Registers R35, OUTBUF_TX_TYPE, OUTBUF_RX_TYPE are used to configure the RF output buffer type between open drain and push-pull. Push-pull is easy to use; all that is required is a DC-blocking capacitor at the output. The output waveform is square wave and therefore, harmonics rich. Open-drain output provides an option to reduce the harmonics using an LC resonant pullup network at its output. Table 8-5 summarizes an example an open-drain vs. push-pull application.

Table 8-5 RF Output Buffer Type

BUFFER TYPE	OPEN-DRAIN			PUSH-PULL
Connection Diagram
Output Power	470 MHz	480 MHz	490 MHz	470 MHz	480 MHz	490 MHz
fo	2.7 dBm	2.8 dBm	2.8 dBm	–0.1 dBm	0 dBm	0.1 dBm
2fo	–31 dBc	–30.7 dBc	–30.5 dBc	–30.4 dBc	–30.2 dBc	–30 dBc
3fo	–17.3 dBc	–17.9 dBc	–18.1 dBc	–11.9 dBc	–12.1 dBc	–12.4 dBc
4fo	–39 dBc	–40.4 dBc	–41.6 dBc	–28.5 dBc	–28.4 dBc	–28.1 dBc
5fo	–18.1 dBc	–17.8 dBc	–17.6 dBc	–15.6 dBc	–15.6 dBc	–15.7 dBc
6fo	–27.6 dBc	–27.2 dBc	–28.5 dBc	–29.5 dBc	–29.8 dBc	–29.3 dBc

Clearly, with a proper LC pull up in open-drain architecture, the 3^rd to 5^th harmonics could be reduced.