NEC N8, Service Manual

Page: 37 / 182

NEC Confidential & Proprietary
5. MODULATOR
The Transmitter shared the RF VCO (OSC121). When in transmission mode, the VCO
generated frequency as:
GSM 3840-3980MHz
DCS 3580-3730MHz
PCS 3860-3980MHz
These two groups of frequency will further divided by a factor of two or four. Which
help to generate the Tx frequency of GSM and PCN band.
The Tx VCO (OSC111) operated on two frequency ranges which is the GSM and
PCN bands.
GSM 880-915MHz
DCS 1710-1785MHz
PCS 1850-1910MHz
These two frequency signals feed back to the PLL circuit of IC121 to achieve the
frequency control task. These feed back signals mixed with the divided frequency signal
from RF VCO. After mixed, a low frequency signal 80MHz came out and sent to the PD
(phase discriminator) to compare with the IF from the modulator circuit.
Modulator and Tx IF
Two differentials I/Q signal. The modulator uses an indirect I/Q modulator, which suitable
for dual band operation. The series transmit IF filters allow improved wide-band noise,
which enables duplexer removal. The I/Q modulator requires no amplitude or phase
calibration to achieve high phase accuracy. It designs for easy for maintenance. The
modulator shared IF frequency (TX IF = 80MHz for GSM900 PCS1900 and DCS1800).
These IF signal will further mixed with Tx synthesiser to pull up the operation frequency to
GSM 900 band,PCS1900 band and DCS 1800 band.
26MHz
Reference
640MHz
80MHz
+pos / -neg
I / Q