Digital Signal Processing Reference
In-Depth Information
Fig. 4.3
Setup used in the measurement of the modulator delay
4.3 Modulator Characterization
Besides verifying that the modulator executes its function correctly, the most im-
portant parameter to be measured is its delay (
t
d
). As it was seen in the last chapter,
this delay and the phase shift of the LC filter determine the maximum switching
frequency according to (3.1), repeated here for convenience:
f
s
t
d
×
360°
+
φ
LC
(f
s
)
=
180°
.
(4.1)
In order to measure the modulator delay, the setup depicted in Fig.
4.3
was used.
A constant voltage of 1 V was applied to one of the inputs (
v
in1
) of the modulator
and its output (
v
sw
) was connected to the other input. The self-oscillation frequency
was measured as if the modulator were operating as a ring oscillator. In this case,
the output
v
sw
oscillates at the maximum frequency as if
φ
LC
(f
s
)
=
0° in (
4.1
).
Figure
4.4
shows the screenshot of the spectrum analyzer
2
from which we identify
the maximum switching frequency to be 112.4 MHz.
Replacing
φ
LC
(f
s
)
=
0° and
f
s
=
112
.
4MHzin(
4.1
), the resulting delay is
4
.
5ns.
3
t
d
=
4.3.1 Envelope Detection and Processing
In Fig. 3.1 on p. 18, it was shown that the dynamic supply voltage must be a function
of the input power and must respect some conditions. There is a minimum supply
voltage that is determined by the minimum power gain desired and the knee voltage
of the power transistor. There is also a maximum voltage that cannot exceed VDD.
2
Agilent's HP8562E, 30 Hz-13.2 GHz [
2
].
3
In [
7
], we published
t
d
=
8
.
9 ns, due to a mathematical mistake. The correct is
t
d
=
4
.
5ns.This
mistake, however, did not affect the results presented in that paper.
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