|
|
Amplitude Control Options
Shaped On-Off Keying
The shaped on-off keying function is enabled/disabled using
the OSK enable bit (CFR1<25>). This function allows the
user to control the ramp-up and ramp-down time when turning
the DAC on or off. This function is primarily used in burst
transmissions of digital data to reduce the adverse spectral
impact of short, abrupt bursts of data.
Both auto and manual shaped on-off keying modes are
supported. CFR1<24> is used to select between auto and
manual on-off keying modes. Figure 20 shows the block
diagram of the OSK circuitry.
Autoshaped On-Off Keying Mode Operation
When autoshaped on-off keying mode is enabled, a single-scale
factor is internally generated and applied to the multiplier input
for scaling the output of the DDS core block (see Figure 20). The
scale factor is the output of a 14-bit counter that increments/
decrements at a rate determined by the contents of the 8-bit
output ramp rate register. The scale factor increments if the
OSK pin is high and decrements if the OSK pin is low. The scale
factor is an unsigned value; all 0s multiply the DDS core output
by 0 (decimal), and 0x3FFF multiplies the DDS core output by
16,383 (decimal).
Table 6 details the increment/decrement step size of the
internally generated scale factor per the ASF<15:14> bits.
Note that the maximum amplitude allowed is limited by the
contents of the amplitude scale factor register, allowing the user
to ramp to a value less than full scale.
Table 6. Autoscale Factor Internal Step Size
ASF<15:14> (Binary) Increment/Decrement Size
00 1
01 2
10 4
11 8
OSK Ramp Rate Timer
The OSK ramp rate timer is a loadable down counter, which
generates the clock signal to the 14-bit counter that generates
the internal scale factor. The ramp rate timer is loaded with
the value of the autoscale factor register (ASFR) every time
the counter reaches 1 (decimal). This load and countdown
operation continues for as long as the timer is enabled, unless
the timer is forced to load before reaching a count of 1.
If the load ARR control bit (CFR1<26>) is set, the ramp rate
timer is loaded upon an I/O update, upon a change in profile
input, or upon reaching a value of 1. The ramp timer can be
loaded before reaching a count of 1 by three methods.
The first method is by toggling the OSK pin or sending a rising
edge to the I/O UPDATE pin (or changing the state of a profile
pin). For this method, the ASFR value is loaded into the ramp
rate timer, which then proceeds to count down as normal.
The second method is if the load ARR control bit (CFR1<26>)
is set and an I/O update (or change in profile) is issued.
The last method is by setting the sweep enable bit. This switches
from inactive autoshaped on-off keying mode to the active
autoshaped on-off keying mode.
(原文件名:onoff shaped keying.jpg) |
|
发表于 2009-7-14 21:27:01
楼主