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The RADAR in Chip Solution (3/5)

Electronic Product Design

Radar

4 mins read

Electrical elements described previously could be considered as a fair addition to one’s knowledge about electronic circuits. Especially when it comes to the RF and RADAR design comprehension. The topology that will be introduced next for the RADAR topic elaboration is in the realm of a higher logical abstraction level. The representation of the electronic circuit in consideration is composed of blocks in a ‘wired’ block diagram, where each block has a dedicated functionality.

As shown by the figure, a block diagram is divided into two domains. A blue-colored domain is related to the signal processing outside the selected RADAR transceiver (BGT24M). Another half of the signal processing chain is carried out by the RADAR transceiver itself and is represented in green color. From the system controller’s point of view, all of the necessary data is in the digital domain. That is why there are two converters deployed at the beginning and the end of the signal processing chain respectively. What happens to the signal after digital-to-analog (DA), and before analog-to-digital (AD) conversion is a subject of upcoming explanation.

 

MCU and TCVR domains

The DA converter output controls the voltage-controlled oscillator (VCO) of the transceiver (TCVR). The VCO generates a signal in the 24GHz range. The signal in consideration can be of constant frequency or modulated frequency. This signal trait depends on how the DA converter is ‘instructed’ by the controller. The generated signal is split into two signal paths. The first signal path leads into a power amplifier and then to the emitter antenna. When the signal reaches the antenna element it is emitted as discussed in the introductory article. The second signal path in question leads to a signal multiplier.

 

Simultaneously, the receiver antenna picks up the relevant EM waves and generates a receiver-side signal. This received signal is amplified and routed to the signal multiplier. Signal multiplication is performed over the transmitted signal and the received signal. The newly obtained combined signal has a frequency that’s a numeric difference between transmitted and received signals frequencies. The combined signal is then amplified and converted by the AD converter. By returning the signal to the digital domain, a full chain of RADAR control is closed. Since a microcontroller (MCU) is selected for this endeavor further explanation of the system developed is to be disclosed next.

 

As stated above, the two domain signal processing is emphasized again. With new information at hand, the domains can be named as the MCU and TCRV domains respectively. As already mentioned, the MCU in this implementation is used in two ways. The first MCU task is to control the RADAR chip. The RADAR chip control information is carried over the SPI communication layer. The second MCU task is to sample the analog down-converted signals with the integrated 12-bit ADC. On the upper layers of the MCU assignments list is the signal processing and that will be discussed in the following articles.

All things considered, the parts list is to be expanded a little bit more with an additional element used in this implementation. The low-noise fractional-N phase-locked-loop (PLL) IC is used to perform frequency control and ramp generation. This is achieved with an external LMX2491 PLL chip. Circuit-wise, the output of the /16 integrated prescaler on the RADAR IC is connected to the PLL RF input pins. Then, the output voltage from the PLL charge pump is connected via a loop filter to the tuning ports of the BGT24M, thereby forming a closed-loop system. Finally, the /65536 integrated prescaler produces a low-frequency output signal (23 kHz), which is connected to the capture and compare unit of the microcontroller for monitoring purposes. This summarizes the signal chain for the BGT24M RADAR functionally.

 

The following articles are an in-depth description of the implementation and debugging procedures.