MT-MR PLATINUM. POINT TO POINT AURAL RADIO RELAY SYSTEM
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IF unit receives the 10.7 MHz I.F. signal, together with all spurious components produced in the non-linear process
of mixing the Local Oscillator and RF signals. In the most used version of MR Platinum receiver, it can be fitted
with an additional IF output connector, to enable it to be used as a component of a repeater station. Other versions
not intended to be used as repeaters, but in STL links only, does not has this IF output enabled.
In the most general version of this unit, IF received signal follows two different paths within this unit: in the first, or
demodulation path, this complex spectrum is adequately amplified and filtered to obtain only the 10.7 MHz IF
component. This signal includes all the modulating baseband, and is amplified and amplitude-limited within
limiter/discriminator integrated circuit TDA 1599. This IC has the multiple functions of amplifying and amplitude-
limiting the incoming IF signal, and can also perform the further process of baseband demodulation through its
discriminator internal section, but in MR Platinum receiver this function is reserved for a CA 3189E in 10.7 MHz IF
unit. This IC also contains an internal squelch or muting circuit that cuts off demodulated baseband signal output
when noise level added to IF signal reaches some threshold, as happens in signal fading propagation conditions.
Mute threshold is obviously adjustable by a potentiometer, having this mute voltage a further amplifier circuit in
order to trigger a relay to cut off baseband output when noise level raises beyond prescribed threshold.
Second path, or repeater path, is the other way followed by IF signal. Here the signal suffers a double filtering
process, being also amplitude-limited and internally filtered again by a limiter IC. Afterwards, this processed and
purified IF signal is amplified and delivered to the IF output connector, to be used in a heterodyne repeater
arrangement. This second path is normally omitted in normal production receivers, and included in IF Processor &
Demodulator unit at Customer’s request.
Microcontroller unit and Display are similar to those used in MT Platinum transmitter.These cards are directly
attached to the back side of Equipment’s Front Panel, containing the main control & supervision elements of
receiver. Microcontroller unit works closely together with PLL section of Local Oscillator, sending it the
corresponding orders, concerning the internal frequency divider programming at PLL circuit, and in accordance with
the chosen reception frequency entered to Microcontroller via the push-buttons Control Panel. Microcontroller unit
also receives information about the PLL’s phase lock condition, allowing audio at receiver’s output only when PLL is
locked in phase. Microcontroller Unit uses a specialized Microprocessor device, or Microcontroller type AT89S8252
having a clock frequency of 11 MHz. This device controls, via ADC08234 integrated circuit, the possible fixing
points for the different binary division rates at the internal programmable frequency divider of PLL integrated circuit,
choosing in this way the receiving frequency. Microcontroller device also has an associated MAX232
interface matching with the RS232 output series interface connector located at Rear Panel to remotely control and
supervise Equipment’s performance. Visual organ of Microcontroller unit is the Liquid Crystal Digital display
V20350, coupled to Microcontroller’s output ports through its “Dx” inputs (D0, D1, D2 ... D7).
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Once audio information merges from IF Unit, this baseband information will be adapted and matched to the external
equipments to which they will go, by modifying its output impedance, level, etc. in order to optimize matching with
external circuits. By the other hand, this baseband information, together with the parameters defining the
operational status of receiver, can be locally supervised in the Equipment itself. Coupling component between the
external outputs or the internal supervision elements, and audio baseband outgoing from IF unit, is the Audio
Output card, and the tiny “Monitor Card”, this late directly mounted over the Headphones jack, giving audio output
both for monitor headphones and loudspeaker. Audio card is mounted at the back side of the Equipment’s Rear
Panel.
