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ARRL QST Article support files
Support Files for QST Article
Raspberry PI FOX Transmitter
Raspberry PI section
ICARC FOX HUNT
foxhunt @ n952 . ooguy . com
Next Fox Hunt
The 09 NOV hunt went well.
We had a much higher success rate than previous
hunts at Kent Park. NO BURRS!
But primarily due to having transmitters that stay on
for much longer. The six KC0JFQ transmitters were
transmitting for about 7 to 8 minutes out of every 10
More Show & Tell at the upcoming meeting.
Operatiog at City Park opens up the opportunity to hunt from
a car for those that get along a bit slower...
Where & When
-- ---- ----     10:00 CST
Hickory Hill Park ???
800 Conklin (off of N. Dubuque Rd just east of HyVee)
Iowa City, Iowa
Reserved use of Park facilities usually requires a paid reservation,
so we will simply meet in the parking lot by the big shelter
by the pond at around 10:00 to begin an informal hunt.
There will be SIX of the new KC0JFQ transmitters in use.
These new KC0JFQ transmitters will all be configured to
transmit on different frequencies and for extended periods of time
to make it a bit easier to "hunt" them.
Suspected transmitter frequencies
-   144.620 MHz
-   144.305 MHz
-   144.280 MHz
-   144.335 MHz
-   144.365 MHz
-   144.380 MHz
-   144.860 MHz
-   144.955 MHz
-   144.285 MHz
-   145.450 MHz
-   146.565 MHz
This is the handout with a log sheet for
the Hickory Hill hunt:
First working model showed up at the November hunts!
Results were very good!
"Differential Time of Arrival" switch schematic.
Our old friend, the 555 timer, is used to generate a square wave
at a convenient audio frequency. This switching signal signal is AC
coupled through C2 to bias D1. D1 is a PIN diode pair so one is
forward biased and one is reverse biased.
Only the forward biased diode carries the RF signal through to
This switching action is demodulated by the attached receiver.
When the antennas are at different distances from the transmitter
the signal to the HT is phase modulated which appears as a
howl at about 1KHz.
You should be able, then, to use the null to point at
(or away from) the hidden transmitter.
The circuit board is the same outline used for the
ICARC Fox Finder so it will be housed in the
same Hammond plastic box. The off/on switch position matches the
ICARC Fox Finder but the coax connector is located in the exact
center of the board to keep pathlength from the right/left antenna
connectors the same.
Either SMA or BNC connectors may be installed on the board. The cables to
the two antenna elements should be very close to the same length or the
null direction will be affected.
The board is provisioned with three methods of adjusting the 555
timer frequency and duty cycle.
A simple one turn pot, a 10 turn pot, or a resistor.
The first prototype unit has been be used to select the resistor
values shown in the schematic.
Production units eliminate the expensive adjustable pots and are not
parts list with links to DigiKey.
Raspberry PI FOX Transmitter
Everything from the FOX Transmitter project below,
but changing the processor out for a Raspberry PI Zero
- Raspberry PI ZERO or Raspberry PI ZERO-W
- ICS307 clock synthesizer.
- Same I2C clock chip as the FOX Transmitter.
- ID EEPROM to configure Raspberry PI GPIO during boot.
- Modulation control from PI PWM channel.
Voice messages become possible.
- Network (WIFI) connection to configure device.
No need to open case and connect USB cable
- BNC, network and power switch match FOX Transmitter locations.
- LVDS clock also provided to the power amplifier daughterboard.
The Raspberry PI Zero-W opens up some interesting possibilities
for applying remote control to the fox during the hunt.
The Raspberry PI Zero-W has Wifi on the circuit board.
The range is rather limited,
but with the addition of an access point to extend range, some interesting
modes can be imagined.
Some added audio capabilities appear on this board revision.
- Audio amplifier and on-board speaker
Allow sequence development without needing an external radio
- R/C Servo channel
"Talkie Toaster" controller for the
"Red Dwarf" fans :-)
- Thermocouple amplifier w/SPI interface.
For this application, the Raspberry-PI Zero is still a power pig.
Using six AAA cells provides (very) roughly 9WH of energy.
The Raspberry-PI, in tight trim (i.e. no WiFi) will draw almost
1W so battery life will be limited, you would need to
replace batteries for each hunt.
- Programmable transmitter
- Base power 12mW to 33mW (without PA)
power amplifier on daughterboard
- ICS525 clock synthesizer
transmit frequency selectable
- Code generator up to 35 WPM
- FRAM sequence memory
- TOY Clock (Time-Of-Year)
- USB host interface
- 6 AAA cell power with SMPS (switch mode power supply)
- Time synchronization network port
A programmable Fox Transmitter with large message storage. We are not limited
to a very short message repeated over and over.
