Additional comments on using an A/D at the front end of a radio.
High Speed A/Ds that can handle the wide dynamic range of radio signals are
still an art form.
Any A/D suffers from the need to have filtering in front of it to avoid a
malady known as aliasing. In A/D theory, without band limiting the signals
into the A/D, it might try to convert signals out of the band you want, as
well as the in band components. You could have the unwanted freqeuncies be
the same spacing from the converter clock frequency, as the desired signals,
and then they "fold" into the band of desired signals, but would be like
noise or interference. The clocking of the A/D conversion, (the switching
speed) must be at least twice your highest desired frequency, and preferably
four times the highest in band frequency. The clock must be stable, and not
have jitter. For a 30 MHz radio, you might then need a clock of 120 MHz.
This has the aircraft band among others, below the clock frequency, and
there is the FM broadcast band, as well as low band public safety
communications.
In other words, a lot of undesired signals that could "fold" into the
desired HF bands when using an A/D.
For this reason, as an "Anti-aliasing filter" we DO put a high quality
filter in front of precision A/D converters, to limit the signals to those
of interest to us as an HF radio.
Thus, the need for a roofing filter is not restricted to just an analog
radio, but enters in for direct digital conversion as well.
If you Google search for "Analog to Digital Conversion" you will get many
white papers and tutorials on all the faults of A/D processes. That is why
they are not common in radios yet at front end RF frequencies. You need
many bits to handle a radio's dynamic range, or you need programmable gain
amplifiers to keep the incoming signal up at the optimum amplitude for the
range of the A/D system.
-Stuart
K5KVH
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