Datasheet

f
O
− Output Frequency − MHz
−250
−200
−150
−100
−50
0
50
100
150
200
250
0 25 50 75 100 125 150 175 200 225 250
f
image
− MHz
IF
G018
IF + f
DAC
/4
IF − f
DAC
/4
IF − f
DAC
/2
IF − f
DAC
y 3/4
f
O
− Output Frequency − MHz
0
50
100
150
200
250
0 25 50 75 100 125 150 175 200 225 250
f
image
− MHz
G019
IF − f
DAC
y 3/4
IF − f
DAC
/4
IF + f
DAC
/4
IF − f
DAC
/2
IF
DAC5686
SLWS147F – APRIL 2003 – REVISED JUNE 2009 ............................................................................................................................................................ www.ti.com
Figure 28 (b) shows the location of the largest spurious signals for f
DAC
= 500 MSPS and 4 × interpolation for a
real output signal. With a real output signal, there is no distinction between negative and positive frequencies,
and therefore the signals that appear at negative frequencies with a complex signal potentially fall near the
wanted signal. In particular, at IFs near f
DAC
/8, f
DAC
/4, and f
DAC
× 3/4 (62.5 MHz, 125 MHz and 187.5 MHz in this
example) the mixing effect results in spurious signals falling near the wanted signal, which may present a
problem depending on the system application. For a frequency-symmetric signal (such as a single WCDMA or
CDMA carrier), operating at exactly f
DAC
/8, f
DAC
/4 and f
DAC
× 3/4, the spurious signal falls completely inside the
wanted signal, which produces a clean spectrum but may result in degradation of the signal quality.
(a) (b)
Figure 28. Spurious Frequency vs IF
The offset between wanted and spurious signals is maximized at low IFs ( < f
DAC
/8) and at f
DAC
× 3/16, f
DAC
×
5/16 and f
DAC
× 7/16. For example, with f
DATA
= 122.88 MSPS and 4 × interpolation, operating with IF = f
DAC
×
5/16 = 153.6 MHz results in spurious signals at offsets of 60 MHz from the wanted signal.
Figure 29 (a) shows the amplitude of each spurious signal as a function of IF in external-clock mode (CLK2
input). The dominant spurious signal is IF – f
DAC
/2. The amplitudes of the IF + f
DAC
/4 and IF – f
DAC
/4 are the
next-highest spurious signals and are approximately at the same amplitude. Finally, at IF frequencies greater
than 100 MHz, small spurious signals at IF f
DAC
/8 and IF – f
DAC
× 3/4 are measurable.
Figure 29 (b) shows the amplitude of each spurious signal as a function of IF in PLL clock mode (CLK1 input).
Generating the DAC clock with the onboard PLL/VCO increases the IF – f
DAC
/2 by 10 dB and the amplitude of
the IF f
DAC
/4 and IF – f
DAC
× 3/4 by 25 dB compared to the external-clock mode. The IF f
DAC
/8 spurs are the
same as in the external-clock mode.
28 Submit Documentation Feedback Copyright © 2003 – 2009, Texas Instruments Incorporated
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