm.
receive path, it is further amplified and then downconverted to analogue baseband. The analogue baseband signal is low-pass filtered, and then the ADC converts it to digital baseband signal. In the digital domain, the signal is filtered to meet the receiving bandwidth requirement and decimated from 163.84MSPS to 10.24MSPS. Then the AD9361 sends the signal to the FPGA.
For the power solution, the module input voltage is 5V. The ADP5014 combines four
Receiver test results
All the receiver test cases defined in the standard were tested, and the AD9361-based ETC RSU module passed the test with great margin. For the receiver sensitivity test, an FM0 coding, ASK modulated signal is downloaded into the signal generator. The demodulation algorithm is implemented inside the FPGA. The ETC RSU module receiver sensitivity is –95dBm, which is much better than the required –70dBm.
Figure 5. ACLR of –50dBc.
Figure 4. Modulation depth of 90%.
Figure 6. Occupied bandwidth of 3.4MHz.
For other test cases such as maximum input power, receiving bandwidth, in-channel interference rejection, adjacent channel interference rejection, and blocking rejection, the module passed all the tests.
The simple RF instrument implementation
This ETC RSU module could be configured as a simple RF instrument to test the ETC RSU module and the antenna module in a
customer’s production line.
The AD9361 has two RF channels. One channel is used to implement the ETC RSU module, and the other RF channel, together with an on-board high directivity microstrip coupler, are used for the return loss test.
Conclusion
A compact-sized ETC RSU module can be designed using the AD9361. Analog Devices offers the complete reference design
including both hardware and firmware. It is easy to integrate this module into a camera system or use it as a standard ETC RSU module alone. It passes all the ETC RSU requirements defined in the standard GB/T 20851-2019. Even more, it could be configured as a simple RF instrument, which could be used in a customer’s production line.
www.analog.com Components in Electronics November 2021 43
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