4.5.1 Transmitter: Modulator and Up-conversion Loop
The modulation is also based on the principle of the
"up-conversion modulation phase locked loop" and is accomplished via the
BRIGHT IC(D800). The internal TX IF-LO provides the quadratic modulator with the
TX IF frequencies (79...81 MHz) by generating 632...648 MHz frequencies,
which are divided by 8.
This so generated IF GMSK RF signal is compared in a phase detector with the
down mixed GMSK RF output from the TX-VCO (Z890).
To get the comparison signal
signal appearing at Pin 10 of the (Z890)
is mixed with the LO1 signal (divided by 2).
The output (PLLOUT) signal of the phase detector passes a discrete loop filter
realised by capacitors and resistors to set the TXVCO to required frequency.
The large loop band width (~1,5MHz) guarantees that the regulating process is
considerably quicker than the changes in the modulation signal.
The required voltage
is provided by the
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ICM MP CCQ ST
C45, M50 AND MT50
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