DAC5687IPZP datasheet(66/72 Pages) TI

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C001

lower sideband

200 MHz

lower sideband

200 MHz

DAC5687

SLWS164B – FEBRUARY 2005 – REVISED JUNE 2005

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 up-conversion 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

up-conversion. 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 side-band and LO feedthrough that falls outside

the signal.

This is shown in Figure 76, which is the RF analog quadrature modulator (AQM) output of an asymmetric three

carrier WCDMA signal with the properties in Table 17. 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 non-harmonic 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 non-harmonic

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

rather than falling inband.

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 76. Analog Quadrature Modulator Output for a Complex IF System

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