Filter Board

The Pico Dev-iCE 📡

DSP Educational Vehicle & iCE40 Mixed-Signal Laboratory

📌 Overview

The Pico Dev-iCE is a low-cost, high-performance "Lab-in-a-Box." It combines the high-speed programmable logic of a Lattice iCE40UP5K FPGA with the flexible C/MicroPython ecosystem of a Raspberry Pi Pico (YD-RP2040 with 16MB Flash).

Designed specifically as an educational platform, it abandons "black box" RF chips in favor of discrete, observable analog blocks. Students can physically probe the RF signal path, design their own LC filters on pluggable sandboxes, and write Verilog to perform Direct Digital Conversion (DDC), digital decimation, and hardware I2S audio routing.

Here is a video of the SDR receiving on 20 meters..

🚀 Key Capabilities

• HF Software Defined Radio (RX/TX): Full-duplex DDC/DUC transceiver covering 0 - 15.36 MHz (1st Nyquist) with Super-Nyquist capabilities up to 30+ MHz.
• Vector Network Analyzer (VNA): Built-in Return Loss Bridge (RLB) for S11 (reflection) antenna tuning, and Port-to-Port S21 (transmission) filter characterization.
• Arbitrary Waveform Generator (AWG): Dedicated DC-coupled and AC-coupled SMA outputs for 30 MSPS generic signal generation.
• Real-Time Spectrum Analyzer: Visualize the entire 15 MHz HF spectrum simultaneously via FPGA-accelerated FFTs.
• Standalone Operation: Onboard Audio DAC/Headphone amp and rotary encoder support allows the board to operate as a standalone radio without a PC.

🧠 Hardware Architecture

Processing Core

• Microcontroller: YD-RP2040 / Pico 2 socket. Handles AGC math, user interface, USB communications, and slow-state hardware toggles. The massive internal Flash (up to 16MB) stores the FPGA bitstream and programs the FPGA directly on every boot, eliminating the need for an external FPGA flash chip.
• FPGA: Lattice iCE40UP5K (SG48). Handles high-speed 30.72 MHz DSP, NCO generation, CIC decimation/interpolation, and I2S master clocking.
• Control Bus: The Pico uses its high-speed SPI0 bus to dual-role: it blasts the bitstream into the FPGA's internal CRAM on boot, and then seamlessly transitions to sending runtime DSP commands to the user's Verilog over the exact same 4 wires.

Clocking & DSP Math

• Master Clock: A clean 30.720 MHz CMOS oscillator drives the FPGA.
• The Magic of 30.72 MHz: The FPGA decimates the 30.72 MSPS ADC data by exactly $R=640$ to yield a mathematically perfect 48.000 kHz baseband I/Q audio stream over I2S to the Pico.
• Process Gain: The 640x decimation provides ~ 28 dB of digital processing gain, turning the raw 8-bit ADC into a highly sensitive 12.6-bit effective receiver!

The Receive (RX) Path

• Topology: SMA ➔ Ethernet Isolation/CMC ➔ Band Sandbox ➔ T/R Switch ➔ LNA 1 ➔ PGA (5/10dB) ➔ LNA 2 ➔ ADC.
• Amplifiers: Uses discrete high-speed op-amps wired as AC-coupled inverting amplifiers.
• Programmable Gain: A Digital Step Attenuator (DSA) built from CMOS switches and precision T-networks, providing 0 to 55 dB of gain control in 5 dB steps. LNA 1 and LNA 2 are both bypassable to prevent clipping on massive signals.
• ADC (MS9280): 8-bit, 32 MSPS. Driven by an RF balun into True Differential Mode with a 4.5V analog supply, yielding a massive 4.0V peak-to-peak input span for maximum dynamic range.

The Transmit (TX) Path

• Topology: DAC ➔ Passive I-V ➔ Ethernet Transformer ➔ TX Switch ➔ Recon Sandbox ➔ TX Op-Amp (+12dB) ➔ T/R Switch ➔ SMA.
• DAC (MS9708): 8-bit, 32 MSPS Current-Steering DAC.
• Passive 50Ω resistors and an Ethernet transformer gracefully map the DAC's strict output compliance limits into a clean 1.0V p-p RF signal, which is then amplified to drive a 50-ohm antenna.

Power Domains (Strict Isolation)

To achieve commercial-grade noise floors, the noisy Switch-Mode Power Supply (SMPS) of standard Picos is bypassed/isolated.

• +4.5V AVDD: Clean LDO powering the RF op-amps, analog switches, and ADC/DAC analog cores.
• +3.3V Digital: LDO powering the FPGA I/O, Pico, and IC logic.
• +3.3V Clean: Ferrite bead + LC filtered rail strictly for the 30.72 MHz oscillator to eliminate phase-noise/jitter.

🎛️ Port & Jumper Reference

Coaxial Connectors

1. SDR_ANT(BNC): Main Transceiver Port (Transformer isolated, AC-Coupled).
2. SII (SMA): Device Under Test port for S11/VSWR Antenna measurements. Connects to the internal Return Loss Bridge.
3. AWG_OUT (BNC): Direct DC-coupled output (0V to +4V) from the DAC. Perfect for baseband signal generation.
4. S21 (SMA): AC-coupled output for S21 Filter sweeps and RF signal generation. Protects the op-amp from DC-shorted filters.

Filter Sandboxes (Plug-and-Play)

The board features IN-GND-GND-OUT 0.1" sockets for inserting custom filter daughterboards.

• Band Sandbox: Shared by both RX and TX (pre-T/R switch). Students calculate and test bandpass filters here.
• TX Pre-Sandbox: Placed before the TX amplifier. Used for mild Nyquist reconstruction filters or super-Nyquist image-selection filters.

