Biomedical Engineering Reference
In-Depth Information
Bragg cell
Detector
array
Laser
P
out
α
P
s
=
I
out
R
L
Signal
W
S
P
S
(a)
BS
Laser
Local
oscillator
P
LO
W
S
+
W
IF
Detector
array
W
S
Signal
W
S
P
S
P
out
α
P
S
P
LO
(b)
FIGURE 3.13
(a) Conventional power spectrum analyzer. (b) Interferometric spectrum analyzer.
be reached in dealing with the number of PDs and their access/integration
time. It is desirable to have many PDs, but it is also desirable to access them
often to detect short pulses, and to most accurately determine time-of-arrival
(TOA) information in ECM applications.
For example, to achieve a 0.5 μs TOA resolution in a 1 GHz wide receiver
with 1 MHz resolution (1000 PDs), a PD must be accessed every 0.5 ns. GaAs
CCDs have been built that contain much of the readout circuitry on the
same die (called self-scanning), but at speeds of 500 MHz, or every 2 ns [26].
Faster speed devices are in development. An alternative approach to access-
ing many PDs quickly is to arrange them in series-parallel readout schemes
with two, four, or more PDs accessed simultaneously. More sophisticated
postprocessing techniques are required, but the faster access time is worth
the hardware cost if access speeds are important.
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