Datasheet
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f
SIG
/f
DAC
−20
−10
0
10
20
30
40
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Amplitude Adjustment − dB
G040
DAC5687
SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006
Figure 75. Amplitude Adjustment Factor for f
SIG
The steps for calculating the nonharmonic spurious signals are:
1. Find the spurious signal frequencies for the appropriate mode from Figure 70 , Figure 71 , or Figure 72 .
2. Find the amplitude for each spurious frequency for the appropriate mode from Figure 73 or Figure 74 .
3. Adjust the amplitude of the spurious signals for f
SIG
using the adjustment factor in Figure 75 .
Consider Example 1 with the following conditions:
1. X4 Mode
2. PLL off
3. Complex output
4. f
DAC
= 500 MHz
5. f
SIG
= 160 MHz = 0.32 × f
DAC
First, the location of the spurious signals is found for the X4 complex output in Figure 71 (a). Three spurious
signals are present in the range – 0.5 × f
DAC
to 0.5 × f
DAC
: two from f
DAC
/4 (35 MHz and – 215 MHz) and one from
f
DAC
/2 ( – 90 MHz). Consulting Figure 73 (c), the raw amplitudes for f
DAC
/2 and f
DAC
/4 are 60 and 58 dBc,
respectively. From Figure 75 , the amplitude adjustment factor for f
SIG
= 0.32 × f
DAC
is estimated at ~1 dB, and so
the f
DAC
/2 and f
DAC
/4 are adjusted to 61 and 59 dBc.
Table 16. Example # 1 for Calculating Spurious Signals
Spurious Frequency
Frequency/f
DAC
Raw Amplitude (dBc) Adjusted Amplitude (dBc)
Signal (MHz)
f
DAC
/4 0.07 35 58 59
f
DAC
/2 – 0.18 – 90 60 61
f
DAC
/4 – 0.43 – 215 58 59
Now consider Example 2 with the following conditions:
1. X2 Mode
2. PLL on
3. Real output
4. f
DAC
= 400 MHz
5. f
SIG
= 70 MHz = 0.175 × f
DAC
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