On Sat, Apr 7, 2018 at 1:31 AM, PRIYANKA PRIYADARSHINI < priyankapriyadarshini...@gmail.com> wrote:
> I am trying to receive a GPS signal and I am using RTL-SDR2832U, and an > antenna with 40dB gain. > > According to my research, the signal coming from the satellite is at RF, > and it gets downconverted in front-end to IF (Intermediate Frequency). > > Now, I need to remove that IF from the signal coming from the front end by > multiplying it with the local generated carrier signal. For this, I want to > know the exact value of the IF. But, I am confused that how can I get this? > I also tried reading the data sheet of RTL-SDR but couldn't find the > helpful information. > You do not need to know the intermediate frequency to use an RTL-SDR. The hardware finishes the conversion and gives you a digital signal at baseband — that is, the center frequency is the frequency you requested, assuming you're using the standard rtlsdr drivers. (Details: There are two different tuner schemes used in RTL devices (depending on the tuner chip, not the RTL2832U chip). One of them actually uses no intermediate frequency; the LO is set exactly to the frequency you request, and the samples you get are directly from the ADC. It can be recognized by the presence of a DC offset (center spike) in the obtained signal. The other uses a nonzero intermediate frequency, but the digital samples are digitally downconverted before they are sent to the host computer.) Understanding IF is important to understanding how receivers are implemented, and what imperfections they can have, but in GNU Radio you generally do not need to consider the hardware's IF, whatever hardware you are using, because the hardware or the driver takes care of it for you. In some cases you need or want to do frequency shifts in software (such as to avoid a DC offset, to receive multiple transmissions, or to work with a subcarrier), but that'll be because of some property if *your* tuning or the signal *you* know you're working with, not the hardware.
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