Datasheet

www.ti.com
Location of the Spurious Signals
f
SIG
/f
DAC
−0.5
−0.4
−0.3
−0.2
−0.1
−0.0
0.1
0.2
0.3
0.4
0.5
0.0 0.1 0.2 0.3 0.4 0.5
Spurious Frequency/f
DAC
G026
f
SIG
f
SIG
− f
DAC
/2
(a) Complex Output in X2 Mode (b) Real Output in X2 Mode
f
SIG
/f
DAC
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.0 0.1 0.2 0.3 0.4 0.5
Spurious Frequency/f
DAC
G027
f
SIG
f
SIG
− f
DAC
/2
DAC5687
SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006
To calculate the nonharmonic clock-related spurious signals for a particular condition, we first determine the
location of the spurious signals and then the amplitude.
The location of the spurious signals is determined by the DAC5687 output frequency (f
SIG
) and whether the
output is used as a dual-output complex signal to be fed to an analog quadrature modulator (AQM) or as a real
IF signal from a single DAC output.
Figure 70 shows the location of spurious signals for X2 mode as a function of f
SIG
/f
DAC
. For complex outputs, the
spurious frequencies cover a range of – 0.5 × f
DAC
to 0.5 × f
DAC
, with the negative complex frequency indicating
that the spurious signal falls in the opposite sideband from the wanted signal at the output of the AQM. For the
real output, the phase information for the spurious signal is lost, and therefore what was a negative frequency for
the complex output is a positive frequency for a real output.
For the X2 mode, there is one spurious frequency with an absolute frequency less than 0.5 × f
DAC
. For a complex
output in X2 mode, the spurious signal always is offset f
DAC
/2 from the wanted signal at f
SIG
– f
DAC
/2. For a real
output, as f
SIG
approaches f
DAC
/4, the spurious signal frequency falls at f
DAC
/2 – f
SIG
, which also approaches
f
DAC
/4.
Figure 70. Frequency of Clock Mixing Spurious Images in X2 Mode
Figure 71 shows the location of spurious signals for X4 and X4L mode as a function of f
SIG
/f
DAC
. The addition of
the f
DAC
/4 clock frequency for the first interpolation filter creates three new spurious signals. For a complex
output, the nearest spurious signals are f
DAC
/4 offset from f
SIG
. For a real output, the signal due to f
SIG
– f
DAC
/4
and f
SIG
– f
DAC
× 3/4 falls in band as f
SIG
approaches f
DAC
/8 and f
DAC
× 3/8. This creates optimum real output
frequencies f
SIG
= f
DAC
× N/16 (N = 1, 3, 5, and 7), where the minimum spurious product offset from f
SIG
is f
DAC
/8.
60 Submit Documentation Feedback Copyright © 2005 – 2006, Texas Instruments Incorporated
Product Folder Link(s): DAC5687