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SDG7000A User Manual
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11.2 Phase Mode
The SDG7000A supports two phase modes.
Independent mode: Two channels can be used as two independent signal
generators, setting one
channel does not affect the output of the other
channel
Locked mode: Synchronous output of both channels, initially phase-aligned
and ensuring that they are
phase-locked when the
frequencies are in
teger
multiples of
each other, with
no drift
In "Independent" mode, the two channels can be used as two independent signal generators,
and setting one channel will not affect the output of the other channel.
In "Locked" mode, setting the time parameter of either channel will force the two channels to
reset to ensure that they are phase aligned. In addition, the frequency calculation of the channels
is specially handled in "Locked" mode to ensure that the two channels are phase-locked when
the frequencies are integer multiples of each other and do not drift.
DDS (Direct Digital Synthesis) technology has the advantage of high-frequency resolution and
fast frequency switching compared to PLL (Phase Locked Loop) technology. However, DDS is
an open-loop system and does not allow for true 'locking' between input and output in the same
way as a phase-locked loop, i.e. the input and output of a DDS structure may not be phase-
locked and the output may drift in phase when viewed over long time durations.
As an example, set CH1 to output a 1 MHz frequency clock and CH2 to output a 10 MHz
frequency clock, according to the relationship between the DDS frequency control word (FTW)
and the output frequency f
out
.
out
N
DDS
f
FTW
F2
=
With the DDS operating frequency F
DDS
= 2.5 GHz and the bit width of the frequency control
word N = 52, the actual output frequencies and frequency errors for both channels can be
calculated as shown in the table below. The error is introduced by the rounding of the frequency
control word. As the FTW is passed to the DDS circuit as a fixed-point integer, an error is
inevitably introduced between the theoretical and actual value of the FTW.
Table 11-1 Frequency errors in independent channel mode
Set frequency
1 MHz
10 MHz
Calculated value of
frequency control word
1801439850948.198
18014398509481.98
Frequency control word
rounding
1801439850948
18014398509482
Actual output frequency
0.999999999999998899 MHz
10.000000000000009 MHz
Frequency error
-0.1101288944482803 μHz
0.009313225746154785 μHz
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