Biomedical Engineering Reference
In-Depth Information
(a)
Rise Time=100ps,PW=0.5ns
-40
-50
-60
-70
-80
(b)
-20
-30
-40
-50
-60
-70
-80
(c)
-40
-50
-60
-70
-80
3
4
5
6
Frequency (GHz)
Fig. 5.10
Transmit spectrums for PRF s a 10 MHz b 100 MHz c 50 MHz
5.2.3 Implementation of the Pulse Generator
A reconfigurable square wave oscillator is used as the source for the pulse gen-
erator circuit. LTC6905 programmable oscillator by Linear Technology is chosen
for this purpose mainly due to its short rise and fall times [
10
]. This is a resistor set
oscillator, in which the oscillation frequency (f
osc
) can be varied by changing the
external resistance value (R
SET
) of the oscillator calibration input using a single
resistor according to the following equation [
10
]:
8
<
9
=
1
1
;
DIV pin
¼
V
þ
2
;
DIV pin
¼
OPEN
4
;
DIV pin
¼
GND
168
:
5 MHz
10 kX
R
SET
f
OSC
¼
þ
1
:
5 MHz
N
;
N
¼
:
;
ð
5
:
6
Þ
In an attempt to change the oscillation frequency without manual intervention,
AD5286 programmable resistor from Analog Devices [
11
] is used to set the
