July 25, 2016 I have created this page to share my knowledge related to electronics and
computer programming. Materials provided here might be
useful for students, engineers and other researchers. My intent is to organize my knowledge, get some
feedback, show my capabilities to potential employers.
December 2, 2016: Using STM32 to acquire FSK signals. Proof of concept firmware was developed to acquire FSK signals using STM32F407 microcontroller. Further research will be performed later.
November 21, 2016: Proof of concept doppler sensor prototypes testing.
Proof of concept doppler sensor prototypes are being
tested. Acceptable performance is achieved. The only issue is analog
power supply noise, which will be filtered out in next prototype. VCO
layout need to be adjusted to achieve better S21 phase matching.
November 11, 2016: STM32F4 preliminary firmware framework is ready.
Buffered ADC / DMA code is ready, it is specially
optimized to perform modulation and signal acquisition of different
waveforms: pulsed, frequency shift keying (FSK), frequency modulated
continuous wave (FMCW) and multiple frequency continuous wave (MFCW).
Only FSK modulation is planned in current project because of VCO
limitations. Next step is to perform some tests on PCB with all crucial
elements soldered on: RF part and DSP part. STM32F4 Discovery board was
used to develop preliminary firmware framework, which could be readily
be used on a custom designed board.
USART of STM32F4 Discovery is connected to PC through ADM3251E IC which provides full isolation for RS232 port.
November 10, 2016: Moving code from STM32F103 to STM32F407.
Added most important parts from my previous project to the new one:
USART code, performance measurement code, FLASH routines, FFT routines.
Need to move ADC code and write new DMA code. Compared results from
STM32F407 and PC FFT code to ensure accuracy. With help of STM32CubeMX
and HAL libraries work is done really fast.
November 7, 2016: Moving from Turbo Delphi to QT.
Today I have ported simple but important piece of
code I used previously in Delphi. It is FFT code for performing two
real-valued FFT transforms using single complex FFT transform.
QTCustomPlot was used for spectrum visualisation (http://qcustomplot.com/).
I also have checked simple webcamera code which I
already made before using QWidget promoted to QCameraViewFinder. The
only part left is to learn some simple RS232 communication. QTCreator
is a fascinating editor, with this piece of software development
becomes much easier. Next step is testing preliminary prototype and
porting some old code from STM32F103 to STM32F407.
November 5, 2016: Moving from Turbo Delphi to QT.
I have used Turbo Delphi
programming tool for many years: making tools for interacting with
different devices over serial port, programming different DSP
algorithms (adsp218x microcontrollers), and also 3D graphics algorithms
as a hobby. A pretty time consuming part was porting DSP source code
written in Pascal language to assembler or C language. To improve my
productivity I decided to move from using Delphi to QT. It will allow
to copy code between platforms in a much easier way. Now I can write
C-style algorithms on a PC platform and easily reuse them as source
code for ARM microcontrollers.
November 2, 2016: DSP board preliminary design.
Some parts of DSP circuit board are already
designed. In my previous project i have used STM32F103RBT6
microcontroller. Although it had enough computing power for my task,
firmware design was too complicated because of many optimizations and
lookup table algorithms. In this project I decided to use STM32F407VET6
microcontroller, which is much more powerful with a little additional
In current project STM32F407VET6 microcontroller would be used to
1) Provide frequency modulation signal for VCO using DAC output (PA4:DAC_OUT1)
2) Acquire IF output from mixers using ADC inputs (PC4:ADC1_IN14, PC5:ADC1_IN15)
3) Perform signal processing using algorithms written in C language
4) Data exchange with host using UART port (PA10:USART1_RX, PA9:USART1_TX)
I configured microcontroller PLL to achieve maximum performance using 8 MHz HSE clock, HCLK is set to 168 MHz:
Configuration is done using STM32CubeMX software.
October 28, 2016: Resistive mixer biasing notes.
It seems that gate does not need to be biased very precisely. I obtained similar results for Vgs≈-1.3
v to -0.8 v. Much more impact is from drain resistor value. Very
useful information on active FET mixer design could be found in this
October 27, 2016: Research continued.
After a little vacation I continued my research.
Mixer IF output amplifier with automatic gain control was successfully
tested. I checked improvised FET mixer performance using different
biasing again. Further conclusions on mixer performance can be made
only after prototypes are built, because current configuration lacks of
high frequency decoupling. I am planning to use single-layer pcb for RF
board prototype, and will use only one high-frequency signal transition VIA to minimize possible losses. VCO prototype must be
built and performance of main RF parts evaluated during a tuning
experiment. VCO control circuit must be designed with special
requirement of signal acquisition synchronized with modulation impulses.
