Re: Direct sampling mode frequency range?

Hi, (from my little knowledge about DSP) RTL2832U (if) sample at 28.8MHz, by Nyquist, usable maximum frequency is 14.4MHz, above you have a aliases (mirrors, ghosts, harmonics). Frequencies above half of sample rate (Nyquist frequency) need to filtered to avoid aliases effect. For example, sound cards at 44.1KHz/48KHz have filter to cut frequencies above 20KHz. Front end tuner (E4000, R820T), used on RTL dongles, have a low pass filter. I don't know if RTL chip have a input filter, probable no. Direct Sampling mode, or IF mode of RTL2832U, use ability of RTL chip to tune to any IF used on front end tuner (3.57MHz for R820T). I suppose, ADCs of RTL3832U always sample at 28.8MHz, and decimate to lower sample rate, and output as I/Q signal. RTL IF can tune from 0 to 28.8MHz and output a small chunk of sample rate frequency as I/Q. If set sample rate at 2400000Hz (or samples per second), and tune IF to 1.2MHz, you have a signal from 0 to 2.4MHz. If tune to 10MHz, signal from 8.8MHz to 11.2MHz. On direct sampling mode (IF mode), use only one ADC, at 28.8MHz, input need to filtered to pass only frequency bellow 14.4MHz. But, IF can tuned above 14.4MHz, entering on zone of aliases. Suppose that you want ti listem a Medium Wave (MW/AM) radio at 1500KHz, you have a station on 1.5MHz and 27.3MHz, or inverse, instead you favorite station, you hear some one talking on CB band channel 29 or 30. So, to use Direct sampling mode need a low pass filter to tune below 14.4MHz and a band pass filter to tune from 14.4MHz to 28.8MHz. On Tue, Jan 28, 2014 at 9:50 PM, Richard Koch <n1gp(a)hotmail.com> wrote: -- Paulino Kenji Sato