Texas Instruments TRF1500 - Manual (Page 49)

SWRA004A
50 TRF1500 Integrated Dual-Band RF Receiver User’s Guide
Low-Band and High-Band Transmit
Low- and High-Band Transmit Mixer
Figure 17 details the block diagram of the transmit mixer. The
TRF1500 provides a transmit mixer for down converting the
system transmit signal (low-band or high-band) to a common IF
for loop-back testing. The LO input for this mixer can be selected
from either the low-band LO input or the high-band LO input by
means of the digital control. The RF input of the transmit mixer
provides a broad-band 200
Ω
differential input impedance over a
band of 800 MHz to 2GHz and thus requires minimal external
matching.
The mixer is a double-balanced Gilbert cell design with open
collector outputs. The Gilbert cell structure was implemented for
its robust isolation and harmonic suppression characteristics.
Figure 17. Transmit Mixer Block Diagram
Low-Band and High-Band Transmit Mixer RF Input
Figure 18 details the transmit mixer RF input configuration. The
transmit mixer can be either driven single-ended or differentially.
For evaluation purposes, a 4:1 transformer balun is used on the
EVM to drive the transmit mixer single-ended. The input requires
very little external matching. A shunt capacitor (C35) near the
input port (J30) and a shunt capacitor (C36) near the transformer
balun (T30) are the only input impedance matching components
required.