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.
|Section||Student version||FiFi SDR|
|Switching signal generation||Flipflops||CPLD|
|RF voltage insulation||yes||yes|
|EMV||optocoupler, DC/DC convertor||separate voltage regulator for digital and analog sections|
- 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
- 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.
- 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
- Si570 from Silicon Labs (datasheet)
- 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
- 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
- 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
- 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).
- two ground layers: HF ground, Analog and Digital ground
- The USB screen earth is connected with high impedance/capacitance
- PCB suitable for Fischer AKG 55 24 80 datasheet