HFSDR Prototype

HFSDR Prototype

Introduction

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 site: 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.

System Architecture

The HFSDR Prototype system diagram is shown in Figure 1 below.

HFSDR System Diagram

Figure 1: HFSDR System Diagram

ADC

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.

FPGA

From the ADC, data passes into the FPGA where it is pre-processed for the MCU. Figure 2 below shows the primary components of the FPGA design.

HFSDR FPGA Diagram

Figure 2: HFSDR FPGA Diagram

Sample Buffer

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. Examples of signals processed thru the sample buffer are shown in Figure 3 and 4.

Sample Buffer Time Waveform

Figure 3: ADC Time Waveform (amplitude scaled to max=+/-1.0)

HFSDR FPGA Diagram

Figure 4: ADC PSD

Input Data Formatting

10-bit 40MSPS offset-binary data from the ADC is reformatted to 14 bit two's complement signed for further processing. This allows future updates to higher dynamic range processing without significant redesign of the the FPGA functions.

Tuning and Real / Complex conversion

14-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 14-bits.

CIC Decimation

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. Since then next step in the processing will provide an additional bit, the output of the CIC is rounded to input plus 5, or 19 bits total at a rate of 1.536MSPS.

FIR Decimator

19-bit decimated data at 1.536MSPS is futher decimated by 4 in a FIR decimator. This subsystem provides up to 250 taps of 18-bit FIR coefficients which allows substantial stop-band rejection and fairly narrow transition bands. Four separate sets of coefficients are available, selectable in real-time. Corner frequencies are currently set at 18kHz, 9kHz, 4kHz and 2.8kHz, but can be easily changed if needed. This filter provides approximately 80dB of stop-band rejection for all four bandwidths. The output signal is 19 bits at 39.0625kSPS, complex.

AGC

After tuning, filtering and decimation, the low-rate baseband signal has a wide dynamic range. Strong input signals will be nearly at the maximum range of the 19-bit data word, while weak signals could occupy only the bottom few lsbs. A Log AGC compresses the dynamic range to a nominal 16 bits by adjusting the gain to ensure that the RMS signal amplitude matches a pre-set target value which allows sufficient headroom for modualtion without clipping, yet maximizes the overall signal amplitude within the 16-bit word. This AGC design has more than 80dB of gain range and settles in approximately 1s. The AGC may be disabled and a fixed, externally set gain value may be applied if desired. The automatically determined gain value can be read back by the MCU to estimate input signal strength.

I2S Master

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 Spartan 6 LX9 FPGA. There is ample room for the addition of transmit support processing as well as the possibility of complex processing for modulation / demodulation of digital waveforms.

MCU

The STM32F427 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, filter and AGC parameters. A background process runs which accepts I2S data from the FPGA, applies user-selected demodulation processing and then returns demodulated audio to the FPGA where it is forwarded to the Audio DAC.

At present the MCU supports three different demodulation types:

These background audio processing algorithms currently require no more than 7% of of the total available CPU cycles. Other demodulation formats are possible, including narrowband FM, synchronous AM and various digital modes.

The foreground process on the MCU is a simple serial command-line interface with simple functions for manipulating the FPGA configuration, tuning setup and background demodulation parameters.

Future Work

At present the HFSDR Prototype 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:

Follow Up

I've had some time to pursue a few of the items from the Future Work list above with some encouraging results

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Last Updated
:2017-03-04
Comments to:
Eric Brombaugh

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