volume55-number1 - Flipbook - Page 13
DR (dB)
IIP2
(FE, dBm)
61
20
30
40
50
60
70
80
Pin (dBm)
–80 –70 –60 –50 –40 –35 –30 –25 –20
41
51
56
51
46 41
36
31
21
41
51
61
56
51 46 41
31
21
41
51
61
59 54 49 44
31
21
41
51
61
60 55 50 45
31
21
41
51
61
60 55 50 45
31
21
41
51
61
60 55 50 45
31
21
41
51
61
60 55 50 45
31
21
–15
31
36
39
40
40
40
40
–10
26
31
34
35
35
35
35
64
20
30
40
50
60
70
80
Pin (dBm)
–80 –70 –60 –50 –40 –35 –30 –25 –20
14 24 34 44 49 50 45 40
4
14 24 34 44 49 54 54 49
4
14 24 34 44 49 54 59 57
4
14 24 34 44 49 54 59 63
4
14 24 34 44 49 54 59 64
4
14 24 34 44 49 54 59 64
4
14 24 34 44 49 54 59 64
4
–15
35
44
52
58
62
63
64
–10
30
39
47
53
57
58
59
DR (dB)
IIP2
(FE, dBm)
Figure 12. Instantaneous spur free dynamic range (DR) vs. RF input level (Pin) and RF front-end
input referenced IP2; high sensitivity (top) and bypass mode (bottom).
Figure 13 shows that for big improvements to NF, which can be very costly to
SWaP-C and linearity, there is a diminishing return to dynamic range using a
mid-range Bv. For lower NF to pay off, Bv needs to decrease with it and the associated trade-offs tolerated. The high sense mode does well with an NF in the
10 dB to 15 dB range. For the bypass mode, the high NF is shown to be a willing
trade-off given the benefit to linearity. Ideally NF can be kept in the 20 dB to
25 dB range for the bypass mode. Better NF in bypass mode doesn’t help dynamic
range, as we are IMD limited.
DR (dB)
NF (dB)
DR (dB)
NF (dB)
Pin (dBm)
61 –80 –70
5 27
37
10 25 35
15 21
31
20 17
27
25 12
22
30 7
17
35 2
12
–60 –50 –40 –35 –30 –25 –20
47 57
61
56
51 46 41
45 55
61
56
51 46 41
41
51
61
56
51 46 41
37 47 57
56
51 46 41
32 42 52 56
51 46 41
27 37 47 52
51 46 41
22 32 42 47
51 46 41
–15
36
36
36
36
36
36
36
–10
31
31
31
31
31
31
31
Pin (dBm)
64 –80 –70
5
14
4
10 4
14
15 4
14
20 4
14
25 3
13
30 2
12
35 0
10
–60 –50 –40 –35 –30 –25 –20
24 34 44 49 54 59 64
24 34 44 49 54 59 64
24 34 44 49 54 59 64
24 34 44 49 54 59 64
23 33 43 48 53 58 63
22 32 42 47 52
57 62
20 30 40 45 50 55 60
–15
64
64
64
64
64
64
64
–10
59
59
59
59
59
59
59
Figure 13. Instantaneous spur free dynamic range (DR) vs. RF input level (Pin) and RF front-end
noise figure (NF); high sensitivity (top) and bypass mode (right).
Analog Dialogue Volume 55, Number 1
Summary
Electronic warfare’s imminent evolution toward multi-octave, multi-GHz
instantaneous bandwidth RF tuners and wideband digital receivers introduce
IMD2 effects that challenge dynamic range. Today’s consideration of SFDR
in terms of IMD3 will broaden to include IMD2, and the designer will use both
the SFDR2 and SFDR3 equations. The system noise floor is dynamic because
processing bandwidth changes on-the-fly based upon waveform detection and
time requirements. When designing the optimal noise floor, decimation M and
FFT depth N together define the FFT bin width, yet they each have separate
important impacts to consider. Example pulse train FFTs of varying M and N
are provided. As ADC performance improves, the front end continues to rely on
high linearity wideband RF components with tunable attributes and frequency
selectivity. The front end should be designed in cascade with the ADC’s RF attributes.
MATLAB® Code
clear all; clc; %close all;
% sampling parameters
fs = 15.36e9; %sampling frequency
ts = 1/fs; % time step
N = 2^9; %FFT bins
m= 2^8; %decimation 1536 max
MN=N*m;
fs_dec=fs/m;
bin = fs_dec/N;
capture_time= N*m*ts; % radar waveform
tau = 100e-9; % pulse width
duty = 0.1;
PRI = tau/duty;
PRF = 1/PRI; % Hz
NSD=-148; %dBFs/Hz
floor=NSD+10*log10(2*fs_dec/N);
mainlobe=2/tau;
line_spacing= PRF;
num_cycles = N*m*ts/PRI;
t = 0:ts:(N*m*ts - ts);
d = tau:PRI:(PRI*(num_cycles));
y = pulstran(t,d,@rectpuls,tau); %pulse train
y=awgn(y, 50);
%plot pulse train in time domain
subplot(2,1,1)
plot(t/1e-6,y)
xlabel('Time (us)'); ylabel('Amplitude')
ylim([-0.5,1.5]);
%filter and decimate data stream
ydec=decimate(y,m);
%window
win = blackman(length(ydec)); % blackman window....
use yup for zero pad
ywin=win'.*ydec;
% FFT
Y = abs(fft(ywin,N));
f = -fs_dec/2:fs_dec/N:(fs_dec/2-fs_dec/N);
%normalize and convert to dB
Y_db = mag2db(Y./max((Y)));
%plot FFT
subplot(2,1,2)
plot(f/1e6,fftshift(Y_db));
xlabel('Frequency (MHz)'); ylabel('Magnitude (dB)');
xlim([-4/tau/1e6 4/tau/1e6])
ylim([-150,0]);
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