Enough storage to have a 15 minute message.
The control processor is a ZiLog zNEO. The program memory in this device is 128KB.
The large program memory allows for a feature-rich control language and
scheduling methodology. A large FRAM is used to store the configuration and
There are no special tools required to load an
operating sequence into the unit.
An existing operating sequence may be erased
using a simple command.
Multiple messages may be stored in memory,
even if they are not actively used.
The scheduling alogrithm allows simply coordination of multiple transmitters
operating on the same frequency.
Sample signon report
Sample status report
Sample initialization commands
Sample S0 commands
For the most part, the clock generator may be used to directly
generate the RF signal. A 7th order Chebyshev filter removes
the overtones. The filter calculation indicates 3rd. should
be 60dB down.
The clock generator when powered from the 3.3V rail allows about
10mW at the antenna connector.
The clock generator may also be powered from the 5V rail to
increase output power to about 25mW.
An RF daughterboard is used to connect the output of the
clock generator to the output filter. One of the
daughterboard designs is configured as a simple
attenuator to reduce power output.
A second design uses an ADL5536 or ADL5534 to raise the
output power to around 100mW.
The clock synthesizer is an ICS525, this is the heart of the transmitter.
This chip is used on many other Fox transmitters and provides for a
relatively broad selection of frequencies in the 2 meter band.
The clock synthesizer is configured through setup commands held in the
FRAM. This allows the operating frequency to be selected as needed,
The ICS525 is used to generate the carrier.
Frequency modulation is achieved by loading the ICS525 crystal.
The zNEO controls all the programming pins into the ICS525.
The code generator is a simple implementation that takes advantage
of one of the timer channels on the zNEO.
The timer is programmed to interrupt at the chipping
(or dit timing) rate. The interrupt service routine then
enables or disables the modulator as required.
The chipping rate and the inter-character, inter-word,
and inter-sentence timing are all commandable.
FRAM (configuration storage)
The FRAM provides non-voaltile storage for all of the configuration
setting for the transmitter. Being non-volatile, there is no need
for a backup battery. Device capacity of up to 2Mb are currently
available. Current builds are making use of a 256Kb device that holds
up to 8192 commands.
This clock stores the time of year to allow multiple transmitters to
time-share one frequency. The TOY clock is battery backed to allow
for configuring the transmitter in the lab, nothing other than switching on
power is required in the field.
The particular device, as DS1672, stores time as a 32 bit number
and the operating software and GSE software assumes this is a
The USB interface provides a means of loading the FRAM and setting the
Ther USB device is powered through the host connection to reduce
power requirements and extend battery life.
Programming the transmitter is accomplished through a USB connection
to a host computer. The USB device used is an FTDI FT232R which is
powered from the host.
You will also note that this USB UART (FTDI Chip FT232R),
being powered from the USB bus, remains visible to the
host system when the Fox Transmitter is not powered.
A simple 3.5mm jacks allows multiple units to have their TOY clocks
synchronized in the field. Connection is through a 3.5mm stereo
As this is simply the second UART in the zNEO, software may
redefine the function of this port. Currently there is the
start of support to allow a master with the USB port to
program additional transmitters that lack the USB port
(slightly redeucing the cost of a set of transmitters)..
Nominally the transmitter is powered either by a single
9V alkaline cell or a six cell AAA pack.
The power supply in the transmitter is a non-isolated buck
convertor allowing for battery voltage up to about 28V.
The nominal six cell alkaline pack is expected to provide
over 24 hours of operation.
Alternate power sources may be substituted, such as
multi-cell'd lithium packs to provide longer operation.
A charging jack appears on the board,
near the antenna connector,
that is intended to allow connecting a charger
when using rechargeable batteries
The board works, two units are up and running.
Some parts values are being updated to improve performance.
A small number of 102-73161-25 boards are on
hand and can be obtained for $10 each plus shipping.
A small number of 102-73161-12 boards are on
hand and can be obtained for $8 each plus shipping.
These are the previous revision and are missing a few features
of the -25 boards.
Complete build documents exist and are available for each revision.
The current software revision is 1.23
This load takes roughly 70K of the available
128KB program flash memory.
This version has a fairly complete command set to allow the
fox transmitter to operate in a variety of modes.
We can operate in a single channel mode where all transmitters
time share one frequency. We can also operate in a
multi channel mode where all transmitters operate on
unique frequencies. A combination of these two modes
may also be programmed.
The TOY clock allows all setup to be performed well in
advance of deployment. The transmitter may be time-locked
any time prior to the fox hunt.
The frequency control table has entries for multiple cyrstals
making it somewhat feasible to select a crystal that
will generate desired frequencies.