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RTL2832

The mystery chip at the heart of RTL-SDR.

Realtek Semiconductor 

6. INPUT IMPEDANCE

The input impedance of the I and Q lines of the RTL2832U is “unknown”. However, much is

known about the Elonics E4000 chip. It is a “Multi-standard CMOS Terrestrial RF Tuner” in a 5 mm

square package (QFN) that has 8 pins per side. It was designed to interface directly to a digital

demodulator. The E4000's analog IF outputs; specifically pin 20 (I+, IVOUTP), pin 19 (I-, IVOUTN),

pin 18 (Q+, QVOUTP), and pin 17 (Q-, QVOUTN) are connected, via a capacitor, directly to pins on

the RTL2832U. And these I/Q baseband outputs can have a differential peak-to-peak output

voltage of 1000 mV. Meaning, the RTL2832U can handle +4 dBm. The E4000 outputs also see an

output load of 15k-ohms and 10 pF. Also telling, the outputs' single ended output impedance is

250-ohms (ROUT). A space-wave has an impedance of 377-ohms. Therefore, there may be no

significant impedance mismatch between the RTL2832U's I/Q pins and a random wire antenna.

7. ELONICS E4000

The E4000's has a 4.5 dB (or better) noise figure and a 50-ohm RF input. Total gain is ~99

dB, which includes: 30 dB LNA gain, 12 dB mixer gain, and 57 dB IF gain. There are six stages of IF

gain, each with a maximum, digitally-programmable gain of: 6 dB, 9 dB, 9 dB, 2 dB, 15 dB, and 15

dB, respectively. The E4000 can handle 2000 mV peak-to-peak or +10 dBm. IIP3 is +5 dBm. It is

important to realize that bypassing the E4000 or R820T also bypasses a huge amount of gain. Like

radios of old, without RF amplification, use a long wire (SW), large ferrite (MW), or box loop (MW).

8. RTL2832U ARCHITECTURE: DIRECT SAMPLING

The heart of the RTL2832U is its ADC (Analog-to-Digital Converter) and DSP (Digital Signal

Processor). It performs Digital Down-Conversion DDC (IF to baseband) via I/Q mixers (phase is 90

degrees apart), digital low-pass filtering, I/Q resamping, and sends 8-bit I/Q data via the USB port.

9. RTL2832U ARCHITECTURE: COFDM

Unused by RTL-SDR is the Fast Fourier Transform FFT unit. This converts time-domain

information into frequency-domain information. Time-domain is when the y-axis is amplitude and

the x-axis is time. Frequency-domain is when the y-axis is amplitude and the x-axis is frequency. I

wrote a FFT algorithm for the PhilSCAN programs. The output seen on programs like GQRX, SDR#,

and SDR Touch is frequency-domain information. Under direct sampling gnuradio does the FFT.

The RTL2832U is a COFDM demodulator, that does things like: symbol synchronization, fine

frequency adjustment, phase rotation, channel estimation and correction, inner and outer

deinterleaving, Viterbi decoding, RS decoding, forward error correction, adjacent and co-channel

interference rejection, pre- and post- and long-echo channel reception, impulse noise cancellation,

automatic carrier recovery, channel equalization, channel state information, guard period removal,

pilot and TPS decoding, sample rate correction, sample rate interpolation and decimation, AGC

delay, measurement of radio frequency levels, SNR estimation, control of the tuner's AGC, MPEG

proportional integral derivative filtering, etc. The chip can automatically detect modulation

parameters (ex. transmission mode, code rate, guard interval) via patent-pending algorithms.

10. POCKET RADIO HF SDR

See Pocket Radio HF SDR for details on how to receive MW and SW using no up-converter

(~$43), no fancy metal case (~$24), no 4:1 balun (~$11), and no cables (~$10). DC to 1.7GHz

coverage opens up a world of sub-$20 DX, including: MW, SW, FM, CB, HAM, CORDLESS, RC, NOAA

WEATHER, TV, MILITARY, AIR, SHIP, RAIL, TRUNKING SYSTEMS, GPS, SATELLITE, and more.

just_rtfm@<NOSPAM>yahoo.com

Dr. Phil's Receiver Designs

Copyright 2015

Dr. Phil

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