Introduction

Transponder is intended for the use in the Cubesat type satellite Bricsat as dual 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.

Mission status

Transponder is onboard the cubesat BRICsat planned for launch on AFSPC-5 mission on May 20 2015 from Cape Canaveral.

Reception reports

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.

Function description

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 four 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.

The receiver board also holds the digital chips 24LC64 and AD7417, the former to store whole orbit data and the latter for the battery voltage measurement - single cell should be measured. Other three channels are at disposition for monitoring of other parts of satellite. The AD chip also monitors the temperature of RX board. Both are connected to I2C bus.

The board is 0.8mm FR4 material with the size 63.4 mm x 63.4 mm.

Fig.2 Transmitter block diagram

The transmitter produces FM modulated signal in UHF frequency band. Output RF power of the transmitter is 24 dBm at 435 MHz. A BPSK modulator with data rate 31.25 bit/s on a sub carrier with frequency 375 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 ADF7012 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. Finally, the signal is filtered by a band pass. On the board are implemented sensors which measure drain voltage, current (MAX4372) and temperature (AD7415) of the PA transistor. If the maximum allowed temperature is reached, the controller immediately shut the PA down to preserve it from overheating. 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 0.8mm FR4 material with the size 63.4 mm x 63.4 mm.

Both boards are interconnected with double row 20pin connector and mechanically mounted with four M3 spacers. The height of the spacers should be approximately 6.5mm to ensure reliable connection in the connector.

Connectors should be connected / disconnected with the special attention to the board flexing to avoid parts stressing.

Fig.3 Output demodulated spectrum with beacon

Fig.4 Output demodulated signal with beacon and CW signal in transponder

Operating frequencies

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 = 28.120030 MHz

And resulting operating passband range:

28.120530MHz - 28.122930MHz

Output center frequency:

435.351 MHz with FM modulation

Beacon specification

The beacon is implemented as added BPSK31 signal into linear audio signal from receiver. The BPSK31 signal is upconverted to 371.5Hz subcarrier frequency to position it in the spectrum below the receiver passband.

aaaaaaa b ccddeeffgghhiijjkkllmm

Where:

*symbol*	width	description	unit
aaaaaaa	5	Call sign assigned by IARU	-
b	1	Mode of operation	-
cc	2	Frame number	-
dd	2	PSK sensing	-
ee	2	RX AGC	-
ff	2	Battery voltage across all 2 cells	10mV
gg	2	Battery voltage across lower cell	10mV
hh	2	voltage1 TBA	10mV
ii	2	voltage2 TBA	10mV
jj	2	voltage3 TBA	10mV
kk	2	Transmitter current consumption	mA
ll	2	Receiver temperature +99	°C
mm	2	Power amplifier temperature +99	°C

aaaaaaa identification of the beacon (callsign W3ADO-6)

b A, B, C (A - transmitter always on, B - transmitter turns on, if BPSK31 signal is present, C beacon only, callsign only in D mode)

cc number of frame (0 ... 804)

dd percentage of BPSK31 detection (0 ... 99%)

ee percentage of AGC operation (0 ... 99%)

ff supply voltage (10 mVolts)

gg voltage across lower cell.(10 mVolts)

hh voltage1 (10 mVolts)

ii voltage2 (10 mVolts)

jj voltage3 (10 mVolts)

kk power amplifier current (mAmps)

ll temperature of receiver +99 (-98 ...156 deg C)

mm temperature of PA transistor +99 ((-98 ...156 deg C)

Full frames are transmitted in a numeral system with base 32. All 32 values are represented by symbols "a" for 0 to "z" for 25 and "A" for 26 to "F" for 31. Such representation allows to encode numeral values in range 0 to 1023, with use of two symbols only.

The telemetry values for dd and ee are averaged throughout the previous 20s interval, the other values for voltage, current and temperature are measured in the time when transmitter is switched on and transmitting the synchronization, then the callsign transmission follows.

The ee 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 = 0.370xee-137.4 [dBm, %]

The temperatures are transmitted with 99 added to eliminate the sign problems. So

Trx=ll-99 [°C]

And the same applies for PA temperature

Tpa=mm-99 [°C]

Example:

W3ADO-6 A cAagbexgaaaaaaaafdeadF

converted to base 10:

W3ADO-6 A 90 6 36 742 0 0 0 0 163 128 127

It means mode A active, frame number 90, BPSK31 06 %, AGC acting 36%, supply voltage 7.42V, voltage across lower cell 0V (actually unconnected), voltage1 to voltage3 0V, PA current 163mA, temperature of the receiver +29°C and temperature of PA transistor of +28°C.

Modes description

Mode A

Mode A means transmitter is always on, receiver is always on.

Mode B

Mode B means, that receiver is always on and transmitter is switched on in the 20s slots depending on the PSK31 activity in the pass band of receiver. When no activity occurs, transmitter is switched on to transmit beacon telemetry every 120s.

Fig.5a Timing of the mode B beacon

As can be seen in Fig. 5a, the beacon is transmitted in approx. 10s and next transmittion is starting every 120s at maximum, or sooner, when the PSK31 signal is detected in the passband.

Mode C

Mode C means, that receiver is always on, however the transmitter is swichted on only every 180s to transmit beacon telemetry.

Fig.5b Timing of the mode C beacon

As can be seen in Fig. 5b, the beacon is transmitted in approx. 10s and next transmittion is starting every 180s

Mode D

Mode D means, that receiver is always on, however the transmitter is swichted on only every 180s to transmit callsign only, without telemetry.

Fig.5c Timing of the mode D beacon

As can be seen in Fig. 5c, the beacon is transmitted in approx. 4s and next transmittion is starting every 180s

Whole orbit data

The transponder includes the EEPROM memory able to store some 804 lines of telemetry. In the every time, telemetry is transmitted over the air, it is also stored into memory. That way it is possible to download the history of the transmissions, only mode letter is not stored, but it can be reconstructed from the step of the frame number. The minimum history time stored in memory is in mode A 20s frame length*804 which results in approx. 4.45hour. In the case of mode B maximum time is 120s repetition*804 means maximum of 26.7hour. In the case of modes C and D. The time would be exactly 180s*804, that is 40.2 hour history. The data can be transmitted in 64 or 125BPSK.

Example:

When proper command is transmitted, transponder acknowledges with cmd 879 700 70 br1 which means WOD data transmission of time and supply voltage was requested, and it will be transmitted by 64BPSK. Then it starts to send data.

Received data:

da xgnonononono

cA xgigioioioio

bw wEioioioioio

as wEioio em zrnono

Data are first sent in the full frame with spaces, and then there are 5 sets of compressed values in one line. The compressed frames are obtained as a difference between last and previous value of each telemetry channel plus 14. If the result lies in 0 to 31 range, the value is transmitted as one symbol, otherwise the value is sent as its full representation preceded by space. Transmission goes from recent time to deeper history reading full memory.