Measurement Setups and Additional Screenshots - Linear CMOS RF Power Amplifiers for Wireless Applications - page 132

Fig. 9.7 Dynamic supply signal for OFDM excitation. Delay of the envelope detection compen-
sated by the oscilloscope
9.4 Additional Screenshots
Figure 9.5 shows a screenshot of the oscilloscope LC534AM with two waveforms.
The upper waveform is the dynamic supply signal for a 2-tone measurement at
5.2 GHz. The waveform at the bottom is the 250 kHz signal from the function gen-
erator HP33250A used to generate the two input tones.
Figure 9.6 shows the screenshot of the oscilloscope Infiniium where the dynamic
supply signal ( top ) and the input RF signal ( bottom ) can be observed. These signals
were obtained with an OFDM measurement at 2.4 GHz. It reveals that there is a
delay between the two waveforms. This delay already appears when we probe the
input of the modulator with the oscilloscope. It indicates that the origin of the delay
is in the envelope detector. Using the deskew function of the oscilloscope and setting
it to 100 ns, the signals are again in phase as shown in Fig. 9.7 . To compensate for
this delay, a very long coaxial cable was used between the output of the power
splitter and the input of the PA.
Figure 9.8 shows the screenshot of the signal analyzer R3681 for the OFDM
measurement of the dynamic supply RF power amplifier at 2.4 GHz. The output
power is 11 dBm (12.6 mW) and the rms value of the EVM is 3%. The constellation
of the output signal can be seen at the bottom right corner of the figure.