QI am trying to design a transmitter in the 50-90 MHz band for satellite
communications. I want to generate a signal in this bandwidth directly using a
DAC. Is there any analog devices converter able to do that? What is the minimum
sample rate necessary for that process? Depending on the frequency, I should
use a sin/x compensation filter, shouldn't I?
ATo generate sinusoidal signals in the range 50-90MHz you need a DAC which has
a sampling frequency >180MSPS. However, it is much easier to design the
anti-image filter if you use a higher sampling frequency, around 270MSPS or
higher will allow you a wider transition band in your anti-image filter.
If the signals you are generating are
then the easiest way to generate them is using a Direct Digital Synthesis (DDS)
device, rather than a DAC. Wheras for a DAC you must calculate the values for
each sample, and send them to the DAC, in a DDS, you simply specify the
frequency, phase shift, amplitude etc and the digital circuitry on board
generates the digital values and converts them to analog at a given sampling
rate. The 50-90 MHZ range is well withing the capability of the AD985x and
AD995x family of DDS devices. sinx/x compensation filters are included in some
of the devices, to keep the amplitude constant.
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