Measurement Results for the Dynamic Supply CMOS RF Power Amplifie - Linear CMOS RF Power Amplifiers for Wireless Applications - page 55

Table 4.4 Two-tone
measurement results
at − 35 dBc IMD3

2 Tone 2.4 GHz 5.2 GHz

Const. Dyn. Const. Dyn.

Output power (dBm) 15 . 9 16 . 9 14 . 3 16

PAE (%) 15 22 . 5 8 . 6 12 . 5

Gain (dB) 8 . 3 7 . 1 4 . 8 3 . 6

Table 4.5 Filter used for the
OFDM measurements

Component Value

L filter 3.3 µH [ 6 ]

C filter 27 pF [ 19 ]

R comp 200

C comp 4.7 nF

Cutoff frequency 3.8 MHz

f s 42 MHz

4.5.2 OFDM Measurements

Due to the rapid envelope variation of an OFDM signal 7 , a modification in the LC
filter, according to the values shown in Table 4.5 , was necessary to increase its cut-
off frequency. A series RC compensation network ( R comp , C comp ) in parallel with
the PA [ 28 , Fig. 5.16] was used to modify Z PA . Simulation results showed that a
cutoff frequency of 3.8 MHz and a switching frequency of 42 MHz could be ob-
tained.
The measurement results 8 presented in this section are only for 2.4 GHz. At
5.2 GHz, the envelope detector did not work properly with an OFDM signal, intro-
ducing too much delay in the envelope signal. Although at 2.4 GHz the envelope
detector also introduced a delay, the delay was smaller and could be compensated
with a delay line added in the RF signal path before the PA input. Screenshots of the
oscilloscope showing this delay are provided in Chap. 9 (Appendix A).
Figure 4.20 shows a linearity measurement in terms of the rms value of the
EVM. The input is an OFDM signal with a 64 Quadrature Amplitude Modula-
tion (QAM) with a data rate of 54 Mbps, 3 / 4 coding rate, and 52 subcarriers. The
IEEE 802.11a/g standards [ 12 , 13 ] define a limit of 5.6% ( 25 dB) for the rms value

of the EVM. However, it is common to adopt a lower limit of 3% for the power am-
plifier to allow for the nonlinearity of other blocks in the transmitter. Figure 4.20
shows that the dynamic supply PA generates more distortion than its constant sup-
ply counterpart. At 3% EVM ( dashed line ), the dynamic supply attains an output

−

7 The envelope bandwidth of an OFDM signal is 20 MHz.
8 The setup used in the OFDM measurements is described in Chap. 9 (Appendix A).