Transponder is intended for the use in the
Cubesat type satellite P-sat as single board with connection to other parts of
the satellite. Single channel 3kHz bandwidth is intended
for multiple PSK31 transmissions through transponder with FM signal
downlink. Additionally the beacon is
implemented to identify the channel and to give info about transponder health.
Transponder
is onboard the cubesat Psat
planned for launch on AFSPC-5 mission on May 20 2015 from
We will be
very grateful for any info about transponder downlink status around the world.
You can send us decoded telemetry frames on our email
psktransponder@centrum.cz. Please specify time of the reception and position of
station. Additional info would be welcomed. If you save the demodulated audio,
send us the wav files, the email should handle great amount of data. If you use
the SDR radio for reception, nondemodulated IF with
bandwidth minimum 40kHz accommodating thermal drift
and doppler would be welcomed.
Fig.1
Receiver block diagram
A block
diagram of the HF receiver part of the band monitor is depicted in Fig. 1.
We use a double conversion super heterodyne, proven in PCSAT2 receiver, with
several modifications - especially the BPSK31 signal sensing circuits.
The
receiver includes low noise preamplifier with BFS17 in order to compensate
electrically short receive antenna, which must be shorter to fit in the small
Cubesat. The LNA is followed by high quality LC filter for the out of band
signal suppression. Then there is first mixer NE602 to intermediate frequency
followed by a crystal filter, which defines actual bandwidth 3 kHz of monitored
HF band. The intermediate frequency amplifier A281D with the gain setting
ability for automatic gain control then amplifies the received signal and it is
followed by the last mixer NE602, which converts the signals to the audio band.
The baseband
signal is then splitted into three ways. The first
signal is rectified and the obtained DC voltage controls the gain of the
intermediate amplifier via the MC34072 amplifier.
The second
signal is rectified to get 31.25 Hz frequency from the received signals. Then
it is amplified and filtered by MC34072. This spectral component is a part of
the BPSK31 signal modulation which is most frequently used digital mode in the
monitored radio-amateur band. After passing through the narrow bandwidth tone
decoder NE567, binary signal carrying information about presence of BPSK31
modulation is obtained. It is monitored by a control microprocessor of
transmitter in order to recognize useful signal and switch the power amplifier
on.
The third
signal is also amplified by MC34072 which also works as amplitude limiter and
through preemphasis filter it is connected to the
transmitter modulation input to modulate the UHF carrier.
Fig.2
Transmitter block diagram
The
transmitter produces FM modulated signal in UHF frequency band. Output RF power
of the transmitter is 27 dBm at 435 MHz. A BPSK
modulator with data rate 31.25 bit/s on a sub carrier with frequency 312.5 Hz
is implemented in order to transmit telemetry data from built-in sensors. Block
diagram of the transmitter is depicted in Fig. 2.
The core of
the transmitter is integrated transceiver IC ADF7021 produced by Analog
Devices. This solution with minimum number of external components results in
minimal dimensions of the PCB board. The ATMega8 3.686MHz quartz oscillator is
directly modulated by a varicap in order to achieve linear
FM modulation. Output power is amplified by one-stage PA with a Mitsubishi RD02MUS1B
MOSFET transistor. On the board are implemented sensors which measure drain
voltage, current (MAX4372) and temperature (AD7415) of the PA transistor and
also the output RF power (LTC5531) - not used. All sensors and the transceiver
IC are controlled by microcontroller ATmega8. The microcontroller also drives a
5-bit parallel DA converter, which provides BPSK modulation of the telemetry
data.
The board
is 1.5mm FR4 material with the size 91.4 mm x 91.4 mm.
Fig.3 Output demodulated spectrum with beacon
Fig.4 Output demodulated signal with beacon and CW signal in transponder
Input frequency:
Figure above shows result of CW signal injection into transponder with frequency 28.TBA MHz. Signal is at the 1000Hz mark. From that we conclude the input frequency:
F LO = 28.119660 MHz
And resulting operating passband range:
28.120160MHz - 28.122560MHz
Output center frequency:
435.350 MHz with
FM modulation
The beacon
is implemented as added BPSK31 signal into linear audio signal from receiver.
The BPSK31 signal is upconverted to 312.5Hz
subcarrier frequency to position it in the spectrum below the receiver passband.
CALL
beacon MODE NOF DET AGC VC IC TMP
Where:
CALL identification of the beacon (callsign)
beacon keyword indicating beginning of data
MODE A
or B (A - transmitter always on, B - transmitter
turns on, if BPSK31 signal is
present)
NOF number of frame (0 ... 999)
DET percentage of BPSK31 detection (0 ... 99%)
AGC percentage of AGC operation (0 ... 99%)
VC supply voltage (10 mVolts)
IC power amplifier current (mAmps)
TMP temperature of PA transistor (deg C)
The AGC measured voltage is highly nonlinearly dependent on the input signal, morover the limiting values of measurement vary with temperature of the chip and its supply voltage. Indicative only conversion equation was computed as from 50Ohm generator to input of receiver
Pin = 0.472xAGC-113.5 [dBm, %]
Example:
W3ADO-5 beacon B 044 03 24 540 198 +28
It means mode B active, frame number 044, BPSK31 03 %, AGC acting 24%,
supply voltage 5.40V, PA current 198mA ant temperature of the PA transistor of
+28°C.
After
reset, the short 100ms RF transmission is implemented.
Mode A means transmitter is always on, receiver is on in both
modes.
Fig.5a
Timing of the mode B beacon, no sig. in transponder
Fig.5b
Timing of the mode B beacon, signal detected at 13:31
As can be seen in Fig. 5, the beacon is transmitted in approx. 12s and
next transmittion is starting every 270s at maximum,
or sooner, when the PSK31 signal is detected in the passband.
Fig. 6
Top side of the transponder board
Fig. 7
Bottom side of the transponder board
Fig. 8
Side view of the board, topside up
Fig.
9
Side view of the board, bottom side up