Transponder
Last Updated: December 11, 1998
The transponder is the device which is carried aboard the rocket to
relay the interrogator's signal back to the ground. This allows the interrogator
to do the following:
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By measuring round-trip time, measure range to the rocket.
-
By accurately pointing at the transponder's signal, measure the azimuth
and elevation angles of the rocket.
-
Send commands to the rocket for firing ordnance, etc.
-
Receive telemetry from the rocket.
.
There is experimental data available on the latest transmitter design:
TX5
Transponder Theory of Operation
The DARTS transponder consists of a 5.7 GHz tuned-detector receiver, a
microcontroller and logic, and a 5.7 GHz pulse transmitter. The tranponder
is extremely small (specs).
Both transponder antennas are patch antennas, printed directly on the transponder's
circuit board, and have omni coverage when the rocket is vertical, which
is most of the time, even when under the parachute
Antenna System
The receiver and transmitter have seperate antenna systems. Both sides
of the 3-layer transponder PCB have two circular-polarization patch antennas
printed on them, one for transmit and one for receive. A 3-dB coupler combines
the signals from each pair of antennas. By using two hemispherical-coverage,
back-to-back antennas, the transponder antenna system has essentially omnidirectional
coverage. Only at the board edges is signal strength reduced slightly.
Receiver
The transponder receiver is a simple, wideband, tuned-radio-frequency (TRF)
receiver. While simple, it is sufficient. A low-noise MMIC serves as an
RF amplifier prior to the detector, increasing its sensitivity significantly.
A full-wave, zero-bias Schottky detector followed by a video amplifier
drives the comparator which detects the received pulses.
Transmitter
The transmitter can be disabled by the on-board computer, allowing the
interrogator to command a specific transponder to transmit.
The DARTS transponder sends out a signal that is both offset in frequency
and delayed in time from the interrogator (uplink) pulse. The delay gives
the ground receiver time to come out of "blocking" (overload due to the
proximity of the transmitter), and switch to the "sum" lobe before the
return signal arrives. In addition, the delay allows tracking the
target into essentially zero range.
We do this digitally, using circuitry that is clocked at least three
times faster than our range measurement clock, to reduce edge placement
error. In addition, in order to be able to send telecommand, the pulse
generator also has to be able to modulate the downlink pulse width.
A 5.7 GHz carrier is produced by a phase-locked oscillator. This
oscillator can change frequency rapidly to scan the ground antenna in azimuth.
The digital pulse-forming circuitry pulse-modulates the 5.7 GHz carrier
by using a shunt-connected PIN diode switch.
On-Board Computer
A MicroChip PIC16C73 microcontroller
performs control functions, and interprets commands from the interrogator
(which is on the ground). The transponder has outputs from its microcontroller
to fire three ordnance (igniter) outputs. These can be used for staging,
parachutes, etc. Preset timer functions are also available as a back-up
to remote control. Also, a user program can modulate the downlink data
to send back telemetry, ordnance status, etc.
The microcontroller can also power down the power-hungry transmitter
circuitry when not transmitting.
Specifications
| Physical Size |
1.25" dia x 7" long (including batteries) |
| Weight |
About 4 oz |
| Uplink Receiver Bandwidth |
10 MHz |
| Receiver Sensitivity (tangential) |
-73 dBm |
| Command / Telemetry Data rate |
500 bits per second |
| Valid Uplink Pulse Width Window |
5-15 uS |
| Receive-to-Transmit Delay |
25.6 uS |
| Uplink Command Modulation |
Pulse Width: <7 uS = logical zero, >9 uS = logical one |
| Downlink Telemetry Modulation |
Pulse Width: 12.8 uS = logical zero, 25.6 uS = logical one |
| Delay/Pulse Width Granularity |
12.5 nS (derived from 80 MHz clock) |
| Power source |
4.8V NiCd pack (internal) |
| Downlink power |
17 dBm (63 mW) CW, 8.4W peak |
| Downlink spurious output |
-50 dBc (target) |
| Current draw (transmitting) |
280 mA |
| Current draw (not transmitting) |
40 mA |
| Interfaces |
(2) digital discrete outputs
(2) digital discrete inputs
(1) bidirectional master-mode SPI-type synchronous serial bus |
| Discrete Outputs |
0V idle, 4.8V @ 20 mA active |
| Discrete Inputs |
Current-limited CMOS, withstand -12V to +12V
Thresholds: <1.6V = LOW, >3.2V = HIGH |
TX5 Engineering Page
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Send comments and suggestions to: steveb@c2-tech.com
URL: http://darts.c2-tech.com/transponder_t.htm
This document copyright Steve Bragg, KA9MVA.