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Keysight Technologies
Improving Radar Performance by Optimizing Overall
Signal-to-Noise Ratio
Application Note
Radar Measurement Series
Better noise-figure
measurements enhance
characterization of excess
noise in receivers
Overview Problem
In radar system design, optimizing the overall signal-to-noise ratio Radars need to extract very small signals--those coming from tar-
(SNR) of the system will improve the overall performance of the gets of interest--in an environment that may include artifacts such
system. This is typically done in one of two ways: by increasing as clutter, jamming signals and spurious noise (e.g., signals from
the signal or by reducing noise. Because a radar is a transmit/ other radars). Any internally generated noise in the radar receiver
receive system, improved SNR can be achieved by increasing the circuit reduces the ability of the radar to discern the targets of
transmitted power by using bigger, more-powerful amplifiers, by interest. Noise generated within a receiver component is indistin-
using larger or higher gain antennas, or a combination of these guishable from any legitimate signal within the signal frequency
changes. band and will be amplified equally along with expected signals in
any subsequent gain stages.
SNR can also be increased by decreasing receiver-contributed noise,
which is usually determined by the quality of the low-noise amplifier
(LNA) at the front end of the receiver. In general, it is easier and less Measuring noise properties is an essential step in the process of
expensive to decrease receiver noise--and achieve a better noise minimizing the noise generated within a receiving system. The fol-
figure (NF)--than to increase transmitter power. lowing equation determines the minimum signal level required to
overcome system noise at the maximum range of the radar:
In the pursuit of a better SNR, NF is a figure-of-merit that describes
the amount of excess noise present in a system. The definition of Smin = kToBnFn(So/No)min
noise figure is very straightforward. The noise factor (F) of a network
Where:
is defined as the input SNR divided by the output SNR:
Smin = the minimum signal level
k= Boltzman's constant
F = (Si/Ni)/(So/No), where To = room temperature
Si = input signal power Bn = receiver noise bandwidth
Fn = noise factor
So = output signal power
(So/No)min = the minimum SNR required by the receiver
Ni = input noise power processor to detect the signal
No = output noise power A close inspection of this equation illustrates the importance of
Noise figure is simply the noise factor expressed in decibels: NF = receiver NF. For example, k and To are effectively constants, Bn is
10*log (F). This definition is true for any electrical network, including dictated by the radar design, and the SNR cannot be improved
those that shift the frequency of the input signal to a different output once the signal arrives at the receiver. Thus, receiver NF becomes
frequency, such as up- and downconverters. the key term for receiver optimization. In reality, this is a somewhat
simplistic model of performance, as other items such as sys-
tem losses and pulse integration will also affect performance.
However, the NF performance of the receive circuit is a key per-
formance factor.
2
The above equation might lead you to believe that improvements Solution: Y-factor noise figure
in noise figure will enable great improvement in system perfor-
mance at modest cost. Today's low-noise amplifiers can deliver measurement
very low NF values. When properly engineered into the receiver
The Y-factor or hot/cold-source method is the most common way to
architecture, the system NF penalty can be minimal. As a result,
measure noise figure. This technique is relatively easy to implement
it may seem more economical to reduce receiver noise figure by
and provides good measurement accuracy in most situations, espe-
3 dB than to increase transmitter power by the same amount.
cially when the noise source has a good source match and can be
However, reality is not quite that simple. Instead, the receiver must
directly connected to the DUT. In addition to the DUT, two pieces of
also provide adequate gain, phase stability, amplitude stability,
test equipment are needed:
dynamic range, and fast recovery from overload and jamming. In
addition, protection must be provided against overload or satura-
tion and burnout from nearby transmitters. As a final point, the