2. Description of Functions
Radio Part MR400
The architecture of MR400 (MR300, MR160) radio modems resolves most of the requirements placed on a top
quality user friendly radio modem with a very short switching time between receiving and transmitting.
Frequency synthesis enables software selection of an arbitrary channel from the given frequency range. The
operation of the radio part is controlled and diagnosed by the microcontroller. The receiving part of the radio
modem is a double-conversion superheterodyne. The overall selectivity is divided between both intermediate
frequency levels. The first filter carries out the necessary channel pre-selection which ensures the linear function
of the following second mixer and intermediate frequency amplifier. The second filter has the attenuation
characteristic necessary for channel selection in the used channel spacing of 25 kHz (12.5 kHz). Logic circuits,
switching the radio part between modes of receiving and transmitting, have high noise immunity and switch
respective blocks sequentially. This minimises transient parasite states and optimises bandwidth during
switching. Switching to the transmitting mode is blocked when the frequency synthesiser is out of lock or the
internal temperature exceeds a hardware set limit or the supply voltage falls below the value ensuring proper
functioning of the device.
Modem Part
The control microcomputer has 4 MB of FLASH memory and 16 MB of RAM memory available. The battery
backup-ed real-time clock, detector of supply voltage failure and watch-dog circuits belong amongst the other
circuits of this block. If there is a supply voltage failure the fact is recorded into memory with the respective
timestamp thanks to the charge stored in electrolytic capacitors. The user therefore has information available
about the time and duration of possible faults caused by power failures. It is possible to connect equipment with
signalling rates up to 115.2 kbit/s to the modem via the RS232 data interface. RS232 interface converters are
protected against overvoltage with TRANSIL elements. A lithium battery is used for backing up in the modem
part.
Important warning – a risk of explosion!
The lithium battery must not be replaced by an incorrect type. When in doubt, leave the replacement to
the manufacturer.
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