Page 15 - Issue 3 2023

INFRASTRUCTURE

and data transmission. Therefore, one ADC and sampling
ADF5904 with four receive channels will Before the FMCW data can be processed and the useful target information deduced,
be used for this example since another the down-converted baseband waveforms must first be filtered and sampled
way to increase the effective receive using an analogue front end (AFE) and ADC, respectively. Outside of the usual ADC
channels is to utilize MIMO operation, considerations such as the number of channels, dynamic range, SNR, capability for
given our choice of a two-channel simultaneous sampling of each channel, and robust filtering options, the ADC selection
transmit configuration. will depend on if the radar will need to utilise fast FMCW ramps to aid in detecting
The signal power of reflected signals many fast-moving targets or if slow ramps are sufficient for the use case.
from targets is a small fraction of the Our choice of the ADF5904 receive has a supported demodulation bandwidth of up
transmitted signal power; therefore, to 10 MHz, so the proposed radar system thus far could support either low speed or
an LNA is typically used to gain up the high speed FMCW ramps.
received signals. Another concern of Low speed ramps will have a low baseband bandwidth in the 500 kHz range whereas
the low reflected signal power is that high speed FMCW ramps would require a high speed signal chain to support the
the noise figure (NF) and the resulting baseband signals of bandwidth 10 MHz and upwards.
output noise of the receive stage will The ADAR7251 was designed to interface to the ADF5904 directly and so it is a good
dictate the minimum detectable signal option here for slow FMCW ramps due to its low noise and dynamic range.
(MDS) and could limit the maximum For applications where the detection of fast-moving targets is required, the AD8285
range of the system. is another viable option. It supports wider input bandwidths of up to 12 MHz and allows
With poor NF, there is the possibility faster sample rates while sacrificing some noise performance, gain, filtering options and
that targets may not be detected resolution when compared with the ADAR7251.
depending on the signal to noise The increased data load from fast FMCW ramps also may require an FPGA to handle
ratio (SNR) required. A traditional the increased data whereas slower speed ramps mean a lower power and cheaper DSP
communication system would typically can instead be used to perform processing and data transmission. For our example
target an SNR of 3 dB. For a radar radar system so far, we have been aiming towards a good balance of performance to
system, this is not quite required, cost and we will continue this by selecting the ADAR7251 as our ADC.
and typical minimum SNRs would
be in the 10 dB to 15 dB region. The Antenna Design
proposed SNR will depend on the Antenna design is a complex topic and beyond the scope of this article. For accurate
specific application. For example, if it angular positioning, the receive elements should not be separated by greater than 0.5λ.
is important to reduce the possibility For this design, identical centre fed patch antennas will be used for each of the transmit
of missing targets, a lower minimum and receive channels. The transmit channels should be separated by greater than 0.5λ
SNR would be required. If instead the to enable MIMO operation. This technique is discussed in the following section, but
potential for false targets needs to be the distances between each antenna must be calibrated and stored to allow the virtual
minimized, then a higher minimum SNR arrays to operate.
is a better option. The ADF5904 has a
noise figure of 10 dBm that results in Other Features
an MDS of around –94 dB for a 1 MHz MIMO has been mentioned a few times in this article. It is a technique that can be used
baseband BW and an SNR of 10 dB. to increase the effective number of receive channels of the radar for increased angular
For the FMCW radar down resolution.
conversion, the received signal must
be compared with the transmit signal
or in this case, a replication of it being
the LO signal. The LO is fed into a
mixer and the receive signal is down-
converted. A common mixer topology in
FMCW radar is direct conversion, also
known as a homodyne or zero-IF mixer.
The ADF5904 incorporates a direct
conversion mixer. The output of the
mixer is non-IQ real data. The phase and
therefore target velocity are deduced by
a series of fast fourier transform (FFT) Figure 3: The top image shows physical antenna positions and separations and the lower image
analyses. shows how these will appear virtually with MIMO operation.

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