3/15/2023 0 Comments Multimon area 52![]() ![]() It's from either the 1/f noise of the electronics or if it's a Zero-IF tuner (E4000) the LO beating with itself in the mixer. You'll almost certainly still notice a stable spike around DC. The samples themselves are unsigned and you subtract 127 (for rtlsdr it's: (x - 127.4f) * (1.0f / 128.0f)) from them to get their actual value. The actual output is interleaved so one byte I, then one byte Q with no header or metadata (timestamps). Check this reddit thread for caveats and details. The minimum resampled output is 0.5 MS/s. ![]() This resampled output can be up to 3.2 MS/s but 2.56 MS/s is the max recommended to avoid losing samples. That can be resampled inside the RTL2832U to present whatever sample rate is desired to the host PC. From this a 28.8 Msps stream at 8 bits is produced. The Sigma-Delta ADC samples at some high rate but low precision. ( Datasheets, general refs: But what is the Fourier Transform? A visual introduction by 3Blue1Brown, Quadrature Signals: Complex, But Not Complicated by Richard Lyons) This is complex-sampled (I and Q) by the ADC. The quadrature mixer produces a complex-baseband output where the signal spans from -bandwidth/2 to +bandwidth/2 and bandwidth is the analog bandwidth of the mixer output stages. The rtlsdr RTL2832U chips use a phased locked loop based synthesizer to produce the local oscillator required by the quadrature mixer. You can cut the leads to the LED to drop usage ~10%. Antti Palosaari's measurements show the R820T use ~300mA of 5v USB power while the E4000 devices use only ~170mA. For the data transfer mode USB 2 is required, 1.1 won't work. Because the devices use complex sampling (I/Q) the sample rate is equal to the bandwidth instead of just half of it. The highest safe sample rate is 2.56 MS/s but in some situations up to 3.2 MS/s works without USB dropping samples (RTL2832U drops them internally). The sensitivity is somewhere around -110 dBm typically. The dynamic range for most dongles is around 45 dB. The RTL2832 ADC differential input impedance is ~3,300 Ohm. ![]() All of the generic dongle antenna inputs are 75 Ohm impedance (some SDR branded versions have 50 ohm input). ![]() For both kinds the tuner error is ~30 +-20 PPM, relatively stable once warmed up, and stable from day to day for a given dongle. The R820T dongles use a 3.57 MHz or 4.57 MHz intermediate frequency (IF) while the E4000s use a IQ pair Zero-IF. The R820T and R820T2 go from 24-1760 MHz (but with reduced performance above 1500 MHz). The dongles with an E4000 tuner can range between 54-2147 MHz (in my experience) with a gap over 1100-1250 MHz in general. Patchvonbraun is the author and maintainer of the build-gnuradio script that made it easy for me, and multitudes of others, to get started with rtlsdr under GNU Radio. Keenerd is the author of many other rtl_* tools: rtl_fm, rtl_power (heatmap.py), rtl_adsb and code changes accepted into the mainline.
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