以特定频率，SDR和FFT获取功率

Question

I am trying to get the power at a particular frequency, with an RTL-SDR. I'm adapting FFT examples I've found online. Abbreviated code here (removed superfluous stuff):

import matplotlib.mlab as mlab
import rtlsdr

NFFT=1024

dwell = 0.016
sample_rate = 2.4e6
offset = 200e3
freq = 100e6

sdr = rtlsdr.RtlSdr(0)
sdr.set_sample_rate(sample_rate)
sdr.set_manual_gain_enabled(1)
sdr.set_gain(22.9)
sdr.freq_correction = 0

numsamples = next_2_to_pow(int(dwell * sample_rate))

freq = freq - offset  # avoid dc spike
sdr.set_center_freq(freq)
samples = sdr.read_samples(numsamples)

powers, freqs = mlab.psd(samples, NFFT=NFFT, Fs=sample_rate/1e6, window=hamming(NFFT))

bin_offset = int(offset / (sample_rate / NFFT))
freq = int(freq + offset)
pwr = float( "{:.2f}".format(10 * math.log10( powers[ int(len(powers)/2) + bin_offset ] )) )

This simply doesn't work. The power at the frequency stays about the same (noise floor level), even when I inject a signal.

I have two theories about why this doesn't work. 1) RTL-SDRs return I/Q data, and this method doesn't account for that (?) 2) My understanding of the FFT is simply not sound enough to perform it right.

Which is it? How can I fix it?

Answer 1

Have you tried using rtl_power then just picking the right bin in the output? It's a relatively recent addition to the rtl_sdr suite. In C, not Python. https://github.com/osmocom/rtl-sdr