DAC5687IPZP datasheet(68/79 Pages) TI

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C001

lower sideband

200 MHz

lower sideband

200 MHz

DAC5687

SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006

A(t) = I(t)cos(

ct) – Q(t)sin(ωct) = m(t)

B(t) = I(t)sin(

ct) + Q(t)cos(ωct) = mh(t)

where m(t) and mh(t) connote a Hilbert transform pair and ωc is the sum of the NCO and CMIX frequencies.

The complex DAC5687 output is input to an analog quadrature modulator (AQM) such as the TRF3701 or

TRF3702. A passive (resistor-only) interface is recommended between the DAC5687 and TRF3701/2 (See the

Passive Interface to TRF3701/2 section). Upper single-sideband upconversion is achieved at the output of the

analog quadrature modulator, whose output is expressed as:

RF(t) = I(t)cos(

c + ωLO)t – Q(t)sin(ωc + ωLO)t

Flexibility is provided to the user by allowing for the selection of –B(t) out, which results in lower-sideband

upconversion. This option is selected by usb in the CONFIG3 register.

Note that the process of complex mixing in FMIX and CMIX to translate the signal frequency from 0 Hz means

that the analog quadrature modulator IQ imbalance produces a sideband and LO feedthrough that falls outside

the signal.

This is shown in Figure 79, which is the RF analog quadrature modulator (AQM) output of an asymmetric

three-carrier WCDMA signal with the properties in Table 18. The wanted signal is offset from the LO frequency

by the DAC5687 complex IF, in this case 122.88 MHz. The nearest spurious signals are ~100 MHz away from

the wanted signal (due to nonharmonic clock-related spurious signals generated by the fDAC/4 digital clock),

providing 200 MHz of spurious-free bandwidth. The AQM phase and gain imbalance produce a lower-sideband

product, which does not affect the quality of the wanted signal. Unlike the real IF architecture, the nonharmonic

clock-related spurious signals generated by the fDAC/2 digital clock fall ±245.76-MHz offset from the wanted,

rather than falling in-band.

As a consequence, in the complex IF system it may be possible that no AQM phase, gain and offset correction is

needed, instead relying on RF filtering to remove the LO feedthrough, sideband, and other spurious products.

Figure 79. Analog Quadrature Modulator Output for a Complex IF System

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