QRP Labs U4B User manual
Please read this manual in conjunction with the operating manual
and other documentation on the U4B product page
http://qrp-labs.com/qdx
Contents
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2
1. Introduction
The U4B tracker was developed over a period of 7 years from 015 to 0 , in collaboration with
Dave VE3KCL who has launched 83 test flights from Toronto, Canada. With flight duration from
hours to 305 days (10 months, almost 17 laps around planet Earth), they all taught us something
and were great fun.
U4B is designed to be an easy to use, lightweight, low-cost module that can be configured as
simply as entering your callsign, yet for more advanced owners can be flexibly extended with more
sensors and as much complexity as you like. Automated tracking maps and utilities are available
on the QRP Labs website, both for simple tracking purposes and downloading your own telemetry.
The U4B PCB contains:
•33.0 x 1 .7mm PCB (plus removable protrusion with micro-USB connector)
•Weight: 1.8g (with micro-USB protrusion removed)
•3 -bit ARM microcontroller running QDOS (QRP Labs isk Operating System)
•1 8K disk (implemented on EEPROM chip)
•7mW (approximately) transmitter using Si5351A synthesizer
•TCXO referenced frequency stability
•Band coverage 00m to m
•LM75 temperature sensor
•Status LED
•USB interface for configuration, programming and easy firmware update (just copy the new
firmware file into the apparent USB Flash drive).
Simplest possible operation:
Just connect to U4B with a PC terminal emulator, and configure it with your callsign. Register the
flight name, details and channel on the QRP Labs website. Fly!
More flexible and advanced features:
U4B contains a wealth of flexibility and hardware expansion options which you can use to
customize your flight:
•19 GPIO pins – of which 9 can be configured as analog inputs and 8 are easily accessible
via PCB edge pads; all 19 can be used as digital input or output control pins
•I C bus for connecting additional sensors e.g. pressure, humidity
•BASIC programming language with full-screen text editor, compiler and debugger
•1 8K Disk storage for your programs and data; BASIC can read/write data files
•Command line utility
•Telemetry over WSPR for relaying your additional sensor data
The U4B radio transmitter can transmit the following modes:
•QRP Labs tracking and telemetry over WSPR
•WSPR (including extended mode and slow 15-minute WSPR)
•JT9 (1, , 5, 10, 30 minutes)
•JT65 (modes A, B, C)
•Hellshreiber (standard, DX, and slow multi-tone FSK)
•CW (standard speed, QRSS, FSKCW and DFCW)
•Customized “Glyph” patterns can produce a unique idenfier on QRSS
3
2. SAFETY FIRST
If you are anyone else gets seriously injured (or worse) that’s seriously going to take the fun out of the
whole thing. So pay massive amounts of attention to safety, PLEASE…
Launch in a wide flat area: First off, the best place to launch is a flat area, a WIDE flat area, with no
obstacles for hundreds of meters in any direction; particularly obstacles involving power lines. A balloon
can often travel along for a while almost horizontally in the slightest breeze you don’t even realize exists, so
don’t assume anything is going straight up. Again, nowhere near or even in sight of power lines!
etermine wind direction: Ideal launch conditions are zero wind. But recognizing that rarely does one
have such a luxury, it is necessary before everything, to determine the wind direction; then choose the
launch site such that the balloon has the longest clear space to ascend into.
Launch from ground level: Don’t be tempted to launch from a roof, or a balcony, even if it seems like a
good idea (maybe you think it will get you higher than nearby obstacles). In the heat of the moment, your
attention could be so focused on getting your precious dream airborne that you’ll take unreasonable risks,
balancing precariously near the edge just to dislodge it or stop that last bit of antenna catching on
something… and before you know it, you’ll fall off to your doom. Better to stay on the ground, do a bit of
extra thinking to find a suitable launch site.
Mind hydrogen: remember, Helium is a non-explosive, non-flammable gas. Hydrogen has greater lift, is
cheaper, a renewable resource, and leaks more slowly – but is explosive. So if you decide to use hydrogen,
do some proper research about how to handle it properly, at the minimum following all safety instructions
directed by whatever container of the stuff you’ve got hold of.
