[Elecraft] here is how to "get a feel" for a roofing

[Stuart Rohre]

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