🛠️ Software & HDL Notes

• Level Shifters (Logic Inversion): The 4.5V analog switches are controlled by 3.3V GPIOs via 2N7002 N-Channel MOSFETs. Note: This results in a logic inversion. Writing 0 to the pin in MicroPython = Switch HIGH (Max Gain / Default Path). Writing 1 = Switch LOW (Attenuated / VNA / TX Path).
• FPGA Boot & SPI0 Bus:
  • The external FPGA Flash chip was removed to simplify the architecture. The Pico boots the FPGA directly by blasting the bitstream into CRAM using its Hardware SPI0 block.
  • Because the Pico drives the bus, the net names match the Pico's hardware roles: SPI0_TX (MOSI) is data entering the FPGA, and SPI0_RX (MISO) is data leaving the FPGA.
  • Runtime: Once booted, the Pico reuses the exact same SPI0 bus and ICE_SSN Chip Select pin to send DSP commands to the user's Verilog.
• ADC OTR: The MS9280 "Out of Range" (Clipping) pin pulses for only 32ns. The FPGA catches this, stretches the pulse, and triggers a hardware interrupt on the Pico to engage the AGC.

The ten-week, verification-first HDL course sequence is documented in DDC_SDR_Lab_Plan.md.

📍 Hardware Pinout Reference

1. Microcontroller (Raspberry Pi Pico) Pinout

This map defines the interface for the software team handling AGC, VNA sweeps, user interface, and I2S audio routing.

2. FPGA (Lattice iCE40UP5K-SG48) Pinout

This map defines the interface for the digital design team writing Verilog and the .pcf constraints file.

Pico SDK Bring-Up

The SDK port is under Software/pico-ice-sdk. It uses the checked-in Pico SDK (currently 2.2.0). The general SDK examples use the TinyUSB revision bundled by that SDK. The DDC firmware below uses a newer TinyUSB checkout because it needs the UAC1 descriptor definitions that are not present in the bundled revision.

The smallest board-specific firmware example programs the FPGA's volatile CRAM directly from the RP2040:

cmake -S Software/pico-ice-sdk/examples/rp2_dev_ice_blinky \
    -B Software/pico-ice-sdk/examples/rp2_dev_ice_blinky/build \
    -DPICO_BOARD=pico_dev_ice -DPICO_NO_PICOTOOL=1
cmake --build Software/pico-ice-sdk/examples/rp2_dev_ice_blinky/build

The generated UF2 is RP2040 firmware that contains the FPGA bitstream. The Dev-iCE has no external FPGA configuration flash, so the board port excludes the external-flash and TinyUF2 FPGA-storage paths. SPI0 GPIO 6 is the CRAM transfer clock and GPIO 7 carries configuration data; normal FPGA operation uses the separate external 30.720 MHz oscillator, not a Pico-generated clock on GPIO 27.

DDC SDR Firmware

The first Dev-iCE SDR application is in Software/ddc_sdr_firmware. It replaces the earlier Si5351a, Tayloe detector, and PCM1808 path with FPGA-generated DDC I/Q data received as PCM1808-compatible I2S. The Pico firmware:

• programs the FPGA's volatile CRAM over SPI0;
• reuses SPI0 for runtime frequency and sample-rate commands;
• captures FPGA I2S with DMA; and
• exposes stereo 24-bit UAC1 audio at 48 kHz and 96 kHz, CDC control, and a DFU interface.

The DDC application requires a TinyUSB checkout with both src/tusb.h and hw/bsp/rp2040. A current checkout at $HOME/tinyusb is detected automatically, or it can be selected explicitly:

cmake -S Software/ddc_sdr_firmware -B Software/ddc_sdr_firmware/build \
        -DPICO_TINYUSB_PATH="$HOME/tinyusb" \
        -DPICO_BOARD=pico_dev_ice \
        -DPICO_NO_PICOTOOL=1
cmake --build Software/ddc_sdr_firmware/build -j2

The Pico firmware keeps FPGA deployment easy to choose: FPGA_BOOT_MODE=DFU waits for a development bitstream from dfu-util, while FPGA_BOOT_MODE=STORED loads an image embedded in the Pico firmware at boot. Two stored transceiver profiles can be included with FPGA_RX_BITSTREAM_BIN and FPGA_TX_BITSTREAM_BIN, with FPGA_DEFAULT_IMAGE selecting the boot profile and FPGA,LOAD,RX or FPGA,LOAD,TX selecting one between sessions. The full commands and build examples are in Software/ddc_sdr_firmware/README.md.

These profile loads are not a VNA sweep mechanism. A VNA needs a single coherent FPGA design containing the stimulus, reference, receive, and measurement paths for the complete sweep. Select a persistent VNA-capable image before a measurement, or implement those paths in one integrated image; do not reconfigure between sweep points.

For development, compile an FPGA bitstream with Yosys, nextpnr-ice40, and icepack, then load it without reflashing the Pico:

python3 Software/ddc_sdr_firmware/tools/dfu_fpga.py /path/to/ddc_sdr.bin

Stop the SDR++ or Quisk audio stream before starting DFU. The helper prepares the shared SPI0 bus, sends the packed bitstream through dfu-util, and lets the Pico restore the runtime FPGA state afterward. Do not reset the Pico after a DFU-only update: FPGA CRAM is volatile and a reset clears the loaded design.

For the complete pinout, SPI protocol, CDC commands, build options, and DFU sequence, see Software/ddc_sdr_firmware/README.md.