Aircraft: An oft-repeated objection to high altitude ballooning is that it’s a danger to aircraft. Having spoken
to numerous commercial and private pilots about this, I have yet to meet one who did other than laugh at
the thought of something that tiny and delicate being any kind of danger to an aircraft. Modern aircraft
simply have too many built-in redundant systems and safety features, over-engineering etc for such a small
thing to be a hazard. Nevertheless, better not tempt fate, or risk any undue attention, by selecting a launch
site anywhere near an airport. Stay even further away from any military facility.
Rules, regulations: It’s your responsibility to find out what laws, rules, regulations etc apply in your
country. QRP Labs is not going to tell you, QRP Labs doesn’t know. No idea at all. So make sure you
undertake all necessary research and figure out what you can, and cannot do. Then make your own
decisions accordingly.
Test test test… OK this isn’t really part of safety. Well maybe it is, if you consider preserving your sanity an
important goal. TEST! Test, re-test, test every possible scenario you can think of. Solar power in the dark,
solar power as the sun comes up, goes down, spinning around, clouds, blah blah. Whatever you can think
of. If you’re writing your own BASIC program try to think of all the possible routes the code can take and
think of a way to check they all work properly. Test everything so thoroughly on the ground. There’s already
enough to go wrong with the wind, clouds, tangled antennas, leaks, miscalculation of gas volume, blah
blah… at least get the U4B set up correctly! Once it’s left your hands, you’re very very unlikely to ever see it
again. So you had better be sure it WORKS before you let go! This includes not breaking off the USB tab
until the last moment – because testing and fixing something is a whole lot harder without that.
Common sense: This list is not exhaustive! Above all, use common sense! Best to delay a launch, and
come back another day – there should be plenty – then risk injury or life.
4
3. Know your U4B
U4B is a 33.0 x 1 .7mm PCB, using 0.6mm thick FR4 PCB for low weight. SMD components are
mounted on both sides of the PCB.
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4. Parts list
Part Value Package escription
C1 10uF 040 Capacitor
C 10uF 040 Capacitor
C3 0.1uF 040 Capacitor
C4 0.1uF 040 Capacitor
C5 0.1uF 040 Capacitor
C6 0.1uF 040 Capacitor
C7 0.1uF 040 Capacitor
C8 0.1uF 040 Capacitor
C9 0.1uF 040 Capacitor
C10 0.1uF 040 Capacitor
C11 0.047uF 040 Capacitor
C1 uF 040 Capacitor
C13 uF 040 Capacitor
C14 10uF 040 Capacitor
C15 0.1uF 040 Capacitor
D1 S4 SOD-3 3 Diodes
D S4 SOD-3 3 Diodes
D3 S4 SOD-3 3 Diodes
D4 Red LED 040 LED
L1 . uH 01 Inductor
L . uH 01 Inductor
R1 1K 040 Resistor
R K 040 Resistor
R3 1.5K 040 Resistor
R4 K 040 Resistor
R5 100K 040 Resistor
R6 80K 040 Resistor
R7 K 040 Resistor
R8 K 040 Resistor
R9 K 040 Resistor
R10 K 040 Resistor
R11 1.5M 040 Resistor
R1 100K 040 Resistor
R14 100K 040 Resistor
R15 5.6K 040 Resistor
R19 1K 040 Resistor
T1 SI1967DH-T1-E3CT-ND SOT363 Dual MOSFET transistor
TCXO 5MHz 3 5 TCXO
U$1 SIM 8M 10x10mm SIM 8M GPS
U1 STM3 F103CBT6 TQFP-48 STM3 chip
U LM75 SOIC-8 I C Temperature sensor IC
U3 4M01 SOIC-8 1 8K I C EEPROM IC
U4 RT9013-33 SOT 3-5 3.3V LDO voltage regulator
U5 SI5351A MSOP-10 Si5351A Synthesizer
USB MICRO-USB MICRO-USB MicroUSB
XTAL 8MHz 3 5 Crystal
6
5. Track layout
6. GPIO pins 8-18 pad locations
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7. Parts layout, top side
8. Parts layout, bottom side
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9. Schematic
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10. Circuit explanation
The U4B tracker has a rather simple design and does not require highly detailed explanation.
