# Spectrum at 1 GHz

I have a query regarding flow graph in GRC.
I am trying to get spectrum from DBS_RX daughter board using GRC. I have
used three blocks as

USRP2 Source --> FFT Filter --> FFT Sink

They have following parameters,

USRP2 Source:
Decimation 1M
Frequency 1G
Gain (dB) 0

FFT Filter:
Decimation 1
Taps firdes.complex_band_pass(1.0, 3000, -0.05e3, 0.05e3, 5,
firdes.WIN_HAMMING, 0)

FFT Sink:
Sample Rate 2K
Baseband Frequency 1K
FFT Size 512
Refresh Rate 30

I need to display shows frequency spectrum centered at 1 GHz. Since I am
using decimation of 1M, the frequency is lowered down to 1K. I filter it
using FFT bandpass filter with 100 Hz bandwidth. The fft sink shows
spectrum at around 1 KHz. The problem is if I do not use decimation, the
GRC seems to be very slow (may be processing 1 GHz at GHz sample rate
is too much). I need to make a FFT plot showing spectrum at 1 GHz. How
Can I accomplish this?

Regards,
Umair N.
MSc Communication Engineering
Chalmers University ot Technology, Sweden.

your samples rates do not match:

usrp2 rate is 100e6/1e6

fft sink rate: 2e3

-Josh

Hi Umair

I believe the decimation factor you have chosen is beyond the maximum
allowed (I think, from memory this is 512, which is well less than 1M =
1 times 10^6).

It seems you are trying to sample the passband signal to show a spectrum
at 1 GHz, however a digital-down conversion is performed at the
receiver, so you should see a signal at baseband. You should not need to
use such a high sampling rate - ideally, Nyquist rate (based on the
signal bandwidth when downconveted to baseband) will suffice.

Regards

Ian.

Hi Umair

There is a setting in the USRP2 source block that lets you set the
carrier frequency. This should be 1e9 (i.e. 1G).

What is the signal you are trying to receive?

Ian

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