Digital Signal Processing Reference
In-Depth Information
Table 7.4
Quiescent point
for the tunable power
amplifier
Voltage/Current
Value
V
bias1
870 mV
I
d1
86 mA
V
bias2
650 mV
I
ctrl
_
q
4.7 mA
Table 7.5
Main measured S parameters for the tunable and conventional PAs
SP
2.4 GHz
3.7 GHz
5.2 GHz
Conv.
Tun.
Conv.
Tun.
Conv.
Tun.
S
11
(dB)
−
3
.
5
−
3
.
4
−
3
.
7
−
3
.
7
−
11
.
8
−
10
.
6
S
22
(dB)
−
11
.
8
−
11
.
3
−
11
.
6
−
11
.
6
−
15
.
7
−
16
.
3
S
21
(dB)
8
.
2
7
.
8
6
.
4
6
3
.
6
3
.
3
−
−
−
−
−
−
S
12
(dB)
27
.
8
28
.
7
26
.
6
25
.
5
26
.
3
26
.
6
4. Perform 2-tone measurements of the tunable PA.
5. Set
V
bias2
to zero to disable the control circuit.
6. Perform S-parameters measurement to verify the input matching and change it if
needed.
7. Perform 2-tone measurements of the conventional PA.
Table
7.4
shows the quiescent currents and voltages for the PA and the control
circuit.
The control voltage
V
bias2
changes the transconductance of the control circuit,
thereby altering the relationship between the currents
I
ctrl
and
I
RF
according to
(6.27)onp.92. Hence, by varying
V
bias2
it was possible to find a quiescent point
corresponding to an optimum performance for the tunable PA at each frequency of
operation.
The 2-port S parameters were measured with the network analyzer HP8719D [
1
]
calibrated from 1 to 6 GHz with the SOLT calibration kit [
2
] provided by the man-
ufacturer. The input (
S
11
) and output (
S
22
) reflection coefficients are shown in the
Smith charts of Figs.
7.15
and
7.16
. The magnitudes of the measured forward (
S
21
)
and reverse (
S
12
) power gains are shown in Fig.
7.17
. These three figures reveal that
there is almost no difference between the behavior of the tunable and conventional
PAs. This indicates that there is a problem with the frequency-tunable PA that must
be investigated. A summary of the results is shown in Table
7.5
.
Figure
7.18
shows the 2-tone measurement results for the frequency tunable PA.
The frequency was swept from 1 to 5 GHz in 0.5 GHz steps. The output power and
PAE were measured at an IMD3 level of
−
35 dBc. The performance of the tunable
PAE is better at every frequency. However, a frequency-tunable behavior—as if the
PA could be tuned for optimum operation at different frequencies—is not observed.
Figure
7.19
shows the same measurement, but for a narrow range of frequencies.
In this figure, the frequency was swept from 2.2 to 2.6 GHz in 50 MHz steps. For
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