The brains of the device is an STM3 F103CBT6 microcontroller. A 3 -bit ARM Cortex M3
processor having 1 8K of program memory. In the U4B, 1 K is used for the bootloader and 116K
for the application firmware. Although the maximum speed of the microcontroller is 7 MHz, it is
run at a much lower speed in this leisurely application, to reduce current consumption. There is an
8MHz crystal as the system oscillator; internally this is used to generate a 4MHz CPU clock during
flight, or 18MHz when a USB connection to a host PC is present.
The USB connector is a micro-USB type, located on a protrusion from the top edge of the
nominally rectangular U4B PCB. This section of the PCB is a break-away tab, in other words a line
of holes in-line with the top PCB edge can be snapped to remove this section for flight, thereby
reducing the weight by several grams. The U4B may be powered from the USB host via a diode
located on the breakaway tab. The other two components present on the breakaway tab are the
necessary 1.5K Fullspeed USB pull-up resistor R3, and a K resistor R7 which the
microcontroller uses to “sense” when a USB connection is present, and enable its USB peripheral
accordingly.
The Si5351A Synthesizer IC is used as the “radio transmitter”, configured by the microcontroller
over the I C bus. It can be used in two ways, under firmware control.
1. Low power mode: Clk0 (connected to output pad RF_1 via a capacitor) is driven at the
operating frequency, Clk1 (connected to output pad RF_ ) is grounded. Power output into a
50-ohm load is found to be approximately 9mW.
. High power mode: Clk0 and Clk1 are driven at the operating frequency in anti-phase (180-
degree phase difference). The power output into a 50-ohm load across RF_1 and RF_ is
found to be approximately 7mW.
If Clk0 and Clk1 are connected to opposing legs of a dipole antenna, the power output can be
conveniently controlled by BASIC commands, set to low or high power mode. There is no Low
Pass Filter in the U4B, which has the advantage of allowing it to operate on any band; but the
disadvantage that there is no harmonic attenuation; however given the very low power output this
is not normally considered a problem. If it troubles you, you could add an external filter on a small
PCB with appropriate SMD capacitors and inductors.
The K resistors R and R4 provide some protection against static build-up on the antenna
during flight.
The synthesizer is powered via half of a dual P-channel MOSFET T1, under control of the
microcontroller via the SW_SYNTH signal. Similarly, the SIM 8M GPS is powered by the other
half of the P-channel MOSFET under microcontroller control. In use, the microcontroller ensures
that the GPS and Synthesizer are never both powered on at the same time; this limits the
maximum current consumption of the device, easing the design of the power supply (normally,
solar panels) particularly during early morning or late afternoon operation when the sun is likely to
have a low angle of incidence on the solar panels if a horizontal panel arrangement is used.
The microcontroller receives serial data from the SIM 8M GPS receiver module via a USART
(serial) connection at the default 9600-baud data rate of the GPS module. Both transmit and
receive serial data signals are implemented so that the microcontroller can write commands to the
GPS; in particular this is necessary to be able put the GPS into “balloon mode” with the
appropriate command.
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Power to the GPS receiver is filtered by the . uH inductor L to try to keep microcontroller digital
noise out of the GPS receiver circuits. Power to the microcontroller Analog Supply pin is similarly
filtered by L1. The battery backup connection to the GPS is always powered via diode D , even
when the GPS main power is switched off. This is essential to retain the GPS memory so that at
next power-up a “warm start” is available, which has a very quick satellite acquisition time.
In addition to the Si5351A Synthesizer chip on the I C bus, two other devices share are connected
to the I C bus: the LM75 temperature sensor IC U , and the 4M01 1 8K Serial EEPROM IC U3.
