This mixer converts the 70MHz IF into the final 455kHz IF by use of 1/10 the frequency of the fractional-
N synthesiser. The 455kHz IF signal is the fed into a one of four analog multiplexer circuit. Via the internal
micro controller the appropriate bandwidth of 6, 12 30 and 100KHz is selectable. The 6, 12 and 30KHz
filters are ceramic filters the 100kHz filter is due to the high relative bandwidth a discrete filter with inductors
and capacitors. OP-amps behind the filters are used to match the filters with the appropriate impedance and
are used to provide almost the same level as at the multiplexer input. In a last gain stage the 455kHz signal is
amplified to the appropriate level for the level detector.
The signal level is measured with a logarithmic amplifier. This level detector has a very high dynamic range
and is also very linear. Output of the level detector is a D/C voltage proportional to the L-band input power.
This voltage is converted into a 16bit digital word by an analogue to digital converter with 16bit resolution.
A on board temperature sensor gives the beacon receiver M&C board the knowledge about the temperature
on the RF circuit board and allows fro temperature compensation.
D/C-input to the beacon receiver is +24V/0.5A provided by the M&C board.
There are two D/C-D/C converters on the board to convert the +24V D/C into +6V for the analog +5V
supply and the digital +3.3V / +5V. The digital D/C voltage regulator provides a DC_ERROR signal which
can be read back to the M&C board.
The 2nd D/C-converter is used for the LNB supply which can be programmed either to 14V or 18V.
Furthermore this LNB voltage can be modulated by the Diseq_22kHz signal to provide a DiseqC interface.
The DiseqC interface is not implemented in the first design stage but the hardware basis is given with this
circuit design and the real DiseqC implementation is later on a question of software in the micro-controller
on the beacon receiver M&C board. At the moment only a 22KHz signal is generated if necessary
In cases where no LNB supply is needed the D/C converter can be switched off by the LNC_ON signal.
In order to reduce interference from the micro-controller to the RF part and also to reduce cost on the PC
board production the circuit for the monitoring and control of the L-Band beacon receiver is designed on a
second printed circuit board.
The main interface to the beacon receiver RF board is via the SPI interface. This means that the analog to
digital converter, the temperature sensor and also all monitoring and alarm input and output ports are placed
on the RF board and the digital information is routed via the SPI bus to the M&C board and its micro-
controller.
In addition to that the following interface is provided, the output of the logarithmic detector provides a
analog output voltage which can be used via two interfaces:
Analog output
Output via 16bit analog to digital converter
The analog output voltage is provided to allow the receiver to interface with other manufacturers antenna
control units. The analogue output voltage is generated in the micro-controller in digital form. This does
have the big advantage that all temperature, linearity and slope compensation is also valid for the analogue
output voltage. Also the slope in dB/V and the input level which generates 0V analogue output voltage can
be set via the web browser interface. Via this connector other vendors antenna control units can receive their
beacon level input signal as analog voltage connected to their analog to digital converter in the ACU. The
SMA connector was selected due to space reasons, as a BNC did not fit on the interface side of the L-Band
beacon receiver housing.
This micro-controller controls also the synthesiser and the other hardware selections like bandwidth control.
The micro-controller communicates to the to the 'outside world' through two serial communication
(C) 2020, SatService GmbH
www.satnms.com
LBRX-1MT-UM-2002 Page 18/21
