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on Thu Dec 10, 2009 4:31 pm
GNU Radio FM Transceiver Manual


----------- -------- ---------- ---------- -----------
Source/ Modulator/ Quarter Sound Card Sound Card 900 MHz
Sink -- Demodulator -- Mixer -- DAC/ADC -- Interface -- Transceiver -- Antenna
----------- -------- ---------- ---------- -----------

Figure 3. Architecture of FM SDR Transceiver

The FM software radio has the following components (Fig. 3):
# Modulator/Demodulator
# Quarter Mixer
# Sound Card DAC/ADC
# Sound Card Interface
# 900 MHz Transceiver Board

The signal to be transmitted is generated using GNU Radio core library. The received signal is played back on PC audio device. The antenna associated with software radio must operate at all of the possible frequencies encompassed by the system. Next, we explain the functionality of each of the transceiver components in detail.

FM Modulator/Demodulator. The modulator takes a real valued input signal and generates complex (I and Q) baseband signals. The carrier is not taken into account here. In mathematical terms, the complex signal x(t) is produced from the real-valued input signal u(t) as

x(t) = e^(2 * π * j * Kc * integral(u(s)ds))

where Kc is the modulation index indicating how much of the carrier spectrum the input signal should fill out, and j is the square root of -1. The real and imaginary parts of the result from this computation are in-phase (I) and quadrature (Q) signals of the baseband. On the receiver, the demodulator performs differentiation to obtain the rate of change (frequency change) of the carrier, which is the baseband signal. GNU Radio core library provides FM demodulator. Also on the receiver side, a low-pass filter is applied on the digital signal after the quarter mixer and before the demodulator. Low-pass filter takes out noise generated by the mixing process.

Quarter-Sampling-Rate Mixer. In frequency modulation (FM), the input signal is encoded into a carrier wave as its (carrier's) rate of change or variation of its instantaneous frequency. A carrier wave is an unmodulated sinusoidal waveform. The mixer multiplies the baseband signal received from the FM modulator by a carrier that is exactly 1/4 of the sampling frequency of the input signal. As a result, the baseband frequency shifts up to 12 kHz. In mathematical terms, the frequency modulated signal x(t) is produced from the baseband signal u(t) as

x(t) = u(t) * e^(2 * π * j * fc * t)

where fc is the carrier frequency. On the receiver, the reverse operation is performed, the digitized signal's frequency is down-converted to 12 kHz.

Sound Card DAC/ADC. For low cost SDR experimentation, a PC sound card with its on-board ADC and DAC is utilized for digital-to-analog conversion. However the relatively narrow bandwidth of the sound card limits the SDR experimentations to voice and low speed data applications. Both the input and output channels (I and Q) of the sound card have 48 kHz frequency.

Narrowband PC Sound Card Interface. Interface for the PC sound card to be used in conjunction with the 900 MHz transceiver board. The interface utilizes quadrature sampling detector to take the received signal down to baseband, i.e. to obtain the in-phase (I) and quadrature (Q) components of the received signal. I and Q signals are sampled by the left and right channels of the PC sound card input. For signal transmission, The left and right channel output (I and Q) with 12 kHz frequency from the PC sound card is fed into the interface module and the result is an IF signal with 10.7 MHz frequency.

900 MHz Transceiver Board. Down-converts a 4 MHz-wide radio channel located anywhere within the range of 902-928 MHz to an intermediate frequency of 10.7 MHz. This FM frequency range is chosen because its unlicensed in the United States and available for our experimentations. Plus the 900 MHz chip set is commercially available. The sampling rate of today's analog to digital converters is not quite high enough to digitize this entire frequency range directly from the antenna. To solve this problem the received signal is converted to an intermediate frequency and vice versa for transmitting.
Posts : 21
Join date : 2011-07-03
Age : 25
Home Address : calabanga, camarines sur


on Sun Jul 03, 2011 3:49 pm
ang haba naman..
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