These latter two devices are kept powered on at all times.
7-bit I C addresses used by devices in the U4B are:
•0x48 U , LM75 Temperature sensor IC
•0x50 U3, 4M01 1 8K Serial EEPROM IC
•0x60 U5, Si5351A Synthesizer IC
It is essential that if additional devices are connected to the I C bus, you must ensure that they do
not use these three I C addresses. These devices may be read and written using BASIC
commands.
There’s a 3.3V LDO voltage regulator IC, U4, RT9013-3.3 which has an enable input, controlled
via a potential divider network R5, R6 and R11. This is set up to provide a hysteresis such that the
“turn-on” voltage is somewhat higher than the “turn-off” voltage; it avoids rapid on/off oscillation of
the power supply in low light conditions. The SOLAR input is connected to the BATT output and
voltage regulator input, via a diode D1. This prevents reverse current into the solar panel in the
dark when a connected battery still has charge.
The maximum rated input voltage of the RT9013-3.3 is 6.0V (absolute maximum rating) and
recommended maximum operating voltage 5.5V. Care must therefore be taken not to use so many
solar cells that 6.0 is exceeded. Remember that solar panels get BETTER as they get colder. In
full sunshine at altitude, -55C temperatures can increase voltages and currents by up to 0%.
The U4B can be used with rechargeable batteries such as LiPo of various capacity; the higher the
battery capacity, the longer operation after dark will be possible, but the higher capacity batteries
are also heavier which will result in lower flight altitudes. There are many trade-offs to consider.
Alternatively for very light-weight flights a battery can be eliminated altogether; an ultra-capacitor
can be used or even nothing at all. However arguably some form of storage is useful to avoid
drop-outs in the early morning and late afternoon, if the panel is rotating and tilted.
The microcontroller can read the available battery voltage V+ on its PB1 port, via potential divider
R8/R9.
The U4B has 8 GPIO pins presented as pads along the lower edge of the PCB on the bottom side;
these are known as pins 0 to 7 in the BASIC programming. They can be written and read as digital
outputs or inputs (0V is “0”, +3.3V is “1”); or they can be read as 1 -bit analog inputs where a
value of 0 is Ground, and 4095 is +3.3V; these inputs can be used for reading analog sensors.
A further 11 GPIO pins are digital-only, and known as pins 8 to 18 in BASIC programs. Most
applications will not need such a large number of GPIO pins but if you do, you will need to solder
very fine wires to very tiny SMD pads on the top side of the board as identified in the diagram in
section 5, above.
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11. Basic flight connections
The HF antenna needs 1/4-wavelength on each leg (for example, each leg is 5m for 0m
operation). A typical flight configuration has the top end of the top leg tied to the balloon, and the
U4B payload suspended directly in the middle of the dipole.
The GPS antenna can be made of simple # 8 (0.3mm) enameled wire; a 1/4-wave at 1575Mhz is
about 45mm. The feedline consists of 1-inch of twisted wire, about 6 turns works well.
The above is just an example of a minimal flight tracking system. You can add your own sensors
and complexity as you see fit!
There are a lot more practical tips about flight system preparation and launch, in the FDIM 0 1
Conference proceedings article at http://qrp-labs.com/u4b
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12. Resources
Refer to the operating manual for details of the terminal applications (for configuration of
U4B, text editing, writing BASIC programs, and testing etc.)
Refer to other pdf publications on the U4B page on the QRP Labs website http://qrp-
labs.com/qdx
The operating manual also explains how to update the U4B firmware (using a USB
connection to a PC), and how to set up tracking on the QRP Labs website.
For updates and tips relating to this kit please visit the QRP Labs QDX kit page http://qrp-
labs.com/ u4b
For any questions regarding the assembly and operation of this kit please join the QRP
Labs group, see http://qrp-labs.com/group for details
13. ocument Revision History
1.00 18-Apr- 0 1 First draft version version 1.00
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Table of contents
Other QRP Labs GPS manuals
