This is a test prototype for experimenting with Software Defined Radio (SDR).
It is composed of several boards that are described in detail elsewhere on this
Combined with suitable firmware and FPGA design, these boards comprise a receiver
capable of capturing 20kHz of signal over 0-20MHz, demodulating it with a variety
of formats and driving high-quality audio.
The iceRadio system diagram is shown in Figure 1 below.
Figure 1: iceRadio System Diagram
RF input from the
antenna first passes thru a 20MHz low-pass anti-aliasing filter (not shown) before
entering the RXADC card where it is first amplified and then digitized in an ADC.
The maximum input signal allowed without exceeing the range of the ADC puts the
0dBfs point of this system at -10dBm in 50 ohms. The ADC runs at 40MSPS with a
resolution of 10 bits, providing approximately 60dB of dynamic range and 20MHz of
bandwidth which places the quantization noise floor at about -70dBm.
From the ADC, data passes into the FPGA. This is an iCE5LP4k part which
provides 20 4kb RAM blocks and 4 16x16 MAC blocks which are essential for the
DSP required for the downconversion. In the FPGA the ADC data is pre-processed
to a sample rate appropriate for the MCU. Figure 2 below shows the primary
components of the FPGA design.
Figure 2: iceRadio FPGA Diagram
For diagnostic and analysis, a 1024x11-bit sample buffer is provided which can
snapshot the ADC input data as well as the overrange bit and store it in SRAM
for analysis by the MCU. This provides the capability to check for overflow and
also to generate wide-band signal analysis via DFT to find strong signals within
the input passband.
Input Data Formatting
10-bit 40MSPS offset-binary data from the ADC is reformatted to 10 bit two's
complement signed for further processing.
Tuning and Real / Complex conversion
10-bit real data passes into a quadrature tuner. Here, a numerically controlled
oscillator (NCO) generates the local tuning reference to mix the incoming sampled
RF signal down to baseband. In the process the real input signal is converted into
complex I and Q. Data precision is maintained at 10-bits.
Baseband I and Q is decimated by a factor of 256 in a 4-stage CIC decimator. This
structure provides 4 bits of additional resolution due to the integration which
takes place. Output is truncated to 16 bits total at a rate of 156.25kSPS.
16-bit decimated data at 156.25kSPS is futher decimated by 8 in a FIR decimator.
This subsystem provides up to 246 taps of 16-bit FIR coefficients which allows
substantial stop-band rejection and fairly narrow transition bands. Corner
frequency of the filter is 9kHz but can be easily changed if needed. The output
signal is 16 bits at 19.531kSPS, complex.
The 16-bit complex I/Q signal is reformatted as a 16-bit stereo I2S data stream
with I on the left channel and Q on the right channel. This signal is sent to
both the MCU and to a mux which can select either the raw I / Q signal for the
DAC output, or the processed audio returned from the MCU over the I2S data input.
SPI Control Interface
The SPI Control interface provides up to 128 32-bit wide read/write registers
which the MCU uses to control the FPGA design and check status. All tuning and
configuration of the RF processing takes place thru this interface, as well as
triggering the 1k sample buffer and reading back its contents.
Overall FPGA design
The current design which supports only receive operations is using about 37% of
the total resources available in the iCE5LP4k. It may be possible to include
additional processing functions on the FPGA to reduce the burden on the MCU.
The STM32F303 processor interfaces to the FPGA via SPI and I2S serial ports to
control the front end processing and exchange baseband and audio data. Firmware
running on the MCU configures the FPGA from a micro-SD card at power up, confirms
the presence of the proper design by reading an ID register in the SPI interface
and then configures the tuning and mux settings. A background process runs which
accepts I2S data from the FPGA, filters it, adjusts gain, applies user-selected
demodulation processing and then returns demodulated audio to the FPGA where it
is forwarded to the Audio DAC.
The first step in the processing is to further filter the input data. The full
9kHz bandwidth is rarely useful for broadcast and amateur radio signals so a
set of real-time selectable 6th-order IIR filters with bandwidths of 8kHz,
6kHz, 4kHz, 2kHz, 1kHz and 500Hz are available.
After decimation and filtering the total signal power can be significantly
reduced so an AGC automatically adjusts the signal power to a pre-determined
level. The attack and decay time constants of the AGC are separate, allowing
for fast attack and slow decay which reduces leading-edge distortion of
signals with wide dynamic range such as amateur SSB.
At present the MCU application supports these demodulation types:
These background audio processing algorithms currently require no more than 30% of
of the total available CPU cycles. Other demodulation formats may be possible
such as various digital modes.
- AM: For broadcast and Short-Wave listening, this algorithm uses
a simple sqrt(I*I+Q*Q), followed by a DC blocker to remove
the carrier component.
- Synchronous AM: For broadcast and Short-Wave listening, this
algorithm regenerates the local carrier reference using an extremely
narrowband PLL for reduced noise in weak signal conditions.
- Upper Sideband: This algorithm performs a phase shift of the I and
Q signals by +/-45 degrees, followed by DC blocking and summation to
cancel out the lower sideband. Phase shifting is performed with a pair of
optimized 6th-order IIR allpass filters.
- Lower Sideband: This uses the same algorithm as the Upper Sideband
described above, but uses a differencing network to cancel upper sideband.
- Upper + Lower: This mode outputs both upper and lower SSB sidebands
simultaneously on the stereo output channels. This produces a unique tuning
experience that may be useful for finding signals.
- Narrowband FM: This is mode differentiates the phase of the
complex I and Q signal to demodulate FM. Standard de-emphasis is included.
- Raw: This is the filtered and AGCed I and Q data applied
directly to the left / right stereo channels.
The foreground process on the MCU is either a simple serial command-line interface
with simple functions for manipulating the FPGA configuration, tuning setup and
background demodulation parameters or a GUI based on a color LCD and rotary
At present the iceRadio system demonstrates basic functionality and provides
a good base for improvement. Here's a list, in no particular order, of things to
explore in the future:
- Additional demodulation algorithms for both audio and digital modes.
- Enhanced front-end RF interfaces, including filtering and pre-amplification
to improve sensitivity and immunity to strong broadcast signals.
- Better user interfaces. The current serial command line interface and
LCD GUI can be improved to make the system more flexible and easier to
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