Page 13 - Issue 3 2023

INFRASTRUCTURE

Frequency considerations algorithms. So, this article will focus on
Higher frequencies of operation do come with several benefits. For example, the the standard FMCW radar topology for
smaller wavelength gives better range detection and object classification data, and the plotting targets in the 2D domain.
shorter wavelength also means the antenna patterns will be smaller, which will lead to
a smaller sized system overall. In some cases, the antennas may be built into the IC, but Ramp generation
we will see that a higher frequency is not always better. As discussed in the “What are you trying
For FMCW radar, the bandwidth of the sweep (that is the ramp start frequency to to detect?” section, the speed of the
stop frequency given as bandwidth here) is directly related to the range resolution. target will dictate how fast the ramp
The range resolution is given in Equation 4. The range resolution is the minimum needs to be.
distance that two targets in the same bearing need to be separated by to be deduced The most straightforward method
as two separate targets. The required range resolution is one of the most important for generating FMCW sweeps is to use a
considerations when choosing the radar’s frequency of operation as it is not possible to PLL and VCO as a frequency synthesiser.
increase this without sweeping a wider frequency range, which is not always viable due Some models of PLLs have in-built
to band restrictions. frequency sweepers. These use internal
25 GHz is an ISM band, meaning there are minimal restrictions in the markets the timers and clocks to automatically
radar can be sold in as a commercial product. There is some variation for each region, increment the PLL N counter internally.
but in general the 24 GHz ISM band covers 24 GHz to 24.25 GHz. Using Equation 4, this Increasing the N counter will increase
equates to a range resolution of approximately 60 cm for the 24 GHz band. the output frequency creating the ramp
profile. The exact profile and timing
can be customised to suit the specific
application – for example, sawtooth vs.
The 77 GHz band has a relatively wide bandwidth allocation up to 5 GHz. This gives triangle waveform, or adding ramp delay
exceptional range resolution but there are some major limitations that should be periods.
noted. The main drawback of the 77 GHz band is that it is predominately restricted An alternative method of generating
to automotive applications. There are certain region dependent exceptions such as FMCW sweeps is to use external wave
industrial tank level sensing but for the most part the 77 GHz radar would be limited to generators to impose the waveform
automotive-only markets. Another drawback is that sweeping a bandwidth of 5 GHz at on the voltage tuning between the PLL
these frequencies, depending on the ramp rate required, is challenging for a standard charge pump and the VCO. Another
analogue phase-locked loop (PLL) and voltage controlled oscillator (VCO) topology to option is to use a PLL in a fixed
generate ramps with acceptable linearity. The result is a complex (and expensive) radar frequency setup and use a digital direct
system from the ramp generation perspective alone. synthesiser (DDS) as its reference input
Other notable drawbacks of operating in the 77 GHz band would be the increased signal. A DDS allows fast frequency
demand for careful PCB design, manufacturing and antenna calibration. switching and so the reference can be
The 60 GHz band, like the 77 GHz band, also has a wide bandwidth allocation and swept to create the ramping waveform
shares many of the advantages while also being an ISM band like the 24 GHz band. That from the PLL.
said, a 60 GHz signal propagating through air will suffer a significant spike in attenuation For FMCW radar applications, due
due to the electromagnetic absorption characteristics of oxygen. Often, 60 GHz radars to the fast frequency hops that make up
have an effective range of less than 20 m. FMCW ramp, the PLL lock time is of high
importance. For a PLL paired with single
Angular resolution band VCO, the largest factor in lock
The angular resolution of a radar is a direct function of the receive antenna aperture (D) time is the bandwidth of the loop filter.
and the number of elements. To find a target’s position, at least two receive channels Higher loop bandwidth gives a fast settle
are required. If the distance between the receive antennas is known, then the delay in time, but also can increase in-band
the reflected signal when it arrives at one channel compared with another can be used phase noise. If the loop bandwidth is
to triangulate the position of a target in relation to the radar. too narrow, the frequency ramp may not
be linear – especially in the down ramp.
There may also be excessive undershoot
that can lead to spectral emission/
Most FMCW radars will show the target in a 2D space only. That is, they will not detect compliance concerns. For fast sweeping
the target’s height. There are some advanced techniques that can be used to estimate FMCW, there is a limit as to how wide
height, such as by monopulse radar. This requires that the transmitted signals have the PLL’s loop filter bandwidth can be.
additional encoding and the height of a target can be calculated based on this encoded A rule of thumb is that it should not
data. This requires a complex ramp profile system and advanced post-processing exceed 10/PFD frequency. In practice,

SUBSCRIBE FREE EngineerIT Issue 3 | 2023 | 13