MOGlabs ARF021, Руководство пользователя

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9.7 Frequency gain
83
Listing 9.7 shows the equivalent formulation using the
TABLE,RAMP
command, which allows the power to be specified in dBm instead of
hexadecimal increments.
TABLE,CLEAR,1
TABLE,XPARAM,1,POW
# define a ramp from off (0x0) to 0dBm
TABLE,RAMP,1,POW,0x0,0dBm,16ns,100
# append the reverse of the ramp
TABLE,RAMP,1,POW,0dBm,0x0,16ns,100
# loop back to the beginning twice more
TABLE,LOOP,1,-1,1,2
# last entry cannot be a loop
TABLE,APPEND,1,POW,0x0,16ns
Listing 9.7:
Alternate specification of triangle wave using
TABLE,RAMP
.
9.7 Frequency gain
The DDS parallel interface is 16-bit, which is sufficient to define the
amplitude (14-bits) and phase (16-bits) but not frequency (32-bits).
This is an inherent restriction of the
DDS
, so in order to specify
frequency on the parallel interface, “gain” is applied by the DDS
when receiving the instruction, which amounts to a
bit-shift
of the
incoming value (Figure 9.4).
LSB MSB
0 11 30 26
FM gain = 11 16-bit parameter
Figure 9.4:
Visualisation of how FM gain allows the frequency word to
be modified by the 16-bit parallel bus. In this example, the gain is 11 and
only the indicated bits can be modified on the parallel bus.
This has the effect of restricting the smallest and largest changes
that can be made in parallel frequency mode through the associated
frequency discretisation (step size) and value range. If large chan-
ges to frequency are required, high gain should be used. If high
resolution is required, small gain should be used. The outcome of
different gain settings are shown in Table 9.1.