hardware concept

This project is based on a student research project by W. Rickert and M. Wiese for Prof. Dr. Guenther Schweppe (DK5DN) at the Technical College in Meschede. As a cost-optimized version using SMD technology was to be created for the Fichten Field Day, a complete new concept was established during this project. The student's work served however as a functional prototype. In addition, it served well to explain the various different approaches. (Diagram: DH1TW)


SectionStudent versionFiFi SDR
MicrocontrollerAVRARM Cortex-M3
Switching signal generationFlipflopsCPLD
Phase reversalTransformerTransistor
RF voltage insulationyesyes
EMVoptocoupler, DC/DC convertorseparate voltage regulator for digital and analog sections

Development goals

  • not to build the best SDR but a particularly economic radio that can be self-built and is motivating
  • economic enough to be affordable for every FiFi participant
  • USB interface
  • power-saving mode (for notebooks)
  • LO frequency must not be radiated
  • board should fit in a standard box
  • optional on-board USB sound card
  • interface for optional (automatic) preselector

First considerations

Input section

  • The HF section must present two signals of equal level and phase-shifted by exactly 180 degress at the mixer input
  • This could be achieved with a HF transformer (the two end terminals provide the same signal but 180 degrees reversed, see the students notes)
  • The problem however: HF signals from the LO also leave the mixer via the input and will be transmitted by the antenna. This must be prevented. A pre-amplifier would provide the necessary isolation.
  • Then you would need two preamps for each output from the transformer ...
  • ... or you put the transformer behind the preamp, but then it can only serve to reverse the phase and not galvanically isolate because the preamp needs power and also needs a reference ground.
  • To avoid needing two transformers a phase-reversal stage is included using an HF transistor.

Filter concept

  • A preselector is important, but doesn't fit in the budget.
  • The contruction of one's own filter would be a rewarding area of activity for experiments. Here it takes little effort and no special prior electronics knowledge to make a noticeable improvement to the receiver's characteristics.
  • We should just make a connection point available.
  • Only a low-pass filter will be fitted as standard.

Selection of components

Local oscillator

  • must provide the quadruple mix frequency
  • must work in a pure low-cost receiver without temperature stabilization
  • programmable via I²C bus, CMOS version
  • can be set for between 10 MHz and 160 MHz
  • 3.3V power supply, c. 100 mA current consumption, no energy-saving mode

Switching signal generation

  • CPLD Xillinx XC9536XL-5VQG44C datasheet, documentation
  • Speed grade 5ns
  • 3.3V power supply
  • 44 pin TQFP casing
  • maximum input frequency c. 178 MHz (simulation, see documentation)
  • possible dividers: 1,4,16,64


  • LPC1758, NXP datasheet, handbook
  • ARM Cortex M3 CPU
  • 3.3 voltage supply
  • 80 pin TGFP casing
  • converts the frequency information from ISB into I²C signals for the LO
  • sets the division factor for the CPLD
  • provides the switching signals for the optional automatic preselector
  • provides a USB sound card as an HID device

RF transformer

  • CX2064, Pulse datasheet
  • 1:4 impedance ratio (50 Ohms at the BNC socket)
  • 3dB bandwidth from 200 kHz to 350 MHz
  • socket-mounted so that it can be removed or rotated

Impedance convertor / preamp

  • On the basis of the BF862 JFET datasheet
  • according to the article by DJ8IL in Funkamateur 12/09 on page 128

Filter concept

  • The frequency information is made available on an expansion plug to control the optional preselector
  • A low-pass filter is installed between the preamp and the phase reversal stage (default: Cauer filter 5th order with 30 MHz limit frequency)
  • Insertion points are available before and after the transformer (on a DIL socket) for expansions and modifications as well as before and after the low pass filter (on an expansion plug)
  • The expansion plug carries all earths as well as 3.3 Volt digital and 4 Volt analog

Phase reversal stage

  • provides an additional signal, phase reversed by 180°
  • solution using the BFR193 RF transistor, datasheet


  • 74CBTLV3126 datasheet
  • 4-Bit FET switch
  • 3.3V supply voltage

Low-noise op amp

  • MAX4477, MAXIM datasheet
  • alternatively AD8656, Analog Devices datasheet or comparable low-noise op amp
  • although it belongs to the analog section it is powered via 3.3V supply

Optional sound card

  • UDA1361TS audio ADC, NXP datasheet
  • 3.3V supply voltage

Power supply

  • All switching elements are supplied via a voltage regulator. This allows complicated filtering of 5V from the USB bus to be omitted.
  • Microcontroller: 3.3 Volt from TDA3663, datasheet. c. 100mA static from the USB bus 5V supply.
  • Remaining digital section: 3.3 Volt (can be switched off) from NCP5500, datasheet. c. 500 mA.
  • Analog section: 4 Volt (can be switched off) from MIC5205, datasheet. Especially high suppression of the 1 kHz signal from the USB bus (ca. 80 dB).

Ground concept

  • two ground layers: HF ground, Analog and Digital ground
  • The USB screen earth is connected with high impedance/capacitance

Case options

  • PCB suitable for Fischer AKG 55 24 80 datasheet
Last modified 4 years ago Last modified on Jun 19, 2013 12:57:56 PM