Table 1. Pin Termination Requirements and Limitations

8.2.2.1 Baseband Inputs

The baseband inputs consist of the in-phase signal (I) and the Quadrature-phase signal (Q). The I and Q lines are differential lines that are driven in quadrature. The nominal drive level is 1-V_PP differential on each branch.

The baseband lines are nominally biased at 0.25-V common-mode voltage (V_CM); however, the device can operate with a V_CM in the range of 0 V to 0.5 V. The baseband input lines are normally terminated in 50 Ω, though it is possible to modify this value if necessary to match to an external filter load impedance requirement.

8.2.2.2 LO Input

The LO inputs can be driven either single-ended or differentially. There is no significant performance difference between either option with the exception of the sideband suppression. If driven single-ended, either input can be used, but LOP (pin 3) is recommended for best broadband performance of sideband suppression. When driving in single-ended configuration, simply ac-couple the unused port and terminate in 50 Ω. The comparison of the sideband suppression performance is shown in Figure 71 for driving the LO single-ended from either pin and for driving the LO input differentially.

8.2.2.3 RF Output

The RF output must be ac-coupled and can drive a 50-Ω load. The suggested output match provides the best broadband performance across the frequency range of the device. It is possible to modify the output match to optimize performance within a selected band if needed. The optimized matching circuits are to match the RF output impedances to 50 Ω.

Figure 72 shows a slightly better OIP3 performance at the frequency above 1850 MHz with an 0.2-pF matching capacitor.

8.2.2.4 350-MHz Operation

A different matching circuit, as shown in Figure 66, could also be applied to improve the performance for the frequency from 300 MHz to 400 MHz.

Figure 66. Matching Components for Operation Centered at 350 MHz

Figure 73 and Figure 74 show a slight improvement in OIP3 performance at frequencies above 1850 MHz with an 0.2-pF matching capacitor.

8.2.2.5 DAC to Modulator Interface Network

For optimum linearity and dynamic range, a digital-to-analog converter (DAC) can interface directly with the TRF3705 modulator. It is imperative that the common-mode voltage of the DAC and the modulator baseband inputs be properly maintained. With the proper interface network, the common-mode voltage of the DAC can be translated to the proper common-mode voltage of the modulator. The TRF3705 common-mode voltage is typically 0.25 V, and is ideally suited to interface with the DAC3482/3484 (DAC348x) family because the common-mode voltages of both devices are the same; there is no translation network required. The interface network is shown in Figure 67.

Figure 67. DAC348x Interface with the TRF3705 Modulator

The DAC348x requires a load resistor of 25 Ω per branch to maintain its optimum voltage swing of 1-V_PP differential with a 20-mA max current setting. The load of the DAC is separated into two parallel 50-Ω resistors placed on the input and output side of the low-pass filter. This configuration provides the proper resistive load to the DAC while also providing a convenient 50-Ω source and load termination for the filter.

8.2.2.6 DAC348x with TRF3705 Modulator Performance

The combination of the DAC348x driving the TRF3705 modulator yields excellent system parameters suitable for high-performance applications. As an example, the following sections illustrate the typical modulated adjacent channel power ratio (ACPR) for common telecom standards and bands. These measurements were taken on the DAC348x evaluation board.

8.2.2.6.1 WCDMA

The adjacent channel power ratio (ACPR) performance using a single-carrier WCDMA signal in the UMTS band is shown in Figure 68.

Figure 68. Single-Carrier WCDMA ACPR, IF = 30 MHz, LO Frequency = 2110 MHz

A marginal improvement in OIP3 and output noise performance can be observed by increasing the LO drive power, resulting in slightly improved ACPR performance. The ACPR performance versus LO drive level is plotted in Figure 75 across common frequencies to illustrate the amount of improvement that is possible.

8.2.2.6.2 LTE

ACPR performance using a 10 MHz LTE signal in the 700-MHz band is shown in Figure 69.

Figure 69. 10 MHz LTE ACPR, IF = 30 MHz, LO Frequency = 718 MHz

8.2.2.6.3 MC-GSM

ACPR performance using a four-carrier MC-GSM signal in the 1800-MHz band is shown in Figure 70.

Figure 70. Four-Carrier MC-GSM, IF = 30 MHz ACPR, LO Frequency = 1812 MHz