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Raspberry PI FOX Transmitter

Circuit Board Image
Raspberry PI section

FOX Transmitter

Circuit Board Image
SOC 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 minute cycle.

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
  1.   144.620 MHz
  2.   144.305 MHz
  3.   144.280 MHz
  4.   144.335 MHz
  5.   144.365 MHz
  6.   144.380 MHz
  7.   144.860 MHz
  8.   144.955 MHz
  9.   144.285 MHz
  10.   145.450 MHz
  11.   146.565 MHz


This is the handout with a log sheet for the Hickory Hill hunt: ICARC_FOXHUNT_HH.pdf



DTOA Switch

First working model showed up at the November hunts!
Results were very good!

Schematic Image
"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 the receiver.
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.

Circuit Board Image
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 adjustable.

The current parts list with links to DigiKey.



Raspberry PI FOX Transmitter

Circuit Board Image


Features

Everything from the FOX Transmitter project below, but changing the processor out for a Raspberry PI Zero board.
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.

The downside

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.



FOX Transmitter

Circuit Board Image

Features



Description

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 message traffic.



Field Prorgammable

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.

Users Manual

Command List

Sample signon report

Sample status report

Sample initialization commands

Sample S0 commands

RF Power

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.

RF Clock

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.

Code Generator

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.

TOY Clock

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 UNIX timestamp.

USB

The USB interface provides a means of loading the FRAM and setting the TOY clock. 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.

Time Network

A simple 3.5mm jacks allows multiple units to have their TOY clocks synchronized in the field. Connection is through a 3.5mm stereo patch cable.
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)..

DC Power

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

Board Status

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.


Software Status

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.