October 12, 2016: Dielectric resonator oscillator design.
Currently I am designing an oscillator. Parallel
feedback configuration was chosen, because it allows to use stable
amplifier topology. Dielectric resonator (DR) will be coupled to drain
and gate lines of a source-grounded FET transistor. To ensure that
unwanted frequencies are damped, gate line will be terminated with 50
Ohm resistor. Output drain line will be matched to 50 Ohm output. DR
will be coupled to open-circuited stub on a drain line, to provide
better coupling between line and DR at the quarterwave distance from
open-end of a stub. In previous project I placed DR between two 50-ohm
terminated lines, which resulted in poor coupling and small output
Gate stub was terminated with 50 Ohm resistor. In this design high
impedance quartarwave open-circuited stubs can be added near resonator
to improve coupling.
The other two choices was series feedback oscillator
and MMIC oscillator. Main problem with series feedback oscillator is
high variation of FR4 dielectric constant value. Unwanted frequency of oscillation will occur in case if negative
resistance achieved at frequency other than frequency of DR. In case of
push-push topology this problem can be solved using less known parallel
feedback push-push configuration, where two parallel feedback
oscillators are combined. MMIC oscillators currently are too expensive,
although could be considered for high-end designs or mass production
designs. Cheapest MMIC with integrated mixer is approximately 100-times
more expensive than single high quality high-gain FET transistor.
Overall price of Dielectric Resonator and 3 FET transistors is
approximately 6% of MMIC's price. Although MMIC oscillators have great
advantage of frequency stability, wide band frequency modulation and
meeting DFM requirement.
In current design three main goals must be achieved:
good frequency stability, good power output, low requirements to DR
positioning. Frequency tuning element will be added later.
October 7, 2016: Design updates 2. Proof of concept resistive mixer was constructed using old diode mixer
with hybrid coupler. Unused microstrip traces was cut, FET transistor
was soldered instead of Schottky diode. To ensure that mixing is
provided by FET, all Schottky diodes was totally soldered out. By quick
tuning I found that gate biasing around Vgs≈-1.3v
satisfactory performance. It is interesting to note, that IF noise
reduced by 3dB, and conversion loss is not as high as expected. There
was unwanted IF frequency of few kHz, it seems that it is some noise
from power circuit. Amplitude of this noise varied periodically while
tuning Vgs. The idea of resistive mixer at X-band proved to be
feasible by experimental test. Very good performance was obtained, even
without low-pass filtering on biasing line and without matching FET for
October 7, 2016: Design updates. Compared two designs of series patch array. First design uses 50 Ohm
feeding line with deep inset (butterfly-shaped patches). Second design
uses ~120 Ohm feeding line without inset. It is interesting to note,
that using different feeding line impedance and varying inset distance
allows change distance between patches without adding additional
phasing delay lines (microstrip "curls"). Currently i prefer first
design, as it gives lower side lobes level and more compact.
Prototyping must be done to be sure if there are some potential
problems, mainly with possible mutual coupling between butterfly-shaped
Mixer is planned to be single-ended FET mixer, or passive/active FET
mixer. Both types will be prototyped. In case of single-ended FET mixer
LO leakage problem must be taken care of. Passive/active FET mixer must
give around ~ 30dB isolation, but require to use different signal path
and separate TX/RX antennas. I am planning to use single FET device for
oscillator and mixer, because it meets DFM goals. Using single FET device also reduces BOM components count, eliminating Schottky diode.
Oscillator is planned to be DRO with FET transistor.
Main disadvantage of using dielectric resonator is that it is not very DFM-friendly,
as it requires positioning, gluing and tuning. In future designs
DRO may be replaced by other type of oscillator. Main reason of using
DRO in current design is poor dielectric constant temperature stability
of generic FR4 substrate.
October 6, 2016: Designing X-band series fed patch array. Two
days was spent designing series fed patch array. Single patch was
designed first. Scattering coefficients S11, S21 was analyzed to
of radiated power. Varying this amount can be used for weighting
reduce side lobes or provide sharper beam. Patches are fed with 50 Ohm
microstrip line. Unlike
traditional 2x1 patch arrays used in cheap motion sensor, such
array may provide higher gain and lower losses.
Usually very thin
microstrip line is used to match high impedance on the radiating edge.
On a FR4 substrate such approach seems to induce more radiation losses.
