IEEE - Aerospace and Electronic Systems - April 2022 - 10

Multifunction Maritime Radar and RF Systems-Technology Challenges and Areas of Development
Table 2.
Example UHF Capable ADCs
Manufacturer
Texas instruments
Teledyne
Analogue devices
Device
ADC12DL3200
EV12AQ600
AD9213
traditional heterodyne receivers that may have a relatively
narrow instantaneous baseband bandwidth.
One issue presented by direct RF digitization is it is
more sensitive to out-of-band interferers. These can be
digitally filtered without issue as long as they do not saturate
the low noise amplifier (LNA) or ADC. If the LNA or
ADC are saturated distortion or clipping will significantly
reduce performance. Dynamic range is therefore a crucial
metric in ADCs, with different RF functions requiring different
dynamic ranges.
Tuneable band-stop front end filters may be employed
to reduce the received power from the interferer while
maintaining the wideband capability of the MFR. Yttrium
iron garnet (YIG) is one technology capable of producing
tunable high Q1 filters at UHFþ frequencies, however,
YIG filters are relatively heavy and have a strong temperature
dependence that may preclude mounting them at the
top of masts. The passband performance of the front end
filter has a direct effect on the SNR performance of the
MFRFS as filtering typically occurs before amplification.
Table 1 sets out the requirements for various naval RF
functions that might be incorporated into an MFRFS.
When compared with the capabilities listed in Table 2, it
can be seen that many ofthe functions cannot be supported
by direct RF digitization using current commercial off the
shelf (COTS) ADCs. For example, radar-volume search
and horizon search require a minimum dynamic range of
90 dB [4], corresponding to approximately 15 bits required
resolution. This dynamic range can be easily achieved but
not at sampling rates required for direct S-band reception.
The sampling rates required for C, X, Ku,K,Ka, etc.
band reception are not fulfilled by commercially available
ADCs, when only the first Nyquist zone is directly digitized.
It is possible to directly digitize RF from higher
Nyquist zones by intentionally causing aliasing. This technique
is known as undersampling or subsampling and
effectively performs the same downconversion function as
an analog mixer. This assumes that the RF has been passed
through an appropriate analog bandpass filter to remove
unwanted signals from the other Nyquist zones.
1The Q factor is the reciprocal of the fractional bandwidth of the
filter.
10
Figure 3.
U.K. ARTIST alongside US ARTIST at Wallops Island. ßNASA.
IEEE A&E SYSTEMS MAGAZINE
APRIL 2022
Resolution (bits) Sample frequency (Gsps)
6.4
12
12
12
6.4
10.25
Figure 4 shows an example of a signal being received
using undersampling. Fs, the sampling frequency, is at
least twice the bandwidth of the signal. The dotted lines
and Fs denote the boundaries between Nyquist Zones.
Modeling [2] suggests that radar performance is comparable
for receivers using either undersampling or traditional
analog downconversion. It should be noted,
however, that undersampling leads to reduced signal-tonoise
ratio (SNR) and dynamic range compared with
direct digitization in the first Nyquist zone.
The performance degradation is partially due to the
increased effect of clock jitter, as shown by the following
equation:
SNRloss ¼ 20 log 10 2p fftjtj
ðÞ (1)
where SNRloss is the signal-to-noise ratio lost solely due to
clock jitter,fis the frequency of interest, and tj is the root
mean square of the clock jitter (typically of the order of
100 fs for these applications).
Noise from clock jitter can be mitigated by using multiple
clocks instead of driving every receiver from the
same clock. In this setup the signals will be combined
coherently while the clock noise will be combined incoherently,
resulting in a gain in SNR. When operating from
a single clock the clock noise will also be combined coherently
so no gain against clock noise is realized. Practically
these multiple clocks would have to be disciplined by a
central clock to prevent long-term phase drift.

IEEE - Aerospace and Electronic Systems - April 2022

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