Datasheet

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C001
lower sideband
200 MHz
LO
lower sideband
200 MHz
LO
DAC5687
SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006
A(t) = I(t)cos( ω
c
t) – Q(t)sin( ω
c
t) = m(t)
B(t) = I(t)sin( ω
c
t) + Q(t)cos( ω
c
t) = m
h
(t)
where m(t) and m
h
(t) connote a Hilbert transform pair and ω c 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( ω
c
+ ω
LO
)t – Q(t)sin( ω
c
+ ω
LO
)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 f
DAC
/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 f
DAC
/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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