Using large inset distance also allows to design
several prototype to tune center frequency of array and using wide
microstrip feeding lines. Small tilt in
radiation pattern of array is exists but it is acceptable in most
cases. Array is terminated with
half-wave open stub to reflect remaining not radiated power back and
Short-circuited quarter-wave stub also can be used for this purposes.
Patches are all equal in size, I did not performed any weighting
techniques. Using equal power divider with one arm having addition
half-wave phase shift, two series fed sub-arrays can be combined in on
corporate feeding array with radiation power maximum at the two center
patches. Such series fed arrays can be
used as sub-arrays in corporate feeding arrays, or in parallel-series
feeding arrays. Prototyping and measurements should be performed.
September 27, 2016: HMC363 prescaler burnt out. Few days ago my prescaler stopped to work. HMC363 IC burnt out while
doing frequency measurements using small wire probe. It seems that too
much power was coupled to prescaler board through the probe. No more
readings on frequency meter. Here is the photo of my prescaler board:
Good news is that new photos will come in better quality, as i
repaired my old phone, which contains OV5640 auto-focus camera
September 19, 2016:
Some news on VCO project. Here
is prototype of 5GHz VCO on generic
1.0mm FR4 substrate. Main build blocks are FET transistor, microstrip
hairpin resonator and varactor diodes. Few hairpin resonators was
manufactured on a PCB without ground plane, so they could be cut out
and positioned to match phase oscillation conditions. My idea was to
use 5GHz first harmonic to build X-Band VCO which can be used for FMCW
sensor. After prototype is done several problems are found. One of the
problems is temperature frequency drift. Heating PCB from 25°C to ≈
80°C leads to ≈ 160 MHz frequency drift at 5GHz, so useful tuning
bandwidth is reduced. Varactor diodes must compensate temperature
frequency drift and still provide some margin for FMCW modulation.
HMC363 divide by 8 prescaler and 2.7GHz frequency
counter was used to measure oscillator frequency at 5 and 10GHz.
Currently I do not have
other measuring equipment. As prescaler was connected
through 20dB microstrip coupler, i conclude that 10GHz microstrip
band-pass filter performance is acceptable. SIW slot antenna
performance is good at 10GHz, more measurements will be
performed. Relative dielectric constant at 10GHz of used FR4 material
is Er = 4.2. It is confirmed by using 10GHz dielectric resonator
positioning. 5200MHz hairpin resonator was replaced by 10500MHz
dielectric resonator. The same frequency is obtained each ≈ 7.8mm. It
is approximately half wavelength 50Ohm microstrip line at 10500MHz on a
1.0mm FR4 substrate. In parallel feedback configuration DR displacement
wavelength results in 180 degree shift in both microstrip "arms", which
gives total phase shift of one wavelength. I conclude that microstrip
resonator oscillator is better to build on Rogers substrate, while
dielectric resonator oscillator can be build on generic FR4 substrate.
Using Rogers substrate requires to minimize board area, because it
affects cost very much. Prototyping on Rogers substrate is also much
more expensive compared to FR4 substrate.
Laminating machine fix. My
laminating machine control circuit board was broken, resulting in
random LED blinking and constant heating. In my opinion schematic is
unnecessary complicated, moreover PCB have two cracked traces and DIP
soldering points which peeled off. Thermocouple is potentially exposed
to electrostatic discharge from laminating sheet, and it seems that
control IC is not protected from such discharge. Mechanical part is
pretty good. Motor and heater work directly from 220V AC. I cut out
control circuit board. To temporarily fix laminating machine, I
connected motor and heater directly to power line. Heating element
connected to power line through "on/off" switch. Thermocouple is
connected to multimeter. Around three thin A4 sheets could be laminated
while multimeter shows resistance of R ≈ 15 kOhm. Then heater must be
on for around 10 seconds to heat up laminating rollers a bit. At power
on around two minutes are required to heat up laminating rollers.
This is an A3 laminating machine, so it has pretty large rollers, which
cools down slowly.
August 16, 2016. Currently I am working on a proof of concept project of cost-effective
microwave sensor system. It must be not only cost-effective, but easy
to assemble and require minimal or no tuning. Main parts are VCO,
mixer, antenna and DSP circuit. Preliminary layout design of VCO is
done already and prototypes performance will be evaluated this month.
Experiments on substrate integrated waveguide slotted array antenna
(SIW) and patch array antenna will be performed. Another important
application of SIW in this project is to realize effective inter-pcb
transition of microwave signal, which will allow to use single layer
PCB without coaxial interconnections between boards.