volume55-number1 - Flipbook - Page 9
Table 1. Comparison of Example Pulsed Doppler and
Pulsed Radar Attributes
Spectral resolution and sensitivity improve as the FFT bin narrows, which requires
increasing N. Longer pulse widths and PRIs require finer resolution to resolve
closer spectral lines, which means larger N for proper detection. Increasing N
improves spectral line resolution, but only within the IF bandwidth defined by M.
If too high a decimation is used, increasing N improves the spectral resolution
within the IF BW set by M, but cannot recover the missing signal bandwidth. For
example, a pulse train with a pulse width below the minimum receiver pulse
width will have a frequency domain sinc function whose main lobe exceeds the
decimation bandwidth. Increasing N will help resolve the PRF of the train, but will
do nothing to resolve the pulse width; that information is lost. The only fix is to
decrease decimation M, increasing the IF bandwidth.
Parameter
100 ns
Longer
10 μs
Short
1 μs
Longer
1 ms
High
1 MHz
Low
1 kHz
Mid/high
10%
Mid/low
1%
Decimation M
Low
256
High
1536
FFT Length N
Low
128 to 512
High
16,384 to 65,536
Time
Quick
2 μs to 9 μs
Longer
2 ms to 7 ms
Sensitivity
Lower
–91 dBFS
Higher
–120 dBFS
The point here is that M and N are not one size fits all, and sophisticated detection algorithms and parallel channelization schemes in any given EW receiver
may employ a wide range of values for each. The EW receiver must be able to
detect both signals, likely at the same time (not shown here), which is why fast,
adaptable configurability is important. Dynamic range and sensitivity are directly
dependent on the pulse attributes that must be detected.
Example: Wideband Digital Receiver Sensing
Pulsed Doppler Radar
Pulse Train FFT Examples
The following two FFTs capture a pulsed doppler scenario.
Two pulse train examples are presented. The first represents a pulsed doppler
radar exhibiting a very short PW (100 ns) at 10% duty cycle, resulting in very high
PRF. The second simulates a pulsed radar exhibiting comparatively longer PW
and PRI (lower duty cycle, lower PRF). The following plots and tables illustrate the
impact of decimation M and FFT length N on time, sensitivity (noise floor), and
spectral resolution. Table 1 summarizes the parameters for easy comparison.
The fictional values do not represent specific radars but are nevertheless in a
realistic ballpark.10
Amplitude
Short
PRI
PRF
EW wideband digital receivers spend a lot of their effort de-interleaving, identifying,
and tracking simultaneous incident radar pulse trains. Carrier frequency, pulse
width, and pulse repetition interval (PRI) are radar signatures that are critical in
figuring out who’s who. Both the time and frequency domain are used in detection
schemes.9 An overarching objective is to sense, process, and react to the pulse
trains in as short a time duration as possible. Dynamic range is critical because
the EW receiver needs to simultaneously track multiple distant targets while being
bombarded with high energy jamming pulses.
The first FFT shown in Figure 6 needs just over 2 pulse cycles to determine the
pulse width of the signal from the width of the FFT main lobe. The decimation M
is set for an IF bandwidth that is adequately wide to capture the main lobe, as
well as some sidelobes. The response time is very fast. The trade-off to quick
response time is a worse noise floor and spectral resolution. Note that due to the
lack of spectral resolution, no PRI information is available in the FFT.
PRI
1.0
PW
0.5
0
–0.5
NMtS
0
0.5
1.0
1.5
2.0
2.5
Magnitude (dB)
Value
Units
FSAMPLE
15.36
GSPS
N
128
M
256
FFT Bin
0
Decimated BW
–30
–20
–10
0
10
20
Frequency (MHz)
30
40
32,768
469
kHz
2
µs
PW
100
ns
Duty
10
%
PRF
1
MHz
PRI
1
µs
–91
dBFS
Time (NMtS)
Main Lobe = 2/PW
–100
–150
–40
Parameter
M×N
Time (µs)
–50
Pulsed Radar
PW
Duty Cycle
Decimation, FFT, and Detection of Pulse Trains
1.5
Pulsed Doppler Radar
Noise Floor
Figure 6. Fast capture of narrow pulse width, high PRF pulse train typical of pulsed doppler radar.
Analog Dialogue Volume 55, Number 1
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