USRP2 & Basic receiver. Unexpected behavior

Hello,

After running my USRP2 with the GRC software I got some results that I
didn’t expect. My configuration was is simple as an USRP2 source
followed by
a FFT sink to see the spectrum (around MHz with a 75cm antenna), but:

  1. I can only see the sprectrum up to 100MHz. I thought that the LTC2284
    ADC
    had a dynamic range of 575MHz…Why can I only see the spectrum lower
    than
    100 MHz?
    (When I say that I can not see the spectrum I mean that at 100MHz I get
    a
    mirror of the spectrum between 0-100MHz).

2)The spectrum I can see is much more clear around 98MHz, where I can
see a
lot of carriers corresponding to FM radio stations that I cannot see for
instance around 90Mhz.

  1. What is the minimum decimation I can set in the USRP2 block?
    Experimentally I saw it is 5 but I don’t understand why.

My specific configuration of GRC is:
USRP Source block: Decimation=5, Frequency=(tunable to cover FM band)
FFT sink: sample rate=21MHz (105M/5), fft size: 512

In fact when I run my FM receiver (also made in GRC), I can only listen
to
89MHz and 108MHz radio stations. I expected to receive the whole FM
spectrum.

If some of you have a answers to my questions I will be very glad
because I
want to understand USRP2 and GNU Radio as soon as possible to became an
active member of this community!

Regards,
Jorge.

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