P-sat Transponder WEB Specification

April 2015

Urbanec, T., Vágner, P., Kasal, M.

Email: psktransponder@centrum.cz




Brno University of Technology, Czech Republic






 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.


Mission status

Transponder is onboard the cubesat Psat 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 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


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 = 28.119660 MHz


And resulting operating passband range:


                                     28.120160MHz - 28.122560MHz


Output center frequency:


435.350 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 312.5Hz subcarrier frequency to position it in the spectrum below the receiver passband.






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, %]




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.


Modes description

After reset, the short 100ms RF transmission is implemented.

Mode A


Mode A means transmitter is always on, receiver is on in both modes.


Mode B


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.



Realized transponder

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