NXP Semiconductors FREESCALE MC13192, Справочное руководство, Страница: 20 / 34

RF Front End
MC13192EVB Reference Manual, Rev. 1.2
3-2 Freescale Semiconductor
3.1 The Transmit (TX) Chain
The TX output is a differential open drain output, which should be loaded with a 200 Ohm differential to
provide 2 dBm output power (nominal). A 50:200 Ohm balun is used to load the output correctly.
A DC path from VDDA is necessary to supply the output stage and is provided via the center tap on the
balun. Output capacitance is 0.56 pF typical, which is tuned out by L102. This value includes a typical 0.2
pF layout and pad capacitance. L102 can be omitted, which results in approximately a 2 dB power loss.
The output impedance is somewhat higher than 200 Ohms, so applying a 50/400 Ohm balun yields slightly
higher output power. The TX power can be adjusted in 16 steps, from 0 to -20 dBm (typical) via software.
3.2 Switch and Bandpass Filter
The switch is a single control device which maintains acceptable performance down to a 1.8 V supply. It
is specified at 2.7 V, but has guaranteed performance with slightly relaxed specifications to below 2 V.
Insertion loss is 0.35 dB typical.
Three series capacitors are needed to maintain the switch bias. The switch typically draws 50 µA supply
current, which is not usually a concern. However, in a battery powered application, where 50 µA
consumption in power-saving mode will shorten the battery life, GPIO1 can be used to turn the switch on
and off. As the GPIO1 signal does not work in some of the test modes, the default is Switch On at all times.
Leaving out R111 and mounting a 0 Ohm resistor at R124 change the switch to GPIO1.
The harmonic content from TX passes the FCC and ETSI requirements, but performance can vary
significantly with differences in the actual layout, and in the components used (especially the baluns). A
small LC bandpass filter has been inserted to provide additional margin to the harmonic suppression
requirements. This filter also provides some rejection of unwanted out-of-band RX frequencies and some
ESD protection. The filter may be left out in some applications. Nominal TX output at the antenna or the
SMA connector is 0 dBm.
3.3 The Receive (RX) Chain
The RX input impedance is high impedance and differential. Typical values are 2-4 kOhm in parallel with
0.63 pF. The latter includes a typical 0.2 pF layout pad capacitance. The inputs may be DC connected
together, but should have no DC connection to other parts of the circuit.
A 50:200 Ohm balun is used to step up the impedance and provide a balanced output. R109 provides
termination and L101 tunes out the input and PCB capacitance. This combination provides a good return
loss, as seen in the measurements. Normally, return loss is of little concern and R109 can be left out. This
provides approximately 5 dB better receiver sensitivity as the balun output voltage approaches the
open-load value. Return loss is low, typically 3.5 dB, but acceptable. All sensitivity measurements have
been made with R109 left out. L101 can be left out, but this results in approximately 4.5-5 dB sensitivity
loss. A 50:400 Ohm balun provides slightly better performance, but is not a commonly available
component.