volume55-number1 - Flipbook - Page 10
short pulse must be detected simultaneously with the long pulse in the same IF
channel, then M must be set to accommodate the short pulse spectral bandwidth
and cannot be increased. Considered on its own, the long pulse requires a lower IF
bandwidth, so M could be set higher to improve the channel noise and resulting
sensitivity. The capture time, or FFT length N, required is a lot longer, however.
So it’s likely the detection algorithm would want to make intermediate decisions
on the short pulse scenario while the system acquires a high enough N to resolve
the long pulse.
The second FFT in Figure 7 shows an improved noise floor and spectral resolution
as sample length N (and time) are increased. M remains the same. By around
nine pulse cycles, the spectral resolution improves enough to determine the PRI
(1/PRF) from the FFT. The noise floor can be seen between sidelobes.
Example: Wideband Digital Receiver Sensing
Pulsed Radar
The following two FFTs capture a wider pulsed scenario.
The much wider PRI, or lower pulse density, in the pulsed radar example in
Figure 8 requires much higher N. Adjusting M is entirely system dependent. If the
Amplitude
1.5
PRI
1.0
PW
0.5
0
–0.5
NMtS
0
1
2
3
4
5
6
7
8
9
PRF
Magnitude (dB)
–50
–100
–150
–40
Main Lobe = 2/PW
Decimated BW
–30
–20
–10
0
Value
Units
FSAMPLE
15.36
GSPS
N
512
M
256
M×N
Time (µs)
0
Parameter
Noise
Floor
10
20
30
40
Frequency (MHz)
131,072
FFT Bin
117
kHz
Time (NMtS)
8.5
µs
PW
100
ns
Duty
10
%
PRF
1
MHz
PRI
1
µs
Noise Floor
–97
dBFS
Parameter
Value
Units
FSAMPLE
15.36
GSPS
N
16,384
M
1536
M×N
25.2M
Figure 7. A longer FFT of a pulsed Doppler example to resolve spectral lines.
Amplitude
1.5
PRI
1.0
PW
0.5
0
–0.5
NMtS
0
200
400
600
800
1k
1.2k
1.4k
1.6k
1.8k
Time (µs)
Magnitude (dB)
0
–50
Main Lobe = 2/PW
–100
–150
–0.2
–0.15
–0.1
–0.05
0
0.05
Frequency (MHz)
0.1
0.15
0.20
FFT Bin
0.6
kHz
Time (NMtS)
1.6
ms
PW
10
µs
Duty
1
%
PRF
1
kHz
PRI
1
ms
–120
dBFS
Noise Floor
Figure 8. Fast capture of longer pulse, lower PRF pulse train typical of pulsed radar.
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Analog Dialogue Volume 